Change desugaring of let-else to ensure temporary is dropped earlier

compiler/rustc_ast_lowering/src/block.rs

@@ -159,6 +159,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             span,
             kind: hir::ExprKind::If(let_expr, then_expr, Some(else_expr)),
         });
+        let drop_temps = self.expr_drop_temps(span, if_expr, AttrVec::new());
         if !self.sess.features_untracked().let_else {
             feature_err(
                 &self.sess.parse_sess,
@@ -168,6 +169,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
             )
             .emit();
         }
-        if_expr
+        drop_temps
     }
 }

compiler/rustc_typeck/src/check/expr.rs

@@ -225,6 +225,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | ExprKind::If(..)
             | ExprKind::Let(..)
             | ExprKind::Loop(..)
+            | ExprKind::DropTemps(..)
             | ExprKind::Match(..) => {}
             // If `expr` is a result of desugaring the try block and is an ok-wrapped
             // diverging expression (e.g. it arose from desugaring of `try { return }`),

src/test/ui/let-else/let-else-temp-borrowck.rs

@@ -0,0 +1,26 @@
+// check-pass
+//
+// from issue #93951, where borrowck complained the temporary that `foo(&x)` was stored in was to
+// be dropped sometime after `x` was. It then suggested adding a semicolon that was already there.
+
+#![feature(let_else)]
+use std::fmt::Debug;
+
+fn foo<'a>(x: &'a str) -> Result<impl Debug + 'a, ()> {
+    Ok(x)
+}
+
+fn let_else() {
+    let x = String::from("Hey");
+    let Ok(s) = foo(&x) else { return };
+}
+
+fn if_let() {
+    let x = String::from("Hey");
+    let s = if let Ok(s) = foo(&x) { s } else { return };
+}
+
+fn main() {
+    let_else();
+    if_let();
+}

src/test/ui/let-else/let-else-then-diverge.rs

@@ -0,0 +1,19 @@
+//
+// popped up in in #94012, where an alternative desugaring was
+// causing unreachable code errors
+
+#![feature(let_else)]
+#![deny(unused_variables)]
+#![deny(unreachable_code)]
+
+fn let_else_diverge() -> bool {
+    let Some(_) = Some("test") else {
+        let x = 5; //~ ERROR unused variable: `x`
+        return false;
+    };
+    return true;
+}
+
+fn main() {
+    let_else_diverge();
+}

src/test/ui/let-else/let-else-then-diverge.stderr

@@ -0,0 +1,14 @@
+error: unused variable: `x`
+  --> $DIR/let-else-then-diverge.rs:11:13
+   |
+LL |         let x = 5;
+   |             ^ help: if this is intentional, prefix it with an underscore: `_x`
+   |
+note: the lint level is defined here
+  --> $DIR/let-else-then-diverge.rs:6:9
+   |
+LL | #![deny(unused_variables)]
+   |         ^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+