Auto merge of #30804 - shssoichiro:deny-warnings-msg, r=nrc

Show clearer error message when #![deny(warnings)] escalates a warning

Addresses #30730

Author: bors

src/librustc/lint/context.rs

@@ -489,9 +489,14 @@ pub trait LintContext: Sized {
 
     fn level_src(&self, lint: &'static Lint) -> Option<LevelSource> {
         self.lints().levels.get(&LintId::of(lint)).map(|ls| match ls {
-            &(Warn, src) => {
+            &(Warn, _) => {
                 let lint_id = LintId::of(builtin::WARNINGS);
-                (self.lints().get_level_source(lint_id).0, src)
+                let warn_src = self.lints().get_level_source(lint_id);
+                if warn_src.0 != Warn {
+                    warn_src
+                } else {
+                    *ls
+                }
             }
             _ => *ls
         })

src/test/compile-fail/issue-30730.rs

@@ -0,0 +1,13 @@
+// Copyright 2016 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(warnings)] //~ NOTE: lint level defined here
+use std::thread; //~ ERROR: unused import
+fn main() {}