let-else: add a test for warnings on let-else with diverging tail

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
+