Phase 4.3 — Conformance: conditional move across branches is rejected

[whoisclebs]

Parent

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

Background

When a value is moved inside one branch of an if/else but not the other, the post-if state is ambiguous — the compiler cannot guarantee whether the value is owned or consumed. This inconsistency must be rejected: the binding must be consistently moved in all branches or in none. The dataflow analysis merges states across branches and flags the discrepancy.

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

What already exists

• compiler/paco-borrow/src/lib.rs — branch-merge logic that requires consistent move state across arms.
• compiler/paco-test-harness/src/lib.rs — golden test harness.

Scope

• Create tests/conformance/phase_04/conditional_move/input.paco — a value moved in the then-branch of an if but used unconditionally after the if.
• Create tests/conformance/phase_04/conditional_move/expected.stderr with the diagnostic on the post-if use.
• Create tests/conformance/phase_04/conditional_move/flags.toml with kind = "fail" and feature_min = 4.

Acceptance Criteria

• Compilation fails with a diagnostic about inconsistent move state.
• The diagnostic span points at the post-if use of the potentially-moved variable.
• expected.stderr matches actual stderr byte-for-byte.
• All previously passing conformance tests remain green.

Key files

• tests/conformance/phase_04/conditional_move/ — new test directory
• compiler/paco-borrow/src/lib.rs — branch-merge move analysis
• compiler/paco-driver/tests/conformance.rs — conformance runner