t27c: optional capture dropped, orelse unparsed, test-local reassign emitted as const, pointer param var rename #8199

Re-filed from #6358. An attempt claimed that issue on 2026-10-06 and left an empty branch, and a dispatch row that has spent its retry ceiling keeps its issue for good - so no bee could take it again, however much the brief improved. The work below is carried over unchanged.

Found by the t27b optionals lane (#6357), Refs #6063. Each is a t27c (parser or gen-zig) defect that makes the reference path refuse a well-formed spec:

1. if (x) |v| { ... } -- gen-zig drops the |v| capture; zig then reports v as undeclared. t27b refuses the shape as StmtIf(capture).
2. orelse does not parse in t27c.
3. In a test block, a plain assignment w = 9; to a name already declared with var is emitted as const w = 9;, a redeclaration zig rejects.
4. A mutated pointer parameter is renamed to var p = p_arg;, which zig rejects as "never mutated" when only p.* is written.

For 3 and 4, t27b currently passes such files while the reference does not compile them; they sit outside the 720 reference passes, so no count changes, but t27b should refuse them by name until the reference is fixed.

Boundary

• bootstrap/src/compiler.rs

User Scenarios

• Given a spec with if (x) |v| { use(v); }, when compiled by t27c, then the generated Zig must declare the capture v so the Zig compiler accepts it.
• Given a spec containing orelse in an optional chain, when parsed by t27c, then the parser must accept the keyword and produce a valid AST node.
• Given a test block where var w = 1; is followed by w = 9;, when codegen runs, then the reassignment must be emitted as a variable assignment, not a const redeclaration.
• Given a function taking a pointer parameter p: *i32 that mutates p.*, when codegen renames the parameter, then the generated Zig must not emit var p = p_arg; if only p.* is written.

Requirements

• FR-001: The gen-zig backend MUST emit a capture variable for optional if patterns so the Zig compiler sees the declared name.
• FR-002: The parser MUST recognize orelse as a valid keyword in optional expressions and produce a corresponding AST node.
• FR-003: In test blocks, assignments to already-declared var names MUST be emitted as mutable assignments, not const redeclarations.
• FR-004: When a pointer parameter is only mutated through dereference (p.*), the codegen MUST NOT emit a var p = p_arg; shadow that Zig flags as never mutated.

Success Criteria

• cargo test -p t27c -- optional_capture exits 0 and the generated Zig for if (x) |v| {} compiles without "undeclared identifier" errors.
• cargo test -p t27c -- orelse_parse exits 0 and a spec containing orelse parses without syntax errors.
• cargo test -p t27c -- test_reassign_const exits 0 and the emitted Zig for w = 9; inside a test block compiles without "redeclaration" errors.
• cargo test -p t27c -- ptr_param_rename exits 0 and the emitted Zig for a mutated pointer parameter compiles without "never mutated" errors.
• All 720 reference specs in specs/tools/tri/test.t27 still pass (t27c exit 0).

The instruments

Every one of these reads and prints; none of them writes. Run them from the repository root, with <spec> replaced by the file in ## Boundary:

.t27 is not Rust and not Zig, and a bee fluent in either writes one by
accident. Measured 2026-09-20: a bee filled eight bodies in
specs/file/watcher.t27 with return Ok(());, Err(FileError::WatcherNotFound)
and for i in 0..watchers.length. The parse ratchet refused the whole file --
Unexpected token in expression: RParen, line 123 -- and a spec that does not
parse generates nothing, so every test it already carried stopped running.

At the top level the parser accepts exactly these eight forms, with an optional
pub, and no others:

const   var   fn   enum   struct   test   invariant   bench

There is no trait, no impl, no type X = ..., no generics, no macro. What
a bee reaches for from another language, and what happens:

• Rust's result sugar (Ok(()), Err(E::V), ?) is not a construct here.
• Rust macros (println!, format!, vec!) are not constructs here.
• A range loop (for i in 0..n) is Rust and Zig, not this language.
• Zig builtins (catch unreachable, @intCast, @constCast) are not constructs here.
• Cross-module reuse (use other::fn;) parses and then generates a comment and an unqualified call, so the Zig fails with use of undeclared identifier (use a::b; generates no import, so no spec can reuse a function from another spec #4298). It does not work yet.

t27c parse <spec> answers in one line whether what you wrote is the language.
Run it before you report, every time.

• t27c spec-status <spec> - the compiler's one-word verdict: IMPLEMENTED, PARTIAL, UNWRITTEN, NOPARSE, NOFN. Exit code is 0 whatever it says, so read the word.
• t27c symbols <spec> - every name the file declares, with its kind. Answers "does this already exist here?" before you add it.
• t27c outline <spec> - per function: its locals, what it calls, what it returns. The contract you are implementing against.
• t27c coverage <spec> - which functions have a test and which do not. The issue asks for tests; this is how you check you wrote them.
• t27c lint <spec> - style and shape warnings. It OVER-REPORTS has no test or invariant: measured 2026-09-20 over 60 specs it printed 677 of those where coverage found 190 untested functions, disagreeing on 59 of the 60 - it warns about bit_to_trit_pair in specs/base/ternary_encoding.t27, which test bit_to_trit_pair_zero calls on line 249. Read it as a hint; coverage is the answer.
• t27c typecheck <spec> - types, before generation. Prints Typecheck OK (0 errors, 0 warnings) or the errors.
• t27c test-report <spec> - builds this spec and runs its own tests, which is what the oracle does. BLOCKED means the generated Zig does not compile, with the error beside it.
• python3 tools/dupe_scan.py --name <function> - where that function already lives, if it does. 576 of 4021 bodies here are byte-identical copies.

The rest of the toolbelt is in docs/BEE_TOOLBELT.md.