Metaprogramming

Source references link to 7e7307e2 (main) in the Logos repository.

Scope: compile-time metaprogramming surface — function-like macros (#[fn_macro], #[token_macro]), quote!/quote_expr!/quote_item!/quote_ty! AST quoting and antiquotation, explicit comptime metacalls, annotations (#[...]) and their cfg!/derive/trigger machinery, the format! family, and the AnyVal/WAny inline value-form + type/enum reflection intrinsics that back them. Sources: tools/peg_gen_cpp/grammars/logos.peg (grammar layer) and src/compiler/mlir_gen_expr.cpp / related codegen (lowering layer); extracted into tools/spec-extract/rules/**/*.json.

Note: expr.macro.fn-style-call and metaprog.macro.fn-style-call both describe function-style macro-call grammar (general form vs. operand/no-struct-lit position) and are extracted from adjacent grammar productions; the first carries domain metaprog despite an expr.*-shaped id. Both are kept verbatim below (grouped under Macro) rather than merged, per extraction-fidelity policy.

Function-style macro call syntax #

metaprog.macro.raw-text-brace-escape — Raw-text macro-call capture honours backslash and backtick escapes #

RAW_TEXT captured from a balanced brace-delimited group on a fn-macro call (RAW_GROUP_*) honours two escapes so literal braces don’t break brace balancing: (a) a backslash immediately preceding {, }, backtick, or \ passes that character through without counting it toward the balance (the backslash itself survives into the captured text, for the handler to de-escape); (b) a backtick-delimited region is skipped as a whole, with no brace counting performed inside it. Both are additive: a well-formed body without these constructs is captured byte-identically.

Divergence: A3

Testability: behavioral

Source: tools/peg_gen_cpp/grammars/logos.peg#L80

expr.macro.fn-style-call — Function-style macro invocation #

Function-style macros invoke as name!(…), name![…], or name!{…}; the contents between balanced delimiters are captured as raw source text and re-interpreted at sema time per the callee’s macro kind (#[fn_macro] re-parses as an expression list; #[token_macro] lexes as a TokenStream). In no-struct-lit (condition) position the brace form name!{…} is excluded.

Source: tools/peg_gen_cpp/grammars/logos.peg#L2743-L2754, tools/peg_gen_cpp/grammars/logos.peg#L2550-L2559

metaprog.macro.fn-style-call — Function-style macro invocation in operand position #

Function-style macros invoke as name!(…) or name![…] in operand/expression position (if cfg!(unix) && … {}, while m!(x) {}); the ordinary (non-no-struct-lit) primary additionally admits the brace form name!{…}, which is excluded from the no-struct-lit grammar since it would swallow the enclosing if/while body block. Contents between the balanced delimiters are captured as raw source text and re-interpreted at sema time per the callee’s macro kind.

Source: tools/peg_gen_cpp/grammars/logos.peg#L2616-L2619, tools/peg_gen_cpp/grammars/logos.peg#L2809-L2814

#[fn_macro] — expression-position function-like macros #

metaprog.fn-macro.expr-and-item-forms — Function-style macros exist in expression and item forms #

name!(args) / name![args] (FN_MACRO_CALL, expression position) resolves CALLEE against #[fn_macro]-annotated functions, passing each argument AST as an ExprBlob. name!{...} at module-item position (FN_MACRO_CALL_ITEM) has the same callee/args shape but routes through the metacall_item-style splice path (callee returns an ItemList / QuoteItemBlob); after splicing the node becomes FN_MACRO_CALL_ITEM_DONE, which sema skips.

Testability: behavioral

Source: tools/peg_gen_cpp/grammars/logos.peg#L294-L296

metaprog.fn-macro.item-position — Item-position fn-macro invocation #

IDENT ! { ... } at item position invokes a function-like macro whose body is captured as raw text (RAW_GROUP_BRACE); brace delimiter only at item position (parens/brackets stay reserved for expression position). The callee resolves through the item-splice pipeline (returns an item list).

my_macro! { struct A; }

Related: metaprog.fn-macro.expr-and-item-forms

Source: tools/peg_gen_cpp/grammars/logos.peg#L579-L580

metaprog.fn-macro.callee-must-be-marked — name!(...) callee must be a #[fn_macro] or #[token_macro] fn #

A name!(...) invocation resolves name against the function overload set; the callee selected must be a fn annotated #[fn_macro] or #[token_macro]. If name is unknown, or no overload bears such an annotation, the call is rejected. Only macro-annotated fns are callable via name!(...) syntax.

Uncertainty: Resolution is restricted to non-generic funcs_ (generic fn_macro out of scope for this slice).

Source: src/compiler/sema_expr.cpp#L18497-L18517

metaprog.fn-macro.signature-shapes — Accepted fn-macro/token-macro signatures #

A #[fn_macro] callee must have exactly one parameter and signature (ExprBlob) -> ExprBlob (single-arg form) or (Vec<ExprBlob>) -> ExprBlob (N-arg packed form). A #[token_macro] callee must have signature (str) -> ExprBlob. Any other signature is rejected.

Source: src/compiler/sema_expr.cpp#L18525-L18555

metaprog.fn-macro.single-arg-arity — Single-arg fn-macro takes exactly one argument #

For a callee with signature (ExprBlob) -> ExprBlob, the name!(...) invocation must supply exactly one argument; supplying any other count is an error.

Source: src/compiler/sema_expr.cpp#L18536-L18539, src/compiler/sema_expr.cpp#L19017-L19022

metaprog.fn-macro.args-are-expr-list — name!(...) arguments parse as a comma-separated expression list #

The raw text between the parentheses of name!(...) is parsed as a comma-separated list of expressions (each becoming one macro ARG). If it does not parse as such, the invocation is rejected.

Source: src/compiler/sema_expr.cpp#L18566-L18681

metaprog.fn-macro.arg-passed-as-ast-blob — Each fn-macro argument is passed as its serialized AST subtree (ExprBlob) #

Each argument expression of name!(...) is passed to the callee unevaluated as an ExprBlob referencing the serialized AST subtree of that argument; the callee receives the syntax tree, not a runtime value. For the Vec form, all argument ExprBlobs are packed into a Vec<ExprBlob> in source order.

Source: src/compiler/sema_expr.cpp#L19028-L19116

metaprog.fn-macro.result-is-exprblob-spliced — fn-macro call expands to the ExprBlob it returns #

A name!(...) expression has the callee’s return type ExprBlob; at expansion the AST produced by the macro (the returned ExprBlob) is spliced in place of the call site before final sema.

Source: src/compiler/sema_expr.cpp#L19118-L19132

metaprog.fn-macro.cfg-builtin — cfg!(...) is a compile-time built-in predicate #

The built-in macro cfg!(...) is evaluated at compile (sema) time to a bool literal, before user fn-macro resolution. It accepts built-in target keys, the boolean combinators all/any/not, and user feature flags supplied via --cfg.

Source: src/compiler/sema_expr.cpp#L18489-L18493

metaprog.fn-macro.callee-marker-required — name!(...) resolves only #[fn_macro]/#[token_macro] callees #

A function-style macro call name!(...) resolves only callees marked #[fn_macro]; a #[token_macro] callee additionally receives its raw source text directly as a str argument.

Testability: transitive

Source: src/compiler/sema_impl.hpp#L2553-L2554

metaprog.fn-macro.builtin-macro-list — Fixed set of compiler-built-in function-style macros checked before user #[fn_macro]s #

Function-style macro calls (name!(args) / name![args]) are first checked against a fixed compiler-built-in set — cfg!, line!, column!, file!, include!, include_str!, include_bytes!, env!, concat!, concat_bytes!, stringify!, compile_error! — handled by lower_builtin_macro; a callee name not in this set falls through to resolution against user #[fn_macro]-tagged functions.

Testability: behavioral

Source: src/compiler/sema_impl.hpp#L4018-L4029

metaprog.fn-macro.item-callee-resolution — Callee resolution and lowering of item-position fn-macro calls #

A function-style macro invoked at module item position as name!{...} resolves its callee against #[fn_macro]-tagged functions (mirroring expression-position fn-macro calls) and routes its arguments through the per-site raw-capture arg-blob shim, mirroring metacall’s item-position lowering.

Related: metaprog.fn-macro.item-position, metaprog.metacall.item-position, metaprog.fn-macro.builtin-macro-list

Source: src/compiler/sema_impl.hpp#L4050-L4055

#[fn_macro] at item position #

metaprog.fn-macro-item.callee-must-be-fn-macro — Item-position macro call callee must be #[fn_macro] or #[token_macro] #

callee!{ ... } / callee!(...){...} at ITEM position resolves callee in the function-overload set; if no overload is found, or none of the overloads carries #[fn_macro] or #[token_macro], the item is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19483-L19508

metaprog.fn-macro-item.return-type — Item-macro callee must return ItemList or QuoteItemBlob #

An item-position #[fn_macro]/#[token_macro] callee’s declared return type must be ItemList or QuoteItemBlob; any other return type is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19510-L19519

metaprog.fn-macro-item.param-signature — Item-macro callee parameter shape is fixed per attribute #

An item-position #[fn_macro] callee must have parameter list (Vec<ExprBlob>) or (). An item-position #[token_macro] callee must have parameter list (str), (name: str, body: str), or (name: str, params: str, body: str). Any other arity/type combination for the given attribute is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19521-L19575

metaprog.fn-macro-item.raw-text-as-expr-list — fn_macro Vec<ExprBlob> item form re-parses RAW_TEXT as a comma-separated expr list #

For an item-position #[fn_macro] callee of shape (Vec<ExprBlob>), the macro’s RAW_TEXT is wrapped as fn __f() { __c(<RAW_TEXT>); }, parsed as Logos source, and the resulting call’s ARGS elements are lowered one-for-one into ExprBlob values (each argument’s AST subtree is copied into its own document, tagged with its original AST node CODE) and passed to the callee as Vec<ExprBlob>.

Testability: behavioral

Source: src/compiler/sema_expr.cpp#L19769-L19825, src/compiler/sema_expr.cpp#L19838-L19876, src/compiler/sema_expr.cpp#L19887-L19901

metaprog.fn-macro-item.zero-arg-arity — Zero-arg fn_macro item form rejects nonzero arg count #

An item-position #[fn_macro] callee declared with zero parameters is invoked with the RAW_TEXT re-parsed as a (possibly empty) comma-separated arg list; if that re-parse yields one or more arguments, the item is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19773-L19825, src/compiler/sema_expr.cpp#L19827-L19832

metaprog.fn-macro-item.arg-must-be-ast-node — Each re-parsed fn_macro item arg must be a valid AST node #

Each element of the re-parsed RAW_TEXT arg list (for the Vec<ExprBlob> item form) must be a non-null pointer value whose root carries a nonzero AST CODE; otherwise the item is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19841-L19855

#[token_macro] — token-stream macros #

metaprog.token-macro.raw-text-as-str — token-macro receives unparsed raw text as str #

For a #[token_macro] callee, the raw bytes between the delimiters of name!(...) are forwarded verbatim as a single str argument, bypassing expression-list parsing and per-argument AST serialization.

Source: src/compiler/sema_expr.cpp#L18576-L18619

#[token_macro] at item position #

metaprog.token-macro-item.name-slot-scope — Resource NAME slot required for 2/3-arg token_macro, optional+dropped for 1-arg #

resource <name> = h!{...} / resource <name> = h!(<params>){...} supplies the LHS binding as a NAME field on the item node. Targeting the 2-arg (name: str, body: str) or 3-arg (name: str, params: str, body: str) #[token_macro] form without a NAME is rejected. Targeting the 1-arg (body: str) form, a supplied NAME is accepted but the value is discarded — never delivered to the callee.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19577-L19592, src/compiler/sema_expr.cpp#L19658-L19663

metaprog.token-macro-item.params-slot-scope — Resource PARAMS slot only valid for the 3-arg token_macro form #

h!(<params>){...} supplies a PARAMS field carrying the raw params source text. Supplying PARAMS against any callee signature other than the 3-arg (name: str, params: str, body: str) #[token_macro] is rejected; conversely, targeting the 3-arg form without a supplied PARAMS is also rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19600-L19620

metaprog.token-macro-item.raw-text-verbatim — token_macro item form receives raw block text unparsed #

For an item-position #[token_macro] callee of shape (str), (name: str, body: str), or (name: str, params: str, body: str), the full RAW_TEXT bytes of the macro’s block (and, where present, the NAME and PARAMS text) are passed to the callee as opaque str values byte-for-byte — the content is NOT re-parsed as Logos source before the call.

Testability: behavioral

Source: src/compiler/sema_expr.cpp#L19630-L19664, src/compiler/sema_expr.cpp#L19670-L19707

Antiquotation (#ident capture into quoted AST) #

metaprog.antiquot.capture-forms — Writ antiquotation capture syntax #

Within a quoted/Writ literal, an antiquotation captures a value either by identifier $name or by expression block ${expr}.

Divergence: Logos metaprogramming antiquotation; no Rust equivalent.

Testability: transitive

Source: src/compiler/sema_render.cpp#L532-L537

metaprog.antiquot.callee-skips-payload-arg — Antiquoted callee drops synthetic first arg #

When a call’s callee is produced by antiquotation substitution, the grammar’s bulk $... capture inserts the antiquot payload as the first argument; semantically that first argument is not a real call argument and is skipped.

Uncertainty: Inferred from renderer mirroring lower_call; exact substitution mechanism defined elsewhere.

Testability: transitive

Source: src/compiler/sema_render.cpp#L141-L170

quote!/quote_item!/quote_ty! — overview #

metaprog.quote.antiquote-placeholder — #ident antiquotation placeholder inside quote_*! bodies #

Inside the body of a quote_*! macro, #ident is an antiquotation placeholder splicing an external value; its AST node carries a NAME_VAR field holding the placeholder’s identifier text. NAME_VAR exists only inside quote_*! bodies (where a where-clause never co-occurs).

Divergence: A3

Testability: behavioral

Source: tools/peg_gen_cpp/grammars/logos.peg#L78

metaprog.quote.typed-ast-literals — quote_item!/quote_expr!/quote_ty! are typed AST literals #

quote_item! { item* } (QUOTE_ITEM), quote_expr! { expr } (QUOTE_EXPR), and quote_ty! { type } (QUOTE_TY) are typed AST-literal forms yielding, respectively, an item list, a single expression AST (root carries a schema_type_code), and a Type AST. Inside a quote body, #(body)sep* (REPEAT_GROUP) repeats body with separator sep in {none, comma, &&}. $ident in type position (ANTIQUOT_TYPE) is legal only inside quote_ty! and refers to a Type-valued binding; $ident... in a type-argument list (ANTIQUOT_PACK) is legal only inside quote_ty! and refers to an Array<Type>-valued binding.

Testability: behavioral

Source: tools/peg_gen_cpp/grammars/logos.peg#L269-L275

metaprog.quote.quote-item — quote_item! typed-AST item literal #

quote_item! { item* } parses zero or more item declarations as its body and produces a QUOTE_ITEM node; sema deep-clones the parsed items into a fresh Writ module, serializes it, and emits a byte-slice (WritStatic) literal value.

Source: tools/peg_gen_cpp/grammars/logos.peg#L3105-L3112

metaprog.quote.quote-expr — quote_expr! typed-AST expression literal #

quote_expr! { expr } parses a single expression as its body and produces a QUOTE_EXPR node; sema deep-clones the expr’s AnyVal as the root TOM of a fresh Writ doc (root schema_type_code set from CODE) and emits an ExprBlob value, spliced via metacall.

Source: tools/peg_gen_cpp/grammars/logos.peg#L3114-L3121

metaprog.quote.quote-ty — quote_ty! typed Type literal #

quote_ty! { type_ref } parses a single type expression as its body and produces a QUOTE_TY node; sema resolves it to a TypeRef and emits the same Type{kind, name, size} struct literal that type_of::<T>() produces.

Source: tools/peg_gen_cpp/grammars/logos.peg#L3123-L3129

quote_expr! — expression-AST quoting #

metaprog.quote-expr.reify-ast-to-exprblob — quote_expr! reifies an expression AST into an ExprBlob #

quote_expr! { e } evaluates to a value of struct type ExprBlob carrying the serialized AST of e. With no antiquots, the AST is emitted as a static rodata blob and wrapped directly as ExprBlob { ptr }.

Divergence: A3/A6 (replaces Rust macro/quote layer)

Source: src/compiler/sema_expr.cpp#L16386-L16423, src/compiler/sema_expr.cpp#L16806-L16813

metaprog.quote-expr.antiquot-must-be-in-scope — Antiquot variable in quote_expr! must be a bound local #

A #name antiquot inside quote_expr! is an error unless name is a variable in scope at the quote site (“#name — variable not in scope”).

Divergence: A3/A6

Source: src/compiler/sema_expr.cpp#L16510-L16514

metaprog.quote-expr.scalar-antiquot-type — Scalar antiquot must be Ident, or Ident/ExprBlob outside ident-only positions #

A #name antiquot outside a repetition group, in a general expression position, must bind a value of type Ident or ExprBlob; in ident-only positions (field names, struct type name, field-read selector) it must bind an Ident. Otherwise it is rejected (“expected Ident” / “expected Ident or ExprBlob”).

Divergence: A3/A6

Source: src/compiler/sema_expr.cpp#L16549-L16560, src/compiler/sema_expr.cpp#L16618-L16621, src/compiler/sema_expr.cpp#L16661-L16685

metaprog.quote-expr.repeat-cursor-type — Repetition cursor must be [Ident;N], Vec<Ident>, or Vec<ExprBlob> #

A #name antiquot inside a #(...)* repetition group (a cursor) must bind a value of type [Ident; N] (fixed count N), Vec<Ident>, or Vec<ExprBlob> (dynamic count); any other type is rejected (“expected [Ident; N], Vec<Ident>, or Vec<ExprBlob>”).

Divergence: A3/A6

Source: src/compiler/sema_expr.cpp#L16523-L16538, src/compiler/sema_expr.cpp#L16469-L16492

metaprog.quote-expr.repeat-cursor-length-agree — Fixed-length cursors in one repetition group must agree on length #

Within a single #(...)* group, all fixed-size [Ident; N] cursors must share the same length N; a sibling cursor with a different N is rejected (“cursor length mismatches sibling cursor in same #(...)*”). A Vec-backed (dynamic) cursor makes the group dynamic and waives the fixed-length agreement check.

Divergence: A3/A6

Source: src/compiler/sema_expr.cpp#L16539-L16548

metaprog.quote-expr.repeat-needs-cursor — A repetition group must contain at least one cursor antiquot #

A #(...)* repetition group body must contain at least one cursor antiquot #x of a cursor type; an empty-cursor body is rejected (“#(...)* body has no cursor #x”).

Divergence: A3/A6

Source: src/compiler/sema_expr.cpp#L16600-L16605

metaprog.quote-expr.no-nested-repeat — Nested repetition groups are not allowed #

A #(...) repetition group may not be nested inside another #(...) group (“nested #(...) repetition not supported”).

Divergence: A3/A6

Testability: untestable

Source: src/compiler/sema_expr.cpp#L16589-L16597

metaprog.quote-expr.antiquot-carrier-positions — Antiquots are recognized only in defined AST carrier positions #

Antiquots and repetition groups are recognized only within the supported carrier set: VAR_REF, BINOP (lhs/rhs), PAREN/UNARY/CAST/DEREF (value), FIELD_READ (selector + receiver), CALL/METHOD_CALL/STATIC_CALL (callee-name-var, receiver, args), STRUCT_LIT/FIELD_INIT/FIELD_SHORTHAND, ARR_LIT/TUPLE_LIT/BLOCK items, statement carriers (LET, LET_DESTRUCT, EXPR_STMT, TAIL_EXPR, RETURN), and control flow (IF, WHILE, FOR, LOOP, ASSIGN, COMPOUND_ASSIGN). Antiquots in unsupported shapes are not substituted.

Divergence: A3/A6

Uncertainty: The exact carrier set is an evolving implementation surface (slices noted in comments), not a frozen spec.

Source: src/compiler/sema_expr.cpp#L16586-L16747

metaprog.quote-expr.subst-runtime — quote_expr! with antiquots substitutes at runtime via logos_quote_expr_subst #

quote_expr! containing N>0 antiquots lowers to a block that binds the static template blob and one IdentSpan { ptr, count, kind } per placeholder, then calls logos_quote_expr_subst(template_ptr, size, &spans[0], N) -> *const u8 and wraps the result as ExprBlob { ptr }. Span kind is 0 for Ident slots, 1 for ExprBlob slots, 2 for Vec<ExprBlob> cursors.

Divergence: A3/A6

Source: src/compiler/sema_expr.cpp#L16815-L16981, src/compiler/sema_expr.cpp#L16866-L16943

quote_item! — item-AST quoting #

metaprog.quote-item.name-antiquot-forms — quote_item! accepts #name and #(expr) name antiquotations #

Within quote_item! { ... }, a NAME_VAR placeholder accepts two forms: #name (shortcut) looks the variable up in the metafn scope and requires type Ident; #(expr) lowers the inner expression in the metafn scope and requires type Ident or ExprBlob. Any other pointee kind is an error.

Divergence: Logos metaprogramming construct; no Rust equivalent.

Source: src/compiler/sema_expr.cpp#L15569-L15625

metaprog.quote-item.repeat-cursor-depth — #(...)* repetition binds cursor placeholders by Vec nesting depth #

Inside a #(...)* repetition group, a #name placeholder becomes a Cursor whose depth equals its Vec nesting (1 for Vec<Ident>, 2 for Vec<Vec<Ident>>); the variable’s cursor depth d must be non-zero and ≤ the current repeat depth, else an error. Outside any repetition group, #name must be a scalar Ident.

Source: src/compiler/sema_expr.cpp#L15582-L15606, src/compiler/sema_expr.cpp#L15523-L15533

metaprog.quote-item.repeat-nesting-limit — #(...) repetition nesting limited to 2 levels #

#(...) repetition groups in quote_item! may nest at most 2 levels deep; deeper nesting is an error.

Source: src/compiler/sema_expr.cpp#L15554-L15568

metaprog.quote-item.placeholder-walk-order — Placeholder index is fixed by source-tree DFS order #

Placeholder indices are assigned by a deterministic depth-first walk of the quoted item subtrees (recursing into all pointer-valued TOM keys and array elements except NAME_VAR), and the destination rewrite mirrors the same recursion so producer indices align with placeholder slots.

Testability: transitive

Source: src/compiler/sema_expr.cpp#L15541-L15661

metaprog.quote-item.synthetic-main-module — quote_item! produces a synthetic package main module #

quote_item! { item* } constructs a synthetic AST module whose root is MODULE with NAME=“main”, empty PATH_PARTS, ITEMS = the deep-cloned quoted items, and SRC_LINE=1. The result is emitted as a serialized WritStatic blob carried by a QuoteItemBlob value.

Divergence: Logos metaprogramming addition (no Rust equivalent).

Testability: untestable

Source: src/compiler/sema_expr.cpp#L15859-L15894, src/compiler/sema_expr.cpp#L15805-L15819

metaprog.quote-item.inherit-import-scope — quote_item! inherits the metafn’s import scope #

The synthetic module inherits the enclosing metafn’s wildcard use packages, plus a self-use of the metafn’s own package (if non-empty), so that unqualified names inside the quoted items resolve through the metafn’s use-list. Each inherited package becomes one USE node carrying the full dotted package name in NAME.

Divergence: Logos metaprogramming addition; controls hygiene/name resolution of quoted items.

Testability: transitive

Source: src/compiler/sema_expr.cpp#L15821-L15857

metaprog.quote-item.placeholder-walk-balance — Source and destination placeholder counts must match #

The number of antiquot placeholders discovered while scanning the source items must equal the number of placeholder slots rewritten in the cloned destination tree; a mismatch is a compile error.

Divergence: Logos metaprogramming addition.

Uncertainty: This is an internal consistency invariant; user-observable only as a diagnostic.

Testability: untestable

Source: src/compiler/sema_expr.cpp#L15797-L15802

metaprog.quote-item.blob-result-type — quote_item! evaluates to a QuoteItemBlob value #

quote_item! evaluates to a QuoteItemBlob struct value with fields { template_ptr, template_size, idents_blob, blobs_blob, cursors_blob }, where template_ptr/template_size address the serialized synthetic-module blob and the *_blob fields carry the packed antiquot substitution data (null when the corresponding placeholder kind has zero occurrences).

Divergence: Logos metaprogramming addition.

Testability: untestable

Source: src/compiler/sema_expr.cpp#L16133-L16144, src/compiler/sema_expr.cpp#L15907-L15910

metaprog.quote-item.ident-antiquot-packing — #name/#(expr) Ident antiquots are packed as Ident pointers #

Each scalar Ident antiquot site (#name shortcut or #(expr) yielding Ident) contributes one *const Ident to the idents blob, in DFS placeholder order; a #(expr) form binds the lowered expression to a fresh local whose address is taken. When there are no Ident sites, idents_blob is null.

Divergence: Logos metaprogramming addition.

Testability: untestable

Related: metaprog.quote-item.blob-result-type

Source: src/compiler/sema_expr.cpp#L15953-L16003

metaprog.quote-item.exprblob-antiquot-packing — ExprBlob antiquots are packed by their .ptr field #

Each #(expr) antiquot whose lowered expression has type ExprBlob contributes one *const u8 (the ExprBlob’s ptr field) to the blobs blob, in DFS placeholder order; the lowered ExprBlob is bound to a local that outlives the array. When there are no ExprBlob sites, blobs_blob is null.

Divergence: Logos metaprogramming addition.

Testability: untestable

Related: metaprog.quote-item.blob-result-type

Source: src/compiler/sema_expr.cpp#L16005-L16054

metaprog.quote-item.cursor-repetition-packing — Cursor (#(...)*) antiquots carry a per-site nesting depth #

Each repetition-cursor antiquot site contributes a *const u8 (the address of a Vec cursor variable) plus a parallel per-site depth byte: depth 1 = Vec<Ident>, depth 2 = Vec<Vec<Ident>> (nested #(...)*). The element type is the neutral *const u8; pack reads each cursor according to its depth. When there are no cursor sites, cursors_blob is null.

Divergence: Logos metaprogramming addition.

Testability: untestable

Related: metaprog.quote-item.blob-result-type

Source: src/compiler/sema_expr.cpp#L16056-L16127, src/compiler/sema_expr.cpp#L15939-L15944

quote_ty! — type-AST quoting #

metaprog.quote-ty.reify-type-to-struct — quote_ty! reifies a type into a runtime Type value #

quote_ty! { T } evaluates to a value of struct type Type whose fields are { kind: u32 = __type_kind_of__::<T>(), name: &[u8] = __type_name_of__::<T>(), size: i64 = size_of::<T>(), align: i64 = align_of::<T>(), uid: u64 = __type_uid_of__::<T>() }.

Divergence: A6 (Logos addition; metaprog reflection intrinsic)

Related: intrinsic.type-reflect.kind, intrinsic.type-reflect.name, intrinsic.type-reflect.uid

Source: src/compiler/sema_expr.cpp#L16357-L16383, src/compiler/sema_expr.cpp#L16179

metaprog.quote-ty.antiquot-type-var — $ident antiquot inside quote_ty! refers to a bound Type value #

An ANTIQUOT_TYPE $x inside quote_ty! lowers to a variable reference of type Type (the in-scope binding named x), instead of being reified from a static type.

Divergence: A6 (Logos addition)

Source: src/compiler/sema_expr.cpp#L16182-L16184, src/compiler/sema_expr.cpp#L16314-L16316

metaprog.quote-ty.tuple-antiquot — quote_ty! tuple with antiquot lowers to tuple_type_apply #

quote_ty! { ($t1, $t2, ...) } where at least one element is an antiquot lowers to __tuple_type_apply__([p1, p2, ...]) where each pi is the per-element Type producer (var-ref for $x, reified Type literal otherwise); mixed literal/antiquot elements are permitted.

Divergence: A6 (Logos addition)

Source: src/compiler/sema_expr.cpp#L16209-L16234

metaprog.quote-ty.array-antiquot-literal-size — quote_ty! array with antiquot element requires literal integer size #

quote_ty! { [$t; N] } lowers to __array_type_apply__(elem_producer, N); the size N MUST be a literal integer (a non-numeric/symbolic size is rejected with “array antiquot requires literal integer size”).

Divergence: A6 (Logos addition)

Source: src/compiler/sema_expr.cpp#L16238-L16263

metaprog.quote-ty.pack-splice — quote_ty! generic pack-splice lowers to type_apply with runtime array #

quote_ty! { Foo<$ts...> }, where the sole generic argument is an ANTIQUOT_PACK $ts..., lowers to __type_apply__("Foo", ts) where ts is a var-ref to a runtime Array<Type>. A pack-splice mixed with any other generic argument (Foo<$t, $ts...>) is rejected (“mixed pack-splice with other args not yet supported”).

Divergence: A6 (Logos addition)

Uncertainty: Mixed-pack rejection is a current implementation limit, not a permanent language rule.

Source: src/compiler/sema_expr.cpp#L16271-L16294

metaprog.quote-ty.generic-inst-antiquot — quote_ty! generic instantiation with antiquot args lowers to type_apply #

quote_ty! { Foo<args...> } with at least one $ident antiquot among the args lowers to __type_apply__("Foo", [elems]), where each elem is a var-ref (for $x) or a reified Type struct literal (for a concrete type arg). Lifetime args and pack-expand args in this position are rejected (“lifetime / pack args not yet supported”).

Divergence: A6 (Logos addition)

Source: src/compiler/sema_expr.cpp#L16299-L16355

Metacall — explicit comptime invocation #

metaprog.metacall.explicit-comptime-call — metacall runs a compile-time callee and splices its result #

metacall <call_expr> (METACALL, expression position) is an explicit compile-time invocation. At item position, metacall <call_expr>; (METACALL_ITEM) is JIT-executed by the driver; the callee must return a QuoteItemBlob, which is spliced into the surrounding item stream via logos_emit_item_blob_subst. After splicing, the node is rewritten to METACALL_ITEM_DONE, which sema’s item-dispatch silently skips (and which is never produced by parsing).

Testability: behavioral

Source: tools/peg_gen_cpp/grammars/logos.peg#L267, tools/peg_gen_cpp/grammars/logos.peg#L278-L279

metaprog.metacall.forms — metacall expression forms #

metacall accepts exactly three operand shapes — a block (metacall { … }), a parenthesized expression (metacall (e)), or a call expression (metacall f(…), including generic f::<T>(…) and static Type::m(…)) — and evaluates its argument at compile time.

Divergence: A1/A6 — Logos addition: metacall is the explicit compile-time evaluation operator, the Logos replacement for const-eval (no implicit const-eval).

Related: metaprog.metacall.block-tail-required

Source: tools/peg_gen_cpp/grammars/logos.peg#L2599-L2604, tools/peg_gen_cpp/grammars/logos.peg#L2731-L2736, tools/peg_gen_cpp/grammars/logos.peg#L2791-L2796, src/compiler/sema_expr.cpp#L17084-L17088

metaprog.metacall.item-decl-syntax — Item-position metacall statement #

metacall <call_expr> ; at item (module top-level) position invokes a metacall whose callee returns a QuoteItemBlob; the driver runs the JIT’d thunk, which splices the returned items into the enclosing program.

Related: metaprog.metacall.explicit-comptime-call

Source: tools/peg_gen_cpp/grammars/logos.peg#L604-L610

metaprog.metacall.item-position-splice — Item-position metacall splices synthesized items #

A metacall foo(); in item position is evaluated during discovery and its result is spliced as synthesized top-level item(s) into the program.

Source: src/compiler/metaprog_dispatch.hpp#L1-L3, src/compiler/metaprog_dispatch.hpp#L97-L98

metaprog.metacall.no-nested-metacall — metacall may not be nested inside another metacall’s operand #

A metacall operand (call args, or the inner subtree for the block/expr forms) must not contain another metacall node; metacall is a one-shot lift to compile time whose result is a runtime value and therefore cannot serve as a compile-time argument to an enclosing metacall. Violation is a compile error.

Divergence: A1/A6: metacall replaces Rust const-eval; rule has no Rust analogue.

Related: metaprog.metacall.forms

Source: src/compiler/sema_expr.cpp#L17090-L17178

metaprog.metacall.no-runtime-capture — metacall block/expr form cannot capture enclosing runtime locals #

In the block and parenthesized-expr forms, every VAR_REF must resolve to a name introduced inside the operand (LET/FOR/FOR_EACH binding, or a match-arm pattern binding), a module-level const, or a known function (concrete or generic). A reference to an enclosing-scope runtime local is a compile error, since the metacall is evaluated at compile time with no access to surrounding locals.

Divergence: A1/A6: compile-time evaluation model specific to metacall.

Related: metaprog.metacall.const-resolver

Source: src/compiler/sema_expr.cpp#L17196-L17302

metaprog.metacall.args-ctfe-constant — Every argument of a metacall call form must be a compile-time constant #

For the call form, each argument expression must be CTFE-evaluable to a constant literal; an argument that cannot be folded is a compile error. CALL stores arguments as a flat array, while GENERIC_CALL/STATIC_CALL wrap them as { ITEMS: [...] }.

Divergence: A1/A6: CTFE of metacall args; replaces Rust const-eval.

Related: metaprog.metacall.const-resolver

Source: src/compiler/sema_expr.cpp#L17334-L17359

metaprog.metacall.const-resolver — metacall argument CTFE resolves bare module-const idents #

CTFE of metacall arguments and operands resolves a bare identifier naming a module-level const (collected into the module const-value map, including cross-package consts) to that const’s value, so expressions like metacall { THRESHOLD + 1 } fold.

Divergence: A1/A6: metacall const folding.

Related: metaprog.metacall.args-ctfe-constant, metaprog.metacall.no-runtime-capture

Source: src/compiler/sema_expr.cpp#L17311-L17332

metaprog.metacall.block-tail-required — metacall block must end in a tail expression #

A metacall { ... } block must terminate with a tail expression (no trailing semicolon) so the metacall yields a value; a block lacking a tail expression is a compile error. The block’s value type is the type of that tail expression.

Divergence: A1/A6.

Related: metaprog.metacall.return-type

Source: src/compiler/sema_expr.cpp#L17366-L17389

metaprog.metacall.return-type — metacall result type must be primitive scalar, WritStatic, Writ, or ExprBlob #

The type produced by a metacall operand must be a primitive scalar (bool; integer kinds i8/i16/i24/i32/i56/i64 and u8/u16/u24/u32/u56/u64; f32/f64; integer/float literal types), a &str / Slice<u8>, WritStatic, Writ (incl. Rc<Writ>), or ExprBlob. Any other result type is a compile error.

Divergence: A1/A6: WritStatic/Writ/ExprBlob returns are Logos additions.

Related: metaprog.metacall.exprblob-deferred-typing, metaprog.metacall.writ-autofreeze

Source: src/compiler/sema_expr.cpp#L17408-L17424

metaprog.metacall.exprblob-deferred-typing — ExprBlob-returning metacall defers result typing to the post-splice pass #

When a metacall returns an ExprBlob (an AST-expression fragment marker), pass-1 typing is deferred: let X: T = metacall foo() accepts any annotated T over an ExprBlob RHS; the actual expression type is recovered after the driver splices the blob and pass-2 sema re-lowers it.

Divergence: A3/A6: ExprBlob is the Logos metaprog AST-fragment return.

Testability: untestable

Related: metaprog.writ-blob.ast-fragment-recurse

Source: src/compiler/sema_expr.cpp#L17400-L17407

metaprog.metacall.writ-autofreeze — Writ-returning metacall auto-freezes to WritStatic and is call-form only #

A metacall whose operand returns a (mutable) Writ / Rc<Writ> is auto-frozen: user code observes the spliced value as WritStatic (the lowered expression is retyped to WritStatic). The Writ return type is supported only on the call form (metacall foo()); using it with the block or expr form is a compile error.

Divergence: A6: Writ/WritStatic is a Logos addition.

Related: metaprog.metacall.return-type

Source: src/compiler/sema_expr.cpp#L17537-L17568, src/compiler/sema_expr.cpp#L17597-L17603

metaprog.metacall.runtime-passthrough — metacall lowers as a runtime pass-through until driver-side splice #

During sema iterations a metacall lowers to its operand’s lowered value (a pass-through), keeping the in-progress IR valid for borrow/type checks. The driver replaces the metacall AST node with the evaluated literal before the final non-metaprog sema pass, so this pass-through lowering never reaches code generation.

Divergence: A1/A6: compile-time splice model.

Testability: untestable

Related: metaprog.metacall.return-type

Source: src/compiler/sema_expr.cpp#L17606-L17610

metaprog.metacall.item-position — Item-position metacall #

metacall <call-expr> ; is legal at module item position (top level). The named metafunction is invoked at compile time and must return an item blob; the items it produces are spliced into the program at the metacall’s position, replacing the metacall item itself.

metacall gen_items::<Foo>();

Source: tools/peg_gen_cpp/grammars/logos.peg#L581-L582, src/compiler/sema_impl.hpp#L4044-L4049

Metacall at item position #

metaprog.metacall-item.callee-form — Item-position metacall callee must be a call expression #

metacall <expr>; at item position requires <expr> to be a free-function call, a generic (turbofish) call, or a static-method call; any other expression form is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19980-L19989

metaprog.metacall-item.args-const-eval — Item-position metacall arguments must be compile-time constants #

Every argument in an item-position metacall <callee>(args...) is evaluated via CTFE (with module-level named consts resolvable through lookup_const); an argument that fails to fold to a constant is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L19991-L20034

metaprog.metacall-item.return-type — Item-position metacall callee must return QuoteItemBlob or ItemList #

The callee of an item-position metacall (after ordinary lowering/type-checking of the inner call) must have return type QuoteItemBlob or ItemList; any other return type is rejected.

Testability: diagnostic

Source: src/compiler/sema_expr.cpp#L20038-L20047

metaprog.metacall-item.turbofish-wstatic-source — WritStatic turbofish type-args re-rendered as source for the metacall thunk #

When building the literal call text for an item-position metacall’s generic/static call, a turbofish type-argument that is a WritStatic literal (Foo::<@{...}>) is re-rendered from its parsed writ-literal AST back into @{...} source syntax, rather than through the ordinary type-to-string path (which would emit the non-reparseable internal form @hs_<hex>).

Uncertainty: Internal @hs_<hex> rendering form is inferred from the comment; the type_str() implementation itself is outside this slice.

Testability: behavioral

Source: src/compiler/sema_expr.cpp#L20050-L20126, src/compiler/sema_expr.cpp#L20140-L20151

Item emission from macro/metacall results #

metaprog.item-emit.itemlist-iteration — ItemList-returning item macro/metacall emits each contained item #

When an item-position fn_macro/token_macro call or item-position metacall resolves to a callee returning ItemList, the compiler synthesises a void thunk that iterates ItemList.blobs and, for each QuoteItemBlob element, emits it into the surrounding module (substituting captured identifiers) then releases its idents/blobs/cursors buffers.

Testability: behavioral

Source: src/compiler/sema_expr.cpp#L19709-L19736, src/compiler/sema_expr.cpp#L19904-L19930, src/compiler/sema_expr.cpp#L20196-L20222

metaprog.item-emit.quoteitemblob-single — QuoteItemBlob-returning item macro/metacall emits a single item #

When an item-position fn_macro/token_macro call or item-position metacall resolves to a callee returning QuoteItemBlob (not ItemList), the compiler synthesises a void thunk that emits that single blob into the surrounding module (substituting captured identifiers) then releases its idents/blobs/cursors buffers.

Testability: behavioral

Source: src/compiler/sema_expr.cpp#L19737-L19757, src/compiler/sema_expr.cpp#L19931-L19950, src/compiler/sema_expr.cpp#L20223-L20241

Macro/handler discovery #

metaprog.discovery.entry-body-skipped — Entry-file function bodies are skipped during discovery #

During the metaprog discovery pass, function bodies of the entry file are not lowered (metaprog_mode); only signatures/items needed for trigger discovery are processed. Name mangling during discovery must stay consistent with the final pass.

Uncertainty: Exact set of skipped work is described only via comment.

Testability: untestable

Source: src/compiler/metaprog_dispatch.hpp#L94-L95, src/compiler/metaprog_dispatch.hpp#L82-L87

Macro expansion dispatch #

metaprog.dispatch.fixpoint-iteration — Metaprog discovery iterates to fixpoint, capped at 16 #

Metaprogram item-generation runs as a discovery loop: each iteration re-lowers and fires triggers/metacalls that may emit new items; the loop repeats until an iteration emits nothing new (fixpoint), bounded by a hard cap of 16 iterations.

Uncertainty: Header doc comment; the loop body lives in the .cpp. Behavior of exceeding the cap (error vs silent stop) is not specified here.

Testability: transitive

Source: src/compiler/metaprog_dispatch.hpp#L94-L100

Macro handler registration #

metaprog.handler.register — #[metaprog_handler(“trigger”)] registers a hook #

#[metaprog_handler("trigger")] on a function registers (trigger, fn_name); the trigger is the annotation’s first positional string-literal argument. The host driver later scans user items for a matching #[trigger] annotation and invokes the registered fn on each. A missing/unresolvable trigger string is recorded as the sentinel <missing> so a later validation pass can surface the misuse as a diagnostic.

Divergence: A6: metaprog handler registration, Logos-only mechanism.

Source: src/compiler/sema_collect.cpp#L1837-L1873

Annotation-triggered expansion #

metaprog.trigger.annotation-scan — top-level annotations matching a registered handler trigger are recorded as metaprog targets #

After all modules complete field/variant/signature collection (so the metaprog-handler trigger registry is final), the compiler scans top-level ANNOTATION items in user (non-binary) ASTs; one or more consecutive annotations immediately preceding a non-annotation item, whose name matches a registered handler trigger, are each recorded as an (ast-index, item-byte-offset, trigger-name) metaprog target for the driver to dispatch against that following item.

Testability: transitive

Source: src/compiler/sema_collect.cpp#L577-L626

Derive-style annotation triggers #

metaprog.derive.trigger-may-emit-items — Derive/handler hooks emit sibling items into the program #

An annotated item bearing a #[derive_*] / metaprog-handler attribute invokes the corresponding handler during discovery; the handler may synthesize and splice additional top-level items into the program (alongside the annotated item).

Source: src/compiler/metaprog_dispatch.hpp#L94-L98, src/compiler/metaprog_dispatch.hpp#L1-L2

metaprog.derive.no-rust-derive-syntax — #[derive(...)] is rejected; use per-trait triggers #

The Rust-style #[derive(Trait, ...)] attribute (a derive annotation carrying args) is not Logos surface syntax and is an error. Logos uses one trigger annotation per derive, #[derive_<trait>], paired with an in-scope #[metaprog_handler("derive_<trait>")] function.

Divergence: Logos replaces Rust #[derive(...)] with #[derive_<trait>] + #[metaprog_handler].

Source: src/compiler/sema_impl.hpp#L1762-L1774

Annotations (#[...]) — literal/argument forms and targets #

metaprog.annot.literal-kinds — Annotation argument literal kinds #

An annotation argument literal is one of: integer, float, bool, string, enum-variant (Name::Variant), or array of literals. Arrays are parsed recursively element-by-element.

#[A(42)]

#[A("s")]

#[A(Color::Red)]

#[A([1, 2, 3])]

Source: src/compiler/sema.cpp#L7165-L7229

metaprog.annot.float-suffix-strip — Float annotation literal: strip underscores and f32/f64 suffix #

A float annotation literal has all ‘_’ separators removed and an optional trailing ‘f32’/‘f64’ suffix stripped before parsing to a double.

#[A(3.14f64)]

#[A(1_000.5)]

Source: src/compiler/sema.cpp#L7173-L7180

metaprog.annot.string-escapes — String annotation literal escape decoding #

A quoted string annotation literal has surrounding quotes stripped and escapes \n \t \r \ " \0 decoded; an unrecognised escape is preserved verbatim (backslash + char). A raw string r"..." strips the r" prefix and trailing quote with no escape processing.

#[A("line\n")]

#[A(r"raw\n")]

Source: src/compiler/sema.cpp#L7186-L7217

metaprog.annot.no-args-bare — Bare annotation #[A] takes no arguments #

An annotation written #[A] (no ARGS, no VALUE) produces an instance with an empty argument list.

#[A]

Source: src/compiler/sema.cpp#L7249-L7250

metaprog.annot.eq-value-first-field — #[A = lit] binds first field #

An annotation #[A = lit] maps the single value to the annotation datatype’s first declared field; if the datatype has no fields it is an error (‘annotation takes no arguments’).

#[A = 5]

Source: src/compiler/sema.cpp#L7252-L7261

metaprog.annot.positional-by-order — Positional annotation args bind by declaration order #

Positional arguments in #[A(arg, ...)] bind to the annotation datatype’s fields in declaration order; supplying more positional args than fields is an error (‘takes at most N positional args’).

#[A(1, 2)]

Source: src/compiler/sema.cpp#L7282-L7292

metaprog.annot.named-by-field-name — Named annotation args bind by field name #

A named argument key=value in #[A(...)] binds to the field whose name equals the key; an unknown field name is an error (‘has no field’) but does not abort parsing of remaining args.

#[A(name = 3)]

Source: src/compiler/sema.cpp#L7271-L7281

metaprog.annot.user-annotation-requires-marker — User annotation NAME must be an #[annotation]-marked datatype #

An annotation whose name is not a compiler-internal key is treated as a user annotation only if NAME resolves to a registered datatype carrying the IS_ANNOTATION_TYPE marker; otherwise it is silently ignored.

Related: metaprog.annot.struct-flags-set-only

Source: src/compiler/sema.cpp#L7327-L7336

metaprog.annot.accumulate-until-item — Annotations and doc-comments accumulate until next item #

Annotation nodes accumulate in a pending buffer and are consumed by the next non-annotation item; doc-comment lines/blocks (///, /**, //!, /*!) similarly accumulate. Both buffers are cleared after each item.

Testability: transitive

Source: src/compiler/sema.cpp#L7304-L7305, src/compiler/sema.cpp#L7424-L7458, src/compiler/sema.cpp#L8055-L8058

metaprog.annot.type-code-on-struct — #[type_code=N] on struct/datatype sets type code #

#[type_code=N] on a struct or datatype sets the type’s TYPE_CODE to N and registers N under the fully-qualified name (pkg::Name) in the explicit-type-code table so type_code_of::<T>() resolves cross-package.

#[type_code=42] datatype Foo { }

Source: src/compiler/sema.cpp#L7313-L7323

metaprog.annot.tag-dispatch-on-trait — #[tag_dispatch(system)] on trait sets dispatch system name #

#[tag_dispatch(system_name)] on a trait records system_name as the trait’s TAG_DISPATCH_SYSTEM, identifying the tagged-dispatch system the trait participates in.

Source: src/compiler/sema.cpp#L7367-L7376

metaprog.annot.type-code-on-trait-genos — #[type_code=N] on trait marks it a genos #

#[type_code=N] on a trait sets the trait’s TYPE_CODE: the code names the logical datatype family, and each impl Trait for Eidos propagates it to the target struct during lowering.

Source: src/compiler/sema.cpp#L7377-L7383

metaprog.annot.type-code-on-template-genos-forbidden — #[type_code] forbidden on parametric (template) genos #

#[type_code=N] on a parametric genos (trait with non-empty type params) is an error; the code must be attached to a concrete specialization, else every specialization would collide in the same tag-system slot.

Source: src/compiler/sema.cpp#L8040-L8048

metaprog.annot.struct-flags-set-only — Structural struct flags are set-only (monotonic) #

Structural attributes #[zone_mut], #[zoned2], #[rel_ptr], #[self_describing], #[borrow_carrying] each set the corresponding boolean struct flag when present; a present flag can only set, never clear, the base value.

Testability: untestable

Source: src/compiler/sema.cpp#L7344-L7360

metaprog.annot.value-int-or-enum-variant — Attribute = value must be an integer or enum variant #

The right-hand value of an attribute #[name = V] read as an integer must be either an integer literal (#[name = 123]) or an enum variant path (#[name = Enum::Variant], resolving to that variant’s discriminant). Any other value form is an error; an unknown enum or unknown variant is also an error.

#[type_code = 42]

#[type_code = Tag::Foo]

Source: src/compiler/sema_impl.hpp#L1372-L1398

metaprog.annot.builtin-target-validity — Builtin attribute must match its allowed targets #

A builtin attribute is valid only on item kinds in its allowed target set; applying a builtin attribute to a target outside that set is an error. Non-builtin attribute names are not diagnosed at this phase (may be user annotations, cross-module metaprog triggers, or deferred typo checks).

Source: src/compiler/sema_impl.hpp#L1775-L1790

metaprog.annot.type-code-not-on-generic — #[type_code] forbidden on generic items #

#[type_code] may not be applied to an item that has type parameters; a type code must be assigned per concrete instantiation, not on the generic definition.

Source: src/compiler/sema_impl.hpp#L1792-L1797

metaprog.annot.type-code-reserved-range — type_code values 1..128 reserved for stdlib #

Explicit #[type_code=N] values with 1 <= N <= 128 are reserved for stdlib primitive tags (TypeTagSystem); user code outside package std/std.* using a reserved value is warned and should use codes >= 129.

Testability: untestable

Source: src/compiler/sema_impl.hpp#L1798-L1811

cfg!/#[cfg(...)] — configuration predicates #

metaprog.cfg.key-value-predicates — cfg key=value predicate resolution #

A cfg key=value predicate matches against compile-target metadata: target_arch, target_os, target_endian, target_family, target_pointer_width resolve to the host/target platform values; feature = "name" matches iff name is in the active feature set. Any unknown key evaluates to false.

Divergence: Unknown-key-false matches Rust per inline comment.

Source: src/compiler/sema.cpp#L3507-L3517, src/compiler/sema.cpp#L3572-L3575

metaprog.cfg.bare-flag-predicates — cfg bare-identifier flag resolution #

A cfg bare identifier matches as: unix/windows against target_family; test/debug_assertions against the feature set; otherwise as a feature-like flag checked against the active feature set.

Source: src/compiler/sema.cpp#L3519-L3528, src/compiler/sema.cpp#L3582

metaprog.cfg.combinators — cfg all/any/not combinators and boolean literals #

cfg predicates compose: all(p...) is the AND of its children, any(p...) the OR, not(p) requires exactly one child and negates it (else error/false). The literals cfg(true)/cfg(false) evaluate to true/false directly. Unknown combinators evaluate to false / raise an error in attribute position.

Divergence: cfg(true)/cfg(false) per Rust 1.80 RFC 3695 (noted inline).

Source: src/compiler/sema.cpp#L3553-L3582, src/compiler/sema.cpp#L3692-L3708, src/compiler/sema.cpp#L3721-L3737

metaprog.cfg.attr-multi-arg-implicit-and — cfg attribute multi-arg implicit AND #

In #[cfg(...)] attribute position, a top-level multi-argument list is an implicit AND of its arguments; #[cfg] with no args matches (true).

Divergence: Multi-arg implicit AND matches Rust (noted inline).

Source: src/compiler/sema.cpp#L3654-L3673

metaprog.cfg.cfg-attr-splice — cfg_attr predicate-gated attribute splicing and item drop #

#[cfg_attr(pred, attrs...)] activates first: when pred is true its wrapped attrs are spliced into the item’s annotation list, when false the entry is dropped (wrapped attrs are NOT re-fed to cfg_attr). After activation, every plain #[cfg(...)] is evaluated; an item is dropped from compilation iff any of its cfg predicates is false. The drop gate applies uniformly to both the collection and lowering walks.

Source: src/compiler/sema.cpp#L3606-L3652

Built-in macros #

metaprog.builtin-macro.vec-desugar — vec![...] desugars via a synthesized vec_from_arr call #

The vec![...] built-in macro’s synthesized body is re-parsed as the tail expression of an ephemeral fn __f() -> i32 { <wrap_body> } and lowered in the current context, routing vec![e1, e2, ..] through a synthesized vec_from_arr([e1, e2, ..]) call; a re-parse failure produces error_expr() after a diagnostic.

Testability: behavioral

Source: src/compiler/sema_impl.hpp#L4032-L4039

Template declarations #

metaprog.template.decl — Template declaration #

template <item> wraps a struct/enum/datatype/trait/impl/fn declaration as inert data (an AST blob) rather than a real binding; the inner names are never registered by sema, so referencing the template as a type yields the ordinary unknown-type diagnostic. Templates are consumed later by metafunctions via apply/metacall.

template struct Pair<A,B> { a: A, b: B }

Divergence: No Rust equivalent.

Source: tools/peg_gen_cpp/grammars/logos.peg#L625-L640

Variadic tuple intrinsics #

metaprog.variadic.tuple-all-eq — __tuple_all_eq__::<T>(a, b) expands to an &&-chain of elementwise eq #

tuple_all_eq with type-arg T substituted to a concrete Tuple and exactly 2 args (a, b) expands to the left-associated &&-chain of per-element equality a.i.eq(&b.i): a Tuple-kind element inlines a recursive chain over the nested field refs (never re-emits tuple_all_eq, since mono does not re-process synthesized intrinsics); a Slice-kind element (canonicalized to element name “str” when it is Slice<u8>) is compared via a direct 2-arg by-value free-function call; any other element resolves a callee symbol by scanning out_.functions then in_.functions for a name containing <elem-type-str>__eq__f__ at a .-delimited-or-leading boundary (falling back to <elem-type-str>__eq) and calls it as a.eq(&b) via method_call. When T is missing/non-Tuple or fewer than 2 args are present, the intrinsic degenerates to the boolean literal true (also the result for an empty tuple).

Divergence: A6

Uncertainty: Symbol resolution by substring scan is an implementation detail; the observable language rule is the &&-chain elementwise-Eq semantics.

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2488-L2584

metaprog.variadic.tuple-each-field-debug — __tuple_each_field_debug__::<T>(self, f) expands variadic tuple Debug #

tuple_each_field_debug with type-arg T substituted to a concrete Tuple and exactly 2 args (self, f: &mut Formatter) expands to a fmt_seq-chained Result: fmt_tuple_open(f), then per element i in order: fmt_tuple_sep(f) folded in when i>0, then either (Tuple-kind element) an inline recursive build over the nested field ref, or (otherwise) a Debug-fmt call resolved as a symbol containing <elem-type-str>__Debug__fmt__f__ else <elem-type-str>__fmt__f__ at a .-delimited-or-leading boundary, else literally <elem-type-str>__fmt (elem-type-str = type_str(elem), with Slice<u8> canonicalized to “str”), invoked as field.fmt(f); finally fmt_tuple_close1(f) if the tuple arity is 1, else fmt_tuple_close(f); each step is folded into the running chain via fmt_seq. Every reuse of the shared f argument after its first use goes through a reborrow wrapper (AddrOfTemp(Deref(f))), since &mut Formatter is a move-type under borrow-check and a bare reuse would consume it on the first call. When T is missing/non-Tuple or fewer than 2 args are present, the intrinsic degenerates to a single close-call.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2591-L2683

format!/println! family — macro expansion shape #

metaprog.format.literal-string-checked — Format-family macros validate a literal format string at compile time #

For the format family (format, print, println, eprint, eprintln, panic, format_args_str) and the write family (write, writeln), when the format-string argument is a string literal it is parsed and validated at sema time (brace balance, placeholder structure). A non-literal format-string argument skips the check (mirroring Rust format_args!). The format string is arg[0] for the format family and arg[1] for the write family.

Source: src/compiler/sema_expr.cpp#L18683-L18728, src/compiler/sema_expr.cpp#L18701-L18722

metaprog.format.arity-check — Format placeholder count must match the argument count #

When no explicit {N} index appears, the number of value placeholders in the format string must equal the number of value arguments provided (args after the format string, and after the sink for write/writeln). When explicit indices are used, the number of value arguments must be at least max(explicit-index)+1. Otherwise the macro is rejected.

Source: src/compiler/sema_expr.cpp#L18754-L18792

metaprog.format.placeholder-trait-dispatch — Each format placeholder dispatches to its format-trait method #

Each format placeholder lowers to a call dispatching to the format trait selected by the placeholder’s trait kind (e.g. {} -> Display, {:?} -> Debug, plus hex/oct/bin/exp variants), invoked on the argument value with a &mut Formatter. Per-placeholder format spec fields (fill, alignment, sign, alternate, zero-pad, width, precision) are applied to the Formatter before each dispatch and reset between placeholders.

Source: src/compiler/sema_expr.cpp#L18843-L18918

metaprog.format.expansion-shape — Format-family expansion builds a String/Formatter block #

format!/format_args_str! expand to a block yielding a String built via a Formatter over a __buf. The print family (println/print/eprintln/eprint) appends a drain of the buffer to stdout/stderr; panic! drains to a panic. write!/writeln! build a Formatter directly over the sink (via (sink).as_formatter()), stream placeholders into it, and yield Result<(),Error> (always Ok — per-placeholder errors are discarded); writeln! additionally writes a trailing newline.

Source: src/compiler/sema_expr.cpp#L18828-L18941

metaprog.format.requires-format-arg — Format/write macros require a format-string argument #

A format-family or write-family invocation must supply at least the format-string argument; an empty argument list is an error.

Testability: untestable

Source: src/compiler/sema_expr.cpp#L19011-L19014

Format-spec mini-language #

metaprog.fmt.trait-spec-types — Format-spec type characters select a formatting trait #

In a format!-family format string, a placeholder’s trailing spec type char selects a formatting trait: no type / absent = Display ({}), ? = Debug ({:?}), x = LowerHex, X = UpperHex, o = Octal, b = Binary, e = LowerExp, E = UpperExp. Each placeholder lowers to a call of that trait’s method on the argument rather than a variadic runtime path.

Uncertainty: LowerExp/UpperExp marked ‘(future)’ in source; presence in enum implies recognized but possibly not yet lowered.

Source: src/compiler/sema_fmt.hpp#L20-L29, src/compiler/sema_fmt.hpp#L52

metaprog.fmt.spec-alignment — Format-spec alignment characters #

A format placeholder spec may carry an alignment: < = Left, > = Right, ^ = Center; when unspecified the default alignment is chosen per formatting trait.

Source: src/compiler/sema_fmt.hpp#L31-L36, src/compiler/sema_fmt.hpp#L46

metaprog.fmt.spec-fill-and-flags — Format-spec fill, sign, alternate, and zero flags #

A format placeholder spec supports: a fill char (default space) used with alignment; a sign flag (+ = always show sign, default none); an alternate flag # (emit base prefix 0x/0o/0b); and a zero flag 0 (pad with zeros, overridden by an explicit fill char).

Source: src/compiler/sema_fmt.hpp#L38-L42, src/compiler/sema_fmt.hpp#L45-L49

metaprog.fmt.spec-width-precision — Format-spec width and precision #

A format placeholder spec may set width and precision; both are unset by default (sentinel -1), and when set precision >= 0 denotes an exact precision.

Source: src/compiler/sema_fmt.hpp#L50-L51

metaprog.fmt.placeholder-arg-selection — Placeholder argument selection: positional, explicit index, or named #

A format placeholder binds to an argument in one of three modes: positional auto-counter ({} consuming 0,1,2,… in order), explicit index ({N}), or named ({name}). Arity validation against positional args applies only when no placeholder uses the named or explicit-index form.

Source: src/compiler/sema_fmt.hpp#L55-L66, src/compiler/sema_fmt.hpp#L71-L75

metaprog.fmt.string-segmentation — Format string parses into literal text + typed placeholder segments #

A format string body (without surrounding quotes) parses into an ordered sequence of segments, each either a literal-text run or a placeholder (carrying its argument selector and spec). Parsing is best-effort: a soft issue emits a diagnostic and continues, while a hard parse error halts at the broken placeholder and sets a failure flag.

Source: src/compiler/sema_fmt.hpp#L68-L89

Writ literal capture (rodata embedding) #

metaprog.writ-lit.size-prefixed-rodata — Capture-free @-literal lowered to size-prefixed rodata #

An @-literal (Writ literal) without runtime captures is serialized to a constant blob laid out as [u64 little-endian size][bytes]; the materialized handle points to the payload (8 bytes past the size prefix), so size() reads *(ptr - 8).

Source: src/compiler/mlir_gen_expr.cpp#L6151-L6196, src/compiler/mlir_gen_expr.cpp#L6234-L6267

metaprog.writ-lit.content-keyed-dedup — Identical @-literal blobs share one global (one address) #

Two capture-free @-literals with byte-identical serialized (size-prefixed) content share a single rodata global, and therefore compare equal by address; capture-bearing @-literals are never deduplicated.

Source: src/compiler/mlir_gen_expr.cpp#L6199-L6202, src/compiler/mlir_gen_expr.cpp#L6247-L6249, src/compiler/mlir_gen_expr.cpp#L6270-L6271

metaprog.writ-lit.capture-zone-alloc-kinds — Capture kinds requiring zone allocation #

An @-literal capture requires zone allocation (rather than an inline WAny word) iff its type is F64/F32/FloatLit, a raw pointer (treated as null-terminated C-string), a slice &[u8]/str (ptr+len), or struct StringView; all other (scalar/AnyVal) captures are stored as inline WAny value words.

Testability: untestable

Source: src/compiler/mlir_gen_expr.cpp#L6311-L6324, src/compiler/mlir_gen_expr.cpp#L6404-L6459

metaprog.writ-lit.float-capture-widens-to-f64 — Float captures widen to f64 #

A zone-allocated float capture is stored as f64: an f32 value is widened via float-extension, and a FloatLit (untyped float literal) defaults to f64.

Testability: untestable

Source: src/compiler/mlir_gen_expr.cpp#L6407-L6422

Writ AST-blob fragments #

metaprog.writ-blob.ast-fragment-recurse — WRIT_BLOB carrying an AST-category root lowers as that expression #

A WRIT_BLOB whose serialized root TinyMap has schema category CAT_AST and whose variant code is a supported expression node (BINOP, LIT_INT, LIT_BOOL, LIT_STR, VAR_REF, CALL, PAREN_EXPR, UNARY, FIELD_READ, METHOD_CALL, CAST, INDEX_READ, STRUCT_LIT, ARR_LIT, TUPLE_LIT, BLOCK, BLOCK_STMT, IF) is lowered by recursively type-checking that root node as an ordinary expression, yielding its recovered expression type. The blob’s arena is retained for the lifetime of sema.

Divergence: A6: Writ/metaprog is a Logos addition (ExprBlob AST fragments spliced from metafunctions).

Related: metaprog.writ-blob.opaque-static-fallback

Source: src/compiler/sema_expr.cpp#L17019-L17050

metaprog.writ-blob.opaque-static-fallback — Non-AST WRIT_BLOB lowers to an opaque WritStatic literal #

A WRIT_BLOB whose root is null, non-TinyMap, or not of an AST expression category is lowered to an opaque data literal of type WritStatic carrying the raw blob bytes verbatim.

Divergence: A6: WritStatic is a Logos addition.

Testability: untestable

Related: metaprog.writ-blob.ast-fragment-recurse

Source: src/compiler/sema_expr.cpp#L17056-L17060

Writ type embedding #

metaprog.writ-type.embed-as-first-class — <type:T> embeds a Type as a first-class Writ value #

<type:T> inside a Writ @-literal (WRIT_TYPE_LIT, NAME = bare type identifier) is sema-embedded as a first-class Writ Type value, i.e. a type usable as ordinary literal data rather than only in type position.

Uncertainty: Comment marks this “slice 1: IDENT only” — the grammar accepted here may not cover the full type-expression surface for this position.

Testability: behavioral

Source: tools/peg_gen_cpp/grammars/logos.peg#L280

AnyVal inline value-form encoding #

metaprog.anyval.bool-value-form — AnyVal bool value-form encoding #

A bool coerced to an inline AnyVal word encodes as (b << 8) | 5 where b in {0,1} (low byte = (WA_BOOL=2)<<1 | 1 = 5).

Uncertainty: Bit-layout is a data-substrate (Writ/Memoria) encoding, not a Rust-language construct.

Source: src/compiler/mlir_gen_expr.cpp#L6002-L6010

metaprog.anyval.int-i24-value-form — AnyVal small-integer value-form (i24 niche) #

Any integer type of width <=32 bits (i8/i16/i32/u8/u16/u32/i24/u24) coerced to an inline AnyVal word encodes as ((v & 0xFFFFFF) << 8) | 0x2F (24-bit payload, type tag 0x2F).

Source: src/compiler/mlir_gen_expr.cpp#L6011-L6022

metaprog.anyval.int64-truncates-to-i24 — AnyVal 64-bit integer truncates to 24 bits #

An i64/u64 coerced to an inline AnyVal word is truncated to its low 24 bits and embedded as the i24 niche ((v & 0xFFFFFF) << 8) | 0x2F; values outside [-2^23, 2^23) lose their high bits in the inline form.

Uncertainty: Lossy truncation is the inline-AnyVal fallback; full-range values require the zone-alloc (C5) path elsewhere.

Source: src/compiler/mlir_gen_expr.cpp#L6023-L6032

metaprog.anyval.float-ptr-not-inline-capturable — Floats and pointers are not inline-capturable as AnyVal #

F32/F64 and pointer/reference types (Ptr/Ref/MutRef) have no inline AnyVal value-form; coercing one yields a null (zero) AnyVal word, and such types must instead use zone-allocated capture encoding.

Testability: untestable

Source: src/compiler/mlir_gen_expr.cpp#L6033-L6040

metaprog.anyval.identity-passthrough — AnyVal value passes through as its first word #

Coercing a value already of struct type AnyVal to an AnyVal word extracts its leading word (field 0) unchanged.

Testability: untestable

Source: src/compiler/mlir_gen_expr.cpp#L6041-L6048

WAny inline value-form encoding #

metaprog.wany.bool-value-form — WAny bool value-form encoding #

A bool coerced to an 8-byte WAny value word encodes as (b << 8) | 5 (low byte = (WA_BOOL=2)<<1 | 1).

Source: src/compiler/mlir_gen_expr.cpp#L6055-L6073

metaprog.wany.int-i56-value-form — WAny integer value-form (i56 niche) #

Any integer type up to 64 bits (i8..i64/u8..u64/i24/u24) coerced to an 8-byte WAny word encodes as (v << 8) | 3 (payload in the high 56 bits, tag = (WA_I56=1)<<1 | 1 = 3).

Source: src/compiler/mlir_gen_expr.cpp#L6074-L6082

metaprog.wany.identity-passthrough — WAny passthrough and AnyVal zero-extension into WAny #

A value already of enum type WAny passes its niche word through unchanged (extracted if struct-typed). A legacy struct AnyVal coerced to WAny is its leading 4-byte word zero-extended to 64 bits (the i24/bool Pod encodings coincide in the low 32 bits).

Testability: transitive

Source: src/compiler/mlir_gen_expr.cpp#L6083-L6104

Type-reflection intrinsics #

metaprog.type-intrinsic.apply-generic — apply_generic instantiates a template struct type #

For apply_generic(tmpl_name, arg0..argN), each arg expr is resolved to a compile-time TypeRef via the type-producer-recover protocol (unresolvable arg aborts compilation with a diagnostic); the intrinsic builds LogosType::Struct{name=tmpl_name, type_args=[recovered...]} (pkg inherited from the matching struct def in out_.structs), interns the type, and replaces the call with that type’s Type reflection value.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2191-L2239

metaprog.type-intrinsic.type-producer-recover — Type-producer recovery protocol for type-apply intrinsics #

To resolve a LIR sub-expr to a compile-time TypeRef (shared by apply_generic/tuple_type_apply/array_type_apply): (1) chase through VarRef by looking up type_let_inits_, up to 8 hops; (2) if the result is a Call to typelist_nth or typelist_head, substitute its type-arg[0] (a type-list) through the current subst map and index into its type_args pack at the literal-int index given as typelist_nth’s value arg (typelist_head implicitly index 0); out-of-range index yields no type; (3) if the result is a StructLit with a field named “uid” whose initializer is a Call to type_uid_of, recover that call’s type-arg[0] substituted through the subst map; (4) otherwise recovery fails (empty TypeRef).

Divergence: A6

Testability: transitive

Source: src/compiler/mono_clone.cpp#L2152-L2189, src/compiler/mono_clone.cpp#L2255-L2315

metaprog.type-intrinsic.tuple-type-apply — tuple_type_apply([Type;N]) builds a tuple type #

tuple_type_apply(arr) requires arg0 to chase to an ArrLit (a non-ArrLit is a compile-time abort); each element is resolved via type-producer-recover (unresolvable element aborts); the intrinsic yields LogosType::Tuple{tuple_elems=[recovered...]}'s Type reflection value.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2244-L2246, src/compiler/mono_clone.cpp#L2316-L2341

metaprog.type-intrinsic.array-type-apply — array_type_apply(Type, N) builds an array type #

array_type_apply(elem, size) requires elem to resolve via type-producer-recover (unresolvable is a compile-time abort) and size (after chase) to be a LitInt literal (a non-literal is a compile-time abort); the intrinsic yields LogosType::Array{elem=recovered, arr_size=size}'s Type reflection value.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2244-L2246, src/compiler/mono_clone.cpp#L2342-L2361

metaprog.type-intrinsic.type-reflection-value — Type reflection value shape and uid #

A reflected Type value (yielded by apply_generic/tuple_type_apply/array_type_apply) is the struct literal Type{kind:u32=T.kind(), name:&[u8]=type_str(T), size:i64=size_of(T), align:i64=align_of(T), uid:u64} where uid = type_hash_64bit(type_hash_23(type_id_canon(T))); T is registered into out_.type_pool and uid_to_type_[uid]=T so a later type_uid_of lookup on that uid recovers T.

Divergence: A6

Testability: transitive

Source: src/compiler/mono_clone.cpp#L2213-L2238, src/compiler/mono_clone.cpp#L2364-L2387

Enum-reflection intrinsics #

metaprog.enum-intrinsic.variant-count-of — __variant_count_of__::<E>() yields variant count #

variant_count_of with type-arg E evaluates to a literal i64 equal to the variant count of E’s enum definition when E resolves to a known LogosType::Enum with a matching def in in_.enums, else 0.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2395-L2417

metaprog.enum-intrinsic.variant-names-of — __variant_names_of__::<E>() yields variant names #

variant_names_of with type-arg E evaluates to a fixed-size array literal of each variant’s name (string literal), in declaration order; empty array if E does not resolve to a known enum def.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2395-L2407, src/compiler/mono_clone.cpp#L2421-L2425, src/compiler/mono_clone.cpp#L2470-L2477

metaprog.enum-intrinsic.variant-payload-counts-of — __variant_payload_counts_of__::<E>() yields per-variant payload arities #

variant_payload_counts_of with type-arg E evaluates to a fixed-size array literal of i64, one per variant in declaration order, each equal to that variant’s payload-field count; empty array if E does not resolve to a known enum def.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2426-L2431, src/compiler/mono_clone.cpp#L2470-L2477

metaprog.enum-intrinsic.variant-payload-types-flat-of — __variant_payload_types_flat_of__::<E>() yields flattened payload-type reflections #

variant_payload_types_flat_of with type-arg E evaluates to a flat fixed-size array literal of Type reflection values (per metaprog.type-intrinsic.type-reflection-value), one per payload type across all variants in declaration order (variant-major, field-minor), each payload type substituted through the current instantiation’s subst map; empty array if E does not resolve to a known enum def.

Divergence: A6

Testability: behavioral

Source: src/compiler/mono_clone.cpp#L2432-L2469

Named resource item declarations #

metaprog.resource.named-item-decl — Named resource item declaration #

resource NAME = CALLEE!{ body }; desugars to the same FN_MACRO_CALL_ITEM node as an item-position fn-macro, but the left-hand NAME rides in the (otherwise-unused) NAME slot: sema passes it to the #[token_macro] callee as a second str arg so the emitted item is named by NAME, not by a fn <name> header inside body. A parameterized form resource NAME = CALLEE!(PARAMS){ body }; additionally captures the parenthesized list as balanced raw source text (PARAMS), passed as a third arg; the callee re-emits it verbatim as the generated fn’s real parameter list, so PARAMS is parsed and type-checked when that fn is compiled. The parameterized alternative is tried first (ordered choice; the extra paren group makes it strictly more specific, so plain resource N = h!{ ... } is unaffected).

resource conn = open_conn!{ ... };

resource h = handler!(req: Request){ ... };

Divergence: no Rust equivalent (ADR 0012 §6 named resource-macro item).

Source: tools/peg_gen_cpp/grammars/logos.peg#L582-L602

Statement-level repeat groups #

metaprog.stmt.repeat-group — Statement repeat-group #(stmt)* #

#(stmt)* at statement position is a metaprogram repeat-group over a statement, expanding to a compile-time-repeated sequence of statements (mirrors the analogous repeat-group forms in type-argument and pattern-group positions).

Divergence: A3

Testability: transitive

Source: tools/peg_gen_cpp/grammars/logos.peg#L1900-L1901

ABI interaction #

metaprog.abi.macro-hook-excluded — fn_macro/token_macro hooks excluded from ABI surface #

A function attributed #[fn_macro] or #[token_macro] is a compiler-invoked metaprogramming hook (discovered by attribute and invoked by the metaprog driver), not a linkable consumer API entry point; it is flagged as a macro hook so ABI-surface extraction (--emit-abi) excludes it from the public pub-allowlist, preventing its churning signature from tripping the ABI-compat gate.

Testability: transitive

Source: src/compiler/sema_decl.cpp#L1138-L1146