Rollup merge of #22224 - sanxiyn:must-use, r=cmr

Fix #20109.

src/test/run-make/rustdoc-must-use/Makefile

@@ -0,0 +1,5 @@
+-include ../tools.mk
+
+all: lib.rs
+	$(HOST_RPATH_ENV) $(RUSTDOC) -w html -o $(TMPDIR)/doc lib.rs
+	$(HTMLDOCCK) $(TMPDIR)/doc lib.rs

src/test/run-make/rustdoc-must-use/lib.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 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.
+
+#![crate_type="lib"]
+
+// @has lib/struct.Struct.html //pre '#[must_use]'
+#[must_use]
+pub struct Struct {
+    field: i32,
+}
+
+// @has lib/enum.Enum.html //pre '#[must_use = "message"]'
+#[must_use = "message"]
+pub enum Enum {
+    Variant(i32),
+}