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Iter 18d.1: Term::ReuseAs schema + linearity check

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Schema floor for explicit reuse hints. Adds Term::ReuseAs
{ source: Box<Term>, body: Box<Term> } with serde tag "reuse-as",
form-A (reuse-as <source> <body>). Schema choice (wrapper, not
modifier-on-Ctor) and rationale recorded in DESIGN.md.

Codegen is identity under all --alloc strategies — lower body,
drop source on the floor. Iter 18d.2 will lower this as the
in-place rewrite under --alloc=rc.

User-facing diagnostics:
- typecheck reuse-as-non-allocating-body: body must be a
  Term::Ctor or Term::Lam, the two AST shapes that allocate.
  Other shapes (literal, var, app, ...) emit this with a
  suggested_rewrite that drops the wrapper.
- linearity reuse-as-source-not-bare-var: source must be a
  Term::Var referring to an in-scope binder. Anything else
  (literal, nested expression) is rejected here. Suggested
  rewrite drops the wrapper.
- linearity use-after-consume at a reuse-as site: source must
  not have been consumed earlier in the body. Suggested rewrite
  drops the wrapper.
- Visiting reuse-as marks source consumed for the rest of the
  body, so a subsequent use of the same binder flags
  use-after-consume against it.

Uniqueness inference treats source as Consume — the consume
shows up in the side table for the codegen consumer.

Tests:
- 4 surface parse-tests (round-trip, no-args, one-arg, full
  parse_term/term_to_form_a round-trip).
- 1 typecheck unit test (non-allocating body).
- 2 linearity unit tests (non-var source, after-consume).
- 1 integration test (happy-path map_inc in all-explicit mode
  is linearity-clean).
- 1 E2E test running the canonical map_inc-via-reuse-as fixture
  under --alloc=gc, asserting stdout 9.
- New fixture examples/reuse_as_demo.{ailx,ail.json}.

Test deltas: E2E 55 -> 56, ailang-check unit 43 -> 46,
ailang-check workspace 8 -> 9, surface 14 -> 18. cargo test
--workspace green. No existing fixture's canonical JSON changed.
This commit is contained in:
Brummel
2026-05-08 10:55:31 +02:00
parent 8f40e8c31f
commit 74379b29e5
15 changed files with 785 additions and 1 deletions
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crates/ailang-surface/src/print.rs
+11
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@@ -382,6 +382,17 @@ fn write_term(out: &mut String, t: &Term, level: usize) {
write_term(out, value, level);
out.push(')');
}
Term::ReuseAs { source, body } => {
// Iter 18d.1: print as `(reuse-as <source> <body>)`. The
// wrapper is identity at codegen in 18d.1 (the `body` is
// lowered, the `source` is dropped); 18d.2 will lower this
// as in-place rewrite under `--alloc=rc`.
out.push_str("(reuse-as ");
write_term(out, source, level);
out.push(' ');
write_term(out, body, level);
out.push(')');
}
}
}