Skip unused variables warning for unreachable code

(cherry picked from commit 2c6b1d3)

Author: chenyukang

compiler/rustc_mir_transform/src/liveness.rs

@@ -880,6 +880,33 @@ impl<'a, 'tcx> AssignmentResult<'a, 'tcx> {
         dead_captures
     }
 
+    /// Check if a local is referenced in any reachable basic block.
+    /// Variables in unreachable code (e.g., after `todo!()`) should not trigger unused warnings.
+    fn is_local_in_reachable_code(&self, local: Local) -> bool {
+        struct LocalVisitor {
+            target_local: Local,
+            found: bool,
+        }
+
+        impl<'tcx> Visitor<'tcx> for LocalVisitor {
+            fn visit_local(&mut self, local: Local, _context: PlaceContext, _location: Location) {
+                if local == self.target_local {
+                    self.found = true;
+                }
+            }
+        }
+
+        let mut visitor = LocalVisitor { target_local: local, found: false };
+        for (bb, bb_data) in traversal::postorder(self.body) {
+            visitor.visit_basic_block_data(bb, bb_data);
+            if visitor.found {
+                return true;
+            }
+        }
+
+        false
+    }
+
     /// Report fully unused locals, and forget the corresponding assignments.
     fn report_fully_unused(&mut self) {
         let tcx = self.tcx;
@@ -933,6 +960,10 @@ impl<'a, 'tcx> AssignmentResult<'a, 'tcx> {
 
             let statements = &mut self.assignments[index];
             if statements.is_empty() {
+                if !self.is_local_in_reachable_code(local) {
+                    continue;
+                }
+
                 let sugg = if from_macro {
                     errors::UnusedVariableSugg::NoSugg { span: def_span, name }
                 } else {

tests/ui/lint/unused/unused-var-in-unreachable-code.rs

@@ -0,0 +1,46 @@
+//@ check-pass
+
+#![allow(unreachable_code)]
+#![allow(dead_code)]
+#![warn(unused_variables)]
+
+fn after_todo() {
+    todo!("not implemented");
+
+    // This should not warn - the code is unreachable
+    let a = 1;
+    if a < 2 {
+        eprintln!("a: {}", a);
+    }
+}
+
+fn after_panic() {
+    panic!("oops");
+
+    // This should not warn - the code is unreachable
+    let b = 2;
+    println!("{}", b);
+}
+
+fn after_unimplemented() {
+    unimplemented!();
+
+    // This should not warn - the code is unreachable
+    let c = 3;
+    println!("{}", c);
+}
+
+fn after_unreachable() {
+    unsafe { std::hint::unreachable_unchecked() }
+
+    // This should not warn - the code is unreachable
+    let d = 4;
+    println!("{}", d);
+}
+
+fn reachable_unused() {
+    // This SHOULD warn - the code is reachable
+    let e = 5; //~ WARN unused variable: `e`
+}
+
+fn main() {}

tests/ui/lint/unused/unused-var-in-unreachable-code.stderr

@@ -0,0 +1,14 @@
+warning: unused variable: `e`
+  --> $DIR/unused-var-in-unreachable-code.rs:43:9
+   |
+LL |     let e = 5;
+   |         ^ help: if this is intentional, prefix it with an underscore: `_e`
+   |
+note: the lint level is defined here
+  --> $DIR/unused-var-in-unreachable-code.rs:5:9
+   |
+LL | #![warn(unused_variables)]
+   |         ^^^^^^^^^^^^^^^^
+
+warning: 1 warning emitted
+

tests/ui/return/early-return-with-unreachable-code-24353.rs

@@ -4,7 +4,6 @@ fn main() {
     return ();
 
     let x = ();
-    //~^ WARN unused variable: `x`
     x
 }