design: lowering string ++ (and slice) to wasm — proposal for sign-off

[hyperpolymath]

Design proposal — lowering string ++ (and slice) to wasm

Design-only, awaiting your sign-off before implementation. This is the write-up you asked for on the last remaining string-wall items (concat / slice), now grounded in investigation.

The headline finding: string ++ silently miscompiles to wasm

++ is type-dispatched in typecheck (string vs array), so string ++ typechecks and the interpreter is correct — but codegen lowers ++ only as list-concat (4-byte element stride), copying a string's [len][bytes] as if they were i32 elements. Measured for "ab" ++ "cd" (should be "abcd"):

byte	interp	wasm
0–1	97, 98	97, 98 (coincidental — "ab" packs into one i32)
2	99 ('c')	2 ← the length word of "cd" leaking through
3	100 ('d')	0

A #555-class silent mis-lowering — so this is a correctness fix, not a feature.

Why "add a string_concat builtin" is ruled out (by evidence)

• ++ is the canonical string-concat surface; string_concat was deliberately removed in STDLIB-04c: string_concat extern → real impl or remove (operator-only) #330 (stdlib/effects.affine:42).
• The stdlib uses string ++ pervasively (encoding/json/io/testing/…) — rejecting it breaks the stdlib.
• It's tested as ++ (test_stdlib_laws.ml concat laws + a conformance suite).

So the only acceptable fix makes the existing ++ lower correctly.

The blocker + recommendation

Codegen is type-blind (ExprBinary carries no type; AST has no spans/ids; Opt.fold rebuilds the AST). The typechecker already decides string-vs-array per ++ node — the work is threading that to codegen.

Recommended (A1): a typecheck-time elaboration that rewrites string ++ into a dedicated internal ExprStringConcat node, which codegen lowers via the established byte-copy idiom (alloc 4 + la + lb, two I32Load8U/I32Store8 loops — the list-++ pattern with a 1-byte stride). ++ stays idiomatic, the stdlib keeps compiling, codegen needs no type logic. slice-on-string is mostly already covered by the shipped string_sub (slice 3); defer its negative-index residual.

Full analysis, alternatives (A2 annotation field; B rejected), implementation plan, and risks are in proposals/DESIGN-string-concat.adoc.

Decision requested

Sign off to proceed with A1 as Phase F slice 8, or redirect. I'll implement only after your go-ahead.

https://claude.ai/code/session_01WoKhFQePiRsAj7aqnxbG8s

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Generated by Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 54 issues detected

Severity	Count
🔴 Critical	2
🟠 High	24
🟡 Medium	28

⚠️ Action Required: Critical security issues found!

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