Rollup merge of #145214 - notJoon:fix/enable-self-assignment, r=petrochenkov

fix: re-enable self-assignment

## Description

Re-enables the self-assignment detection that was previously disabled due to unrelated regressions. The fix detects useless assignments like `x = x` and `foo.field = foo.field`.

## History

The original regressions (rust-lang/rust#81626, rust-lang/rust#81658) were specifically about false positives in write-only field detection, not self-assignment detection. Belows are brief history for the rule that I understand.

- Self-assignment detection was originally implemented in rust-lang/rust#87129 to address rust-lang/rust#75356
- The implementation was disabled alongside the revert of rust-lang/rust#81473's "write-only fields" detection
- rust-lang/rust#81473 was reverted via rust-lang/rust#86212 and rust-lang/rust#83171 due to false positives in write-only field detection (rust-lang/rust#81626, rust-lang/rust#81658)
- The self-assignment detection feature got removed, even though it wasn't the reason for the problems

This PR only re-enables the self-assignment checks, which are orthogonal to the problematic write-only field analysis.

## Changes
- Removed `#[allow(dead_code)]` from `compiler/rustc_passes/src/dead.rs` file
    - `handle_assign` and
    - `check_for_self_assign`
- Added `ExprKind::Assign` handling in `visit_expr` to call both methods
- Updated test expectations in `tests/ui/lint/dead-code/self-assign.rs`

1 files changed, 4 insertions, 2 deletions

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index fa9d0c7b1b7..d5d7cc5dc2e 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -172,7 +172,6 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
         }
     }
 
-    #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
     fn handle_assign(&mut self, expr: &'tcx hir::Expr<'tcx>) {
         if self
             .typeck_results()
@@ -189,7 +188,6 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
         }
     }
 
-    #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
     fn check_for_self_assign(&mut self, assign: &'tcx hir::Expr<'tcx>) {
         fn check_for_self_assign_helper<'tcx>(
             typeck_results: &'tcx ty::TypeckResults<'tcx>,
@@ -576,6 +574,10 @@ impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
             hir::ExprKind::OffsetOf(..) => {
                 self.handle_offset_of(expr);
             }
+            hir::ExprKind::Assign(ref lhs, ..) => {
+                self.handle_assign(lhs);
+                self.check_for_self_assign(expr);
+            }
             _ => (),
         }