Fix missing unused_variables lint when using a match guard

Within a binding pattern match guard, only real reads of a bound local
impact its liveness analysis - not the fake read that is injected.

Author: eggyal

compiler/rustc_mir_transform/src/liveness.rs

@@ -1243,9 +1243,12 @@ struct TransferFunction<'a, 'tcx> {
 impl<'tcx> Visitor<'tcx> for TransferFunction<'_, 'tcx> {
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
         match statement.kind {
-            // `ForLet(None)` fake read erroneously marks the just-assigned local as live.
-            // This defeats the purpose of the analysis for `let` bindings.
-            StatementKind::FakeRead(box (FakeReadCause::ForLet(None), _)) => return,
+            // `ForLet(None)` and `ForGuardBinding` fake reads erroneously mark the just-assigned
+            // locals as live. This defeats the purpose of the analysis for such bindings.
+            StatementKind::FakeRead(box (
+                FakeReadCause::ForLet(None) | FakeReadCause::ForGuardBinding,
+                _,
+            )) => return,
             // Handle self-assignment by restricting the read/write they do.
             StatementKind::Assign(box (ref dest, ref rvalue))
                 if self.self_assignment.contains(&location) =>

tests/ui/lint/unused/match_with_guard.rs

@@ -0,0 +1,10 @@
+//! The mere presence of a match guard should not deem bound variables "used".
+//! Regression test for https://github.com/rust-lang/rust/issues/151983
+//@ check-pass
+#![warn(unused)]
+fn main() {
+    match Some(42) {
+        Some(unused) if true => (), //~WARN unused variable: `unused`
+        _ => (),
+    }
+}

tests/ui/lint/unused/match_with_guard.stderr

@@ -0,0 +1,15 @@
+warning: unused variable: `unused`
+  --> $DIR/match_with_guard.rs:7:14
+   |
+LL |         Some(unused) if true => (),
+   |              ^^^^^^ help: if this is intentional, prefix it with an underscore: `_unused`
+   |
+note: the lint level is defined here
+  --> $DIR/match_with_guard.rs:4:9
+   |
+LL | #![warn(unused)]
+   |         ^^^^^^
+   = note: `#[warn(unused_variables)]` implied by `#[warn(unused)]`
+
+warning: 1 warning emitted
+