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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`