enforce unused-must-use lint in macros

src/librustc_lint/unused.rs

@@ -29,7 +29,8 @@ use rustc::hir;
 declare_lint! {
     pub UNUSED_MUST_USE,
     Warn,
-    "unused result of a type flagged as #[must_use]"
+    "unused result of a type flagged as #[must_use]",
+    report_in_external_macro: true
 }
 
 declare_lint! {

src/test/ui/macros/must-use-in-macro-55516.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 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.
+
+// compile-pass
+// compile-flags: -Wunused
+
+// make sure write!() can't hide its unused Result
+
+fn main() {
+    use std::fmt::Write;
+    let mut example = String::new();
+    write!(&mut example, "{}", 42); //~WARN must be used
+}
+

src/test/ui/macros/must-use-in-macro-55516.stderr

@@ -0,0 +1,10 @@
+warning: unused `std::result::Result` that must be used
+  --> $DIR/must-use-in-macro-55516.rs:19:5
+   |
+LL |     write!(&mut example, "{}", 42); //~WARN must be used
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-W unused-must-use` implied by `-W unused`
+   = note: this `Result` may be an `Err` variant, which should be handled
+   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)
+