Introduce the welded comptime `compiler` library (`#library "compiler"` +
`abi(.zig) extern compiler`), per design/comptime-compiler-api.md, and unify
`callconv(...)` into the new `abi(...)` annotation.
abi(...) replaces callconv(...):
- New ABI enum { default, c, zig, pure }; `abi(.c|.zig|.pure)` parses in the
postfix slot before extern/export (and standalone). `kw_callconv` -> `kw_abi`.
- Migrated 52 sx files, the call-convention-mismatch diagnostic, and docs
(readme/specs) from `callconv(.c)` to `abi(.c)`.
Phase 1 — welded compiler library (parse -> registry -> validation -> bridge):
- `abi(.zig) extern compiler` parses on fn decls (carries abi/extern_lib) and
struct decls (StructDecl.abi/extern_lib).
- `#library "compiler"` is the comptime-only internal surface — never dlopen'd.
- src/ir/compiler_lib.zig: the binding registry (the safety boundary). `Field`
welded to StructInfo.Field with layout baked from the real Zig type
(@offsetOf/@sizeOf); `findType`/`findFn`. Welded structs are layout-validated
at registration (field set + total size) as a header checked against the impl.
- Host-call bridge: a `fn abi(.zig) extern compiler` dispatches under the
comptime interp to its registered Zig handler (intern/text_of round-trip),
never dlsym. IR Function.compiler_welded; validated in declareFunction.
- Comptime-only enforcement: a runtime call to a welded fn is a clean
build-gating error (emitCall), not an undefined-symbol link failure.
Phase 2.1 — byte-layout weld foundation:
- Decision: full byte-layout weld (sx struct laid out byte-identically to the
bound Zig type). Registered StructInfo (first non-natural / Zig-reordered
layout). `computeWeldPlan` — pure offset-ordered element plan + padding +
sx-field->LLVM-element remap; unit-tested. Emit/interp wiring is the next
sub-step (2.2+, see current/CHECKPOINT-COMPILER-API.md).
Examples: 0625/0626 (welded struct + fn round-trip), 1183/1184/1185
(layout-mismatch, unexported-fn, runtime-call diagnostics).
16 fn/global examples across categories (0415/0602/0603/1024/1025/1605/1607-1609/
1611/1616/1619/1622/1628/1635/1636): bare '#foreign'→'extern'. All cls=0 (no class
forms). Marker'd ones (1605/1609/1611 + the rest) corpus-validated; the 3 unmarked
uikit importers (1607/1608/1616) verified byte-identical via 'sx ir' probes.
Empty snapshot diff; suite green (647 corpus / 444 unit, 0 failed).
LEFT comment-only/provenance #foreign (0716/0729 + issues/0030-extern-global +
extern-test files 1223-1231/1332/1348/1349/1426) and the keep-list (identity
ffi-foreign-* + foreign-asserting diagnostics 1172/1174/1219/1228/1620) for Phase 8.
Both halves of the C2 contract already work in JIT and AOT; these
examples pin them. 1635: libc qsort drives an sx callconv(.c)
comparator passed by name as a typed fn-pointer param. 1636: a real
pthread enters sx through a callconv(.c) entry, fabricates its own
Context (push Context with a local GPA), and runs default-conv sx code
that allocates through it — the re-entry contract std.thread (S6)
stands on. Also unblocks the sqlite callback APIs (hooks/UDFs) left
unbound by design in P5.1.
