A scalar `::` constant folds to its value and has no storage. The
unary `.address_of` lowering (src/ir/lower/expr.zig) skipped the
alloca path (is_alloca == false) and resolveGlobalRef (scalar consts
get no storage global), falling through to the generic addr_of arm,
which reinterpreted the folded value as a pointer:
`inttoptr (i64 <value> to ptr)`. That wild pointer segfaulted on
deref and emitted invalid stores for inline-asm `-> @const`.
Diagnose instead, in the address_of(identifier) path: a non-alloca,
non-ref-capture, non-pack-elem scope binding (local scalar const) and
a module_const_map name not backed by storage (module scalar const)
both report "cannot take the address of constant '<name>' — a scalar
'::' constant has no storage …" and return a placeholder Ref. Chose
diagnose over materializing read-only storage (consistent with the
fold-only scalar model). Array/struct consts keep real storage and
stay addressable (@K/@LIT unchanged).
Also gives the ASM stream's planned output-to-const rejection for
free — asm `-> @const` lowers through the same path. Regression:
examples/1177-diagnostics-addr-of-const-rejected.sx. Resolves 0138.
2a954ceeb6