Phase 4.2 — Conformance: double-move is rejected with use-after-move diagnostic

[whoisclebs]

Parent

Part of #6 (Phase 4: Implement ownership and move semantics).

Background

A double-move — moving the same value into two separate consumers — must be rejected at compile time. The borrow checker tracks the move state of each binding; once a value is marked MovedOut, any further use emits use-after-move. This test targets the most explicit form: two consecutive function calls each consuming the same variable.

Spec reference: docs/design/spec.md §3 · Lint: use-after-move · Crate: paco-borrow

What already exists

• compiler/paco-borrow/src/lib.rs — definite-assignment dataflow; MovedOut state blocks subsequent reads.
• compiler/paco-test-harness/src/lib.rs — golden test harness.

Scope

• Create tests/conformance/phase_04/double_move/input.paco — a consume(s: string) function and main that passes the same string to consume twice.
• Create tests/conformance/phase_04/double_move/expected.stderr with the use-after-move diagnostic on the second call.
• Create tests/conformance/phase_04/double_move/flags.toml with kind = "fail" and feature_min = 4.

Acceptance Criteria

• Compilation fails with use-after-move on the second call.
• Span points at the second use (not the first).
• expected.stderr matches actual stderr byte-for-byte.
• All previously passing conformance tests remain green.

Key files

• tests/conformance/phase_04/double_move/ — new test directory
• compiler/paco-borrow/src/lib.rs — move-state tracking
• compiler/paco-driver/tests/conformance.rs — conformance runner