Derives Apple's runtime type-encoding string from an IR method
signature. Called by class_addMethod(cls, sel, imp, types) when
M1.2 A.4+ synthesise IMPs for sx-defined classes.
Layout: <ret> @ : <param0> <param1> ... — @ is the receiver,
: is _cmd. Caller passes user-declared params AFTER stripping
'self: *Self'.
Encoding table:
v=void B=bool c=s8/BOOL s=s16 i=s32 q=s64
C=u8 S=u16 I=u32 Q=u64 f=f32 d=f64
@=foreign Obj-C class ptr #=Class :=SEL
*=[*]u8 (C string) ^v=any other ptr
bool (sx i1) maps to 'B' (C99 _Bool); s8 to 'c' (Apple's BOOL).
Foreign-class pointers detected via foreign_class_map lookup on
the pointee struct name. Other pointers fall to ^v — encoding is
metadata, not ABI, so conservative is safe.
Struct / slice / closure / etc. BAIL via diagnostic
(ObjcEncodingUnsupported) rather than silently mis-encoding, per
CLAUDE.md rejected-patterns rule. Future passes will widen the
table as new shapes show up in real IMPs.
Dead code at this commit — helper isn't called yet. Three unit
tests in src/ir/lower.test.zig pin the primitive / pointer /
Obj-C-class-pointer encodings before A.2 wires the helper in.
170 example tests + zig build test green.
6cc016cd4f