A sub-slice `base[lo:hi]` now sets cap to base_cap - lo (the storage
remaining to the underlying end; Go/Hare-identical) instead of hi - lo
(== len). base_cap is the array length N for [N]T, or the .capacity
word carried in a slice/str header at +16. Authored once per stage in
the cg_base_cap / cgbasecap helper, applied at both cap sites: the
N_SLICE value path (which serves let-init since the prior commit) and
the call-arg push. Both stages stay byte-identical (find-4 closed).
cap arithmetic per ref/harec/src/eval.c:1017 (slice: slice.cap -=
start) and eval.c:1024 (array: cap = array.length - start); capacity
is a distinct field per ref/hare/rt/ensure.ha:4-8 and cap >= len per
ref/harec/src/check.c:596. Only the cap arithmetic transfers: the ptr
stays unscaled (lo*esz is #76) and eval.c's stricter start>=end bound
is not ported (ww's runtime bound is start>end).
str[lo:hi] yields str with a real .capacity (D1), so the str base uses
the same +16 load -- no downgrade to []u8. base_cap falls back to len
(prior behavior) where it isn't cleanly available: a non-ident base
(its header cap was discarded by cgexpr; len is likewise wrong for a
defaulted hi there, pre-existing) and a global str base (wwstage
cgslice has no global-str load, #73 -- the carve-out keeps both
stages byte-identical).
Test: 942_subslice_cap_run, table-driven over both drivers, array /
slice / str base + an append-no-realloc row, each shape chosen so
base_cap-lo != hi-lo.
Fold in three pre-existing fixtures that asserted the old cap == len
and so failed under the corrected semantics (project #20):
681_arr_elem_field_write (slice_field_value_write,
slice_field_ptr_write, slice_field_distinct_bytes),
693_dot_tagged_source (local_struct_slice_variant,
via_ptr_slice_variant, letinit_slice_roundtrip, top_level_global_slice),
and 695_match_bind_struct (slice_neg_control). Each cap word updated to
base_cap - lo: a [8]u8 base sliced at lo=0 yields cap 8 (5->8, 3->8);
distinct_bytes slices a [16]u8 at lo=0, yielding cap 16 (6->16). len /
mark / ptr assertions are unchanged -- only the cap word moved.
Match-arm bind size in wwstage hardcoded str=16, []T=24, else=8 in
both cgmatch (emit) and scanlocals (frame pre-scan). A TY_STRUCT
variant fell into the 8B fallback: only the first quadword reached
the bind, and the prologue SUBQ underbooked the frame so the
emit-time localalloc(bsz=24+) wrote past SP.
Replace the hand-rolled table with slotsize(c, pat) at both sites.
slotsize already covers N_TNAME named structs (returns si.totsize),
str (16), []T (24), tuples, aliases, and primitives (8). Mirrors
cstage cgen.c cgmatch which falls through to bu->size for TY_STRUCT.
Cstage is correct; no mirror needed.
Test 695 covers seven shapes: the 3xi64 headline repro, a 4xi64
struct via let-init scrut (exercises >24B bind), a mixed-quadword
struct (i32+i32+i64+i64), str/slice/i32 negative controls, and a
direct let-init scrutinee variant. The wider 4xi64 row uses the
let-init shape because the N_DOT spill path in cgmatch tops out at
AX/DX/CX/R8 — a separate, unrelated gap from the bind size.
