t27o: a refusal caused by a refused self receiver names the receiver type (656 cascades cannot be followed today) #11128

Goal

When t27o refuses a fn because its self receiver has a type it cannot convert, the refusal line must name that type, so the sweep (and a reader) can follow the cascade to the type's own refusal. Today it prints "uses refused self" and the chain ends there.

Why

Measured 2026-10-20 over all 1,388 tracked Rust files of the five repositories plus the five big files: 563 first refusals are "uses refused self" (3.5% of 16,057), and after the root attribution of PR #11127 (issue #11122) 656 of the 696 cascades that cannot be followed are self or Self. They are methods of types the converter refuses (struct with a heap field, a refused derive, a refused impl item); the refusal line does not say which type, so a lookup by name finds nothing. These are the largest block of unexplained refusals left after the heap-type families, and the only one the sweep cannot attribute.

Boundary

• specs/tools/t27o/rust_convert.t27
• gen/rust/tools/t27o/rust_convert.rs
• .trinity/seals/t27o_T27oRust.json

User Scenarios

• Given impl Foo { fn len(&self) -> usize { .. } } where Foo has a refused field, when t27o converts the file, then the method's refusal names Foo and its reason.
• Given a Self-returning constructor of the same type, when it is refused, then the line names the same type.
• Given a type that converts, when its methods convert, then nothing changes.

Requirements

• FR-001: A refusal caused by a refused receiver type MUST name that type in the // t27o: refused fn line, in the same position the token occupies for other refusals.
• FR-002: The rule MUST live in rust_convert.t27 with a failing test first and a negative control; hand Rust MUST NOT grow (own-language net <= 0).

Success Criteria

• After the change and the sweep attribution of t27o-sweep: attribute cascade refusals to their root and count files that convert whole #11122, the unattributed cascades fall from 696 to under 100 over the same files.
• t27b test specs/tools/t27o/rust_convert.t27 ends with exit 0 and 0 tests failing.