Replace the str-only XOR (e0_is_str ^ e1_is_str) at the tuple send
(N_RETURN) and receive (N_MLET/N_MASSIGN) sites with a positional
per-element register cursor, mirroring harec create_unpack_bindings
(ref/harec/src/check.c:1354-1416). Each element rides consecutive
eightbytes over [AX,DX,CX,R8]; a slice/str rides its 3-word
{ptr,len,cap} header (ref/hare/rt/ensure.ha:4-8), a scalar rides 1.
Send and receive walk the SAME type-table widths so element->register
agrees. This routes []u8 elements through the 3-word path (the XOR was
slice-blind, dropping len+cap to the scalar fallback) and closes the
pre-existing (scalar,slice) cs!=ww divergence by construction. cstage
and wwstage emit byte-identical asm.
Both receive sites derive each element's width from the rhs tuple's
element types (n->rhs->type->params / the callee return type) -- the
SAME producer view the send site walks -- NOT the binding type: a `_`
lvalue is an N_IDENT with empty str the checker never type-stamps, so a
binding-typed width mis-sized a wide `_` and desynced the cursor for the
next element (cstage read DX, wwstage R8). harec `_` skips the store but
CONSUMES its tuple offset; the cursor advance honours that.
Loud-stop (rule 7): the register file holds 4 eightbytes; a tuple whose
elements sum to >4 (([]u8,[]u8)/(str,str)=6) cannot be register-returned,
so the send site aborts at compile time citing the return-ABI capacity
(#10) rather than silently miscompiling. The receive loop guards the
same predicate (defense-in-depth). Routed through each stage's EXISTING
pinned-fatal idiom: cstage fatal() (cmd/wcc/err.c), wwstage the inline
os.write(2,...)+os.exit(1) at cgen.ww:604 -- no new diagnostics path.
N_MASSIGN (`a,b=f()`, bare comma, pre-declared) is a retained
ww-EXTENSION beyond Hare's binding-only tuple-unpack (Go/rob-pike
multi-assign, rule-9 carve-out); the loop covers it identically to
N_MLET.
Test 945_tuple_nary_destructure_run: (i64,[]u8)+(i64,str) store+read
len/cap for both N_MLET and N_MASSIGN, a single-str control, a wide-
first blank `_,a=f()` row (the cursor-desync discriminator), and a
([]u8,[]u8) row asserting the loud BUILDERR carries the cited
diagnostic; dual ww/ww_ww drivers.
Hojun-Cho
5a0427ef32