Phase 5: prove the backend on real corpus programs #78

Why this phase

The backend rows (BE-01..BE-28) are the parts of the matrix with no official-suite evidence, because no harness has ever compiled and run a corpus passing file. Phase 1 builds that harness; this phase uses it.

The backend is not the bottleneck in most cases — the frontend is. What the backend needs is proof that a representation decision holds for real programs, not more representations. BE-27 currently has vertical execution tests for the implemented slice and nothing else.

Sub-issues

Tracked as children of this epic. Known gaps at planning time:

• Backend: Wasmtime execution harness for synthesized WIT ABI fixtures #73 — a Wasmtime harness that executes synthesized WIT aggregate fixtures. All 20 fixtures under crates/psrs-backend/src/mir/indirect_tests/aggregate/ (result, option, variant, nested payloads, flags, non-byte list, list<record>, resource-in-result, large return area, indirect parameter record) currently end at validate_all. Only list<tuple<string,string>> executes, because it is the one shape reachable from source. The blocker is that mir/gc_tests' run_gc is private, pinned to the real WASI command world, and the fixtures import a synthesized wasi:io/streams function with no host implementation; wasm/lower/post_return/tests.rs shows the pattern that works for an import-free fixture world.
• BE-13..BE-16 — cover the remaining MIR instruction forms and pass the L6/M7 gate with per-operation validation.
• BE-05..BE-10 — broaden source-path coverage for enum tags, field-bearing ADTs, newtype erasure, parameterized ADTs, records, and arrays past the cases the acceptance records cover.
• BE-01, BE-02, BE-03 — general control flow beyond enum-tag Switch, remaining partial applications, recursive and mixed-field layouts.

Gate

Every in-scope passing file for an implemented backend feature compiles, validates, and runs with the expected observable result, and the runtime scoreboard reports per-file agreement.

References

• docs/design/D-04-suite-roadmap.md — M7, BE matrix, CC/MIR crosswalk
• docs/implementation/backend/ — topic acceptance records
• crates/psrs-backend/src/mir/gc_tests/mod.rs — the existing execution harness and its limits

Validation

cargo fmt --all --check
cargo test --workspace
cargo clippy --workspace --all-targets -- -D warnings
PSRS_REQUIRE_WASMTIME=1 cargo test --workspace