Auto merge of #30374 - durka:issue-30371, r=alexcrichton

Fixes #30371.

Author: bors

src/librustc_front/lowering.rs

@@ -1502,8 +1502,9 @@ pub fn lower_expr(lctx: &LoweringContext, e: &Expr) -> P<hir::Expr> {
                                                 hir::MatchSource::ForLoopDesugar,
                                                 None);
 
-                    // `{ let result = ...; result }`
-                    let result_ident = lctx.str_to_ident("result");
+                    // `{ let _result = ...; _result }`
+                    // underscore prevents an unused_variables lint if the head diverges
+                    let result_ident = lctx.str_to_ident("_result");
                     let let_stmt = stmt_let(lctx, e.span, false, result_ident, match_expr, None);
                     let result = expr_ident(lctx, e.span, result_ident, None);
                     let block = block_all(lctx, e.span, hir_vec![let_stmt], Some(result));

src/test/run-pass/issue-30371.rs

@@ -0,0 +1,18 @@
+// Copyright 2012 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![deny(unused_variables)]
+
+fn main() {
+    for _ in match return () {
+        () => Some(0),
+    } {}
+}
+