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ff6dc928c5
Perform unused assignment and unused variables lints on MIR. Rebase of https://github.com/rust-lang/rust/pull/101500 Fixes https://github.com/rust-lang/rust/issues/51003. The first commit moves detection of uninhabited types from the current liveness pass to MIR building. In order to keep the same level of diagnostics, I had to instrument MIR a little more: - keep for which original local a guard local is created; - store in the `VarBindingForm` the list of introducer places and whether this was a shorthand pattern. I am not very proud of the handling of self-assignments. The proposed scheme is in two parts: first detect probable self-assignments, by pattern matching on MIR, and second treat them specially during dataflow analysis. I welcome ideas. Please review carefully the changes in tests. There are many small changes to behaviour, and I'm not sure all of them are desirable.
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