Handle generic reborrow in expression-use adjustment walking

[P8L1]

Fixes an ICE in expression-use adjustment walking where Adjust::GenericReborrow could reach a match arm that assumed generic reborrow was unreachable.

GenericReborrow is already emitted by typeck and classified as rvalue-producing elsewhere in expr_use_visitor.rs, so the adjustment walker must handle it explicitly instead of panicking.

This PR models GenericReborrow as a borrow-like use of the source expression:

• Mutability::Mut is treated like an exclusive/mutable reborrow use.
• Mutability::Not is treated like a shared/coerce-shared borrow-like use.
• The source is not moved or treated as a mere copy.

cc @aapoalas
@rustbot label F-reborrow
Fixes #156339
Tracking: #145612

[rustbot]

r? @nikomatsakis

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[aapoalas]

LGTM.

View changes since this review

[theemathas]

Does this fix #156339 ?

[P8L1]

Does this fix #156339 ?

Yes, this should fix #156339. I tested the repro from that issue on this branch: the Reborrow trait usage during adjustment walk ICE is gone. I also checked the old behavior by restoring the previous GenericReborrow unreachable in expr_use_visitor, and the repro hits the old ICE again there.

The reduced repro can still produce ordinary diagnostics unrelated to this PR, including the trait-method and async-block type errors from the issue, but the ICE from #156339 is the same expression-use GenericReborrow path and is fixed by this change.

[aapoalas]

r? @dingxiangfei2009

[P8L1]

@bors squash

[rust-bors]

🔨 2 commits were squashed into a5e4dc9.