Auto merge of rust-lang#142390 - cjgillot:mir-liveness, r=davidtwco

Perform unused assignment and unused variables lints on MIR.

Rebase of rust-lang#101500

Fixes rust-lang#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.

Author: bors

std/src/sync/poison.rs

@@ -60,6 +60,9 @@
 //!   while it is locked exclusively (write mode). If a panic occurs in any reader,
 //!   then the lock will not be poisoned.
 
+// If we are not unwinding, `PoisonError` is uninhabited.
+#![cfg_attr(not(panic = "unwind"), expect(unreachable_code))]
+
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use self::condvar::Condvar;
 #[unstable(feature = "mapped_lock_guards", issue = "117108")]