steins-infer: PHP's compile-time return fatals are return.compile-fatal (#952, #1051 S3)

[zonuexe]

Slice S3 of the type/arity reproduction run (#1051). Fixes #952 k1/k2: PHP's three compile-time return fatals are now reported under one new Proof/Default id, return.compile-fatal (decision D5).

What changed

• IR (steins-syntax): a scope lists its return statements with the form PHP's compiler tells apart: bare, valued, or arrow body. The list is read off the syntax tree, so dead code and opaque constructs are covered. Nested function-likes are not descended into, and the list is empty when the scope has no written return type.
• Pass (crates/steins-infer/src/compile_fatal_returns.rs): judges each listed return against the scope's written return type. It needs no env, no folder and no dead-region filter. The full rule table is in the module's doc comment.
• Id: return.compile-fatal is Proof/Default. The message quotes PHP's own sentence.
• Docs: an ADR-0078 amendment (PENDING), the trace-IR spec, the profile counts in the user manual, and a changelog fragment.

Rule table (every row run on PHP 8.5.11)

scope type	statement	verdict
: void (function, method, closure or arrow fn)	return <expr>;, return null; included	fires: "A void function must not return a value"
: never	any return	fires: "A never-returning function must not return"
fn(): never => 5	arrow body	silent: PHP compiles it
any other written type (17 tried, including null, false, true, static, self, mixed and unions)	bare return;	fires: "A function with return type must return a value"
: int	return null;	silent for this id; stays type.return-mismatch
constructor or destructor	return 5;	silent
nested closure	any	judged by its own type, since the rule is per scope
generator, property hook, ?void, int|void	—	declined (different fatal, or out of scope)
trait, enum or anonymous-class method	—	silent until S8 gives them scopes

Tests

• compile_fatal_returns.rs has 30 tests, at least one per row.
• union_and_return::return_without_value_is_silent used a bare return; under : int. That is now this id's finding, so the test now asserts only return.compile-fatal.
• The syntax, infer, cli and xtask suites pass. Workspace clippy and rustdoc pass with -D warnings, and so does xtask changelog.

Measurement (ten public packages)

• Strict with vendor diagnostics, and default: byte-identical, with 0 return.compile-fatal hits (vendor included).
• effect-diff: 0 events.

Not measured

• The private half of the local fp-gate. The orchestrator runs it before merge.

Review outcome

Adversarial review: one blocker, now fixed.

• Blocker (false positive): a return inside a property hook, in a class nested in a typed scope, was charged to the enclosing scope. Fixed in 917f50d5. The function-like boundary is now one helper (opens_function_like: function, method, closure, arrow fn, property hook), shared by scan_returns and node_is_generator. Regression tests cover hooks in named and anonymous classes inside void and typed scopes, and a yield in a hook.
• Re-check on the fixed head (73 witnesses, each run through php -l): no witness fires where PHP compiles. The only PHP fatals Steins stays silent on are the declared declines:
  • trait, enum and anonymous-class methods, which S8 picks up;
  • ?void and int|void;
  • generators;
  • one unrelated abstract-method fatal.
• Should-fix applied: the profile counts in 04-findings.md, and the constructor/destructor row of the module table (__construct(): void { return 5; } fires, as PHP does).