[bug] t27c gen-rust: the use splice matches words inside string literals and pulls in a whole module (13,432 vs 4,111 lines) #10824

Summary

t27c gen-rust's use splice decides which used module's code to pull into the generated file by matching words, and it matches words inside string literals. In specs/tools/t27o/zig_convert.t27 (PR #10819) a string literal containing convert or at line pulled the whole Rust front end into gen/rust/tools/t27o/zig_convert.rs: 13,432 lines instead of 4,111. The lane worked around it by writing those strings in two pieces.

Expected vs actual

Expected: the splice follows identifiers the spec references, never text inside a string literal. Actual: a literal that happens to contain a used module's fn name pulls that module in.

Steps to reproduce

A spec that uses one fn of a module (use tools::t27o::rust_convert::puts;) and has a string literal containing another of that module's fn names; run t27c gen-rust and count lines with and without the literal.

Risks / severity

Medium: silent bloat of generated code (and of every crate that includes it by #[path]); the workaround (split strings) is invisible to the next reader. Related: #10556 (two imported modules that each declare a helper of the same name make the splicer drop both).

Boundary

• May touch: the gen-rust splice in the compiler (bootstrap/src/compiler.rs or its spec, per the FROZEN_HASH port rule), a regression fixture.

Links

#10819, #10765, #10556, #5980.

User Scenarios

• Given a spec specs/tools/t27o/zig_convert.t27 that uses tools::t27o::rust_convert::puts and contains a string literal with the word convert, When t27c gen-rust runs, Then the generated gen/rust/tools/t27o/zig_convert.rs must not include code for tools::t27o::rust_convert::convert (the file stays near 4,111 lines, not 13,432).

Requirements

• FR-001: The gen-rust use-splice MUST identify referenced identifiers by parsing the spec's AST, not by textual word matching.
• FR-002: The splice MUST ignore tokens that appear inside string literals, character literals, or comments.
• FR-003: A regression test fixture MUST be added that fails before the fix and passes after, exercising a spec with a literal containing a used module's function name.

Success Criteria

• t27c gen-rust specs/tools/t27o/zig_convert.t27 produces gen/rust/tools/t27o/zig_convert.rs with ≤ 4,500 lines (wc -l gen/rust/tools/t27o/zig_convert.rs exits 0 and shows ≤ 4500).
• The new regression fixture in the test suite runs and exits 0 (cargo test -p t27c gen_rust_splice_ignores_string_literals exits 0).
• No existing gen-rust tests regress (cargo test -p t27c gen_rust exits 0).