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

Perform unused assignment and unused variables lints on MIR.

Rebase of rust-lang/rust#101500

Fixes rust-lang/rust#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

tests/ui/needless_match.fixed

@@ -1,7 +1,7 @@
 #![warn(clippy::needless_match)]
 #![allow(clippy::manual_map)]
 #![allow(dead_code)]
-
+#![allow(unused)]
 #[derive(Clone, Copy)]
 enum Simple {
     A,

tests/ui/needless_match.rs

@@ -1,7 +1,7 @@
 #![warn(clippy::needless_match)]
 #![allow(clippy::manual_map)]
 #![allow(dead_code)]
-
+#![allow(unused)]
 #[derive(Clone, Copy)]
 enum Simple {
     A,

tests/ui/useless_conversion.fixed

@@ -1,5 +1,5 @@
 #![deny(clippy::useless_conversion)]
-#![allow(clippy::needless_if, clippy::unnecessary_wraps)]
+#![allow(clippy::needless_if, clippy::unnecessary_wraps, unused)]
 // FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
 #![allow(static_mut_refs)]
 

tests/ui/useless_conversion.rs

@@ -1,5 +1,5 @@
 #![deny(clippy::useless_conversion)]
-#![allow(clippy::needless_if, clippy::unnecessary_wraps)]
+#![allow(clippy::needless_if, clippy::unnecessary_wraps, unused)]
 // FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
 #![allow(static_mut_refs)]