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/fail/function_calls/arg_inplace_locals_alias.rs

@@ -4,7 +4,10 @@
 //@[tree]compile-flags: -Zmiri-tree-borrows
 // Validation forces more things into memory, which we can't have here.
 //@compile-flags: -Zmiri-disable-validation
+
 #![feature(custom_mir, core_intrinsics)]
+#![allow(unused)]
+
 use std::intrinsics::mir::*;
 
 pub struct S(i32);

tests/fail/function_calls/arg_inplace_observe_after.rs

@@ -22,6 +22,7 @@ fn main() {
     }
 }
 
+#[expect(unused_variables, unused_assignments)]
 pub fn change_arg(mut x: S) {
     x.0 = 0;
 }

tests/fail/function_calls/arg_inplace_observe_during.rs

@@ -23,6 +23,7 @@ fn main() {
     }
 }
 
+#[expect(unused_variables, unused_assignments)]
 pub fn change_arg(mut x: S, ptr: *mut S) {
     x.0 = 0;
     // If `x` got passed in-place, we'd see the write through `ptr`!

tests/pass/align_repeat_into_well_aligned_array.rs

@@ -2,6 +2,7 @@
 
 use std::mem::size_of;
 
+#[expect(unused_variables, unused_assignments)]
 fn main() {
     let mut a = Params::new();
     // The array itself here happens to be quite well-aligned, but not all its elements have that