Fix panics involving mutable references to ZSTs

[coord-e]

fix #39
fix #40

1. The temporary variables are referenced by another part of the environment and therefore should not be omitted in Env::dependencies.
2. local_def::Analyzer::reassign_local_mutabilities adjusts the mutability of locals for optimization purposes. It treats variables that are defined (alive) at the entry of a BB and mutated within that BB as mutable ones. However, for ZSTs, it is possible for them to be mut-borrowed without initialization. (To be precise, let _1: (); bb0: { _1 = (); } should not mark _1 as mut, so this fix is imprecise, but it's a rare case anyway and shouldn't have a performance impact...)

[Copilot]

Pull request overview

Fixes panics when the analyzer/refinement pipeline encounters mutable references to zero-sized types (ZSTs), specifically around closure types with no captures and the unit type ().

Changes:

• Update Env::dependencies() to stop dropping singleton-sorted temp vars, preventing “unbound var” panics during clause building.
• Update local_def::Analyzer::reassign_local_mutabilities() to treat ZST locals as mutable when needed, avoiding incorrect optimization assumptions.
• Add new UI tests covering &mut () and &mut to a no-capture closure in both pass/fail suites.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/refine/env.rs	Keeps singleton temp vars in dependency mapping to avoid missing mapped variables during clause construction.
src/analyze/local_def.rs	Marks ZST locals as mutable in mutability reassignment to avoid panics/incorrect handling of mut-borrows.
tests/ui/pass/mut_empty_tuple.rs	New passing regression test for &mut ().
tests/ui/pass/closure_mut_param.rs	New passing regression test for &mut to a ZST closure type.
tests/ui/fail/mut_empty_tuple.rs	New failing (Unsat) test variant for &mut ().
tests/ui/fail/closure_mut_param.rs	New failing (Unsat) test variant for &mut to a closure parameter that is invoked.

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