add a lowercase suggestion to unknown_lints

src/librustc/lint/levels.rs

@@ -247,13 +247,27 @@ pub fn push(&mut self, attrs: &[ast::Attribute]) -> BuilderPush {
                                                                     self.cur,
                                                                     Some(&specs));
                         let msg = format!("unknown lint: `{}`", name);
-                        lint::struct_lint_level(self.sess,
+                        let mut db = lint::struct_lint_level(self.sess,
                                                 lint,
                                                 level,
                                                 src,
                                                 Some(li.span.into()),
-                                                &msg)
-                            .emit();
+                                                &msg);
+                        if name.as_str().chars().any(|c| c.is_uppercase()) {
+                            let name_lower = name.as_str().to_lowercase();
+                            if let CheckLintNameResult::NoLint =
+                                    store.check_lint_name(&name_lower) {
+                                db.emit();
+                            } else {
+                                db.span_suggestion(
+                                    li.span,
+                                    "lowercase the lint name",
+                                    name_lower
+                                ).emit();
+                            }
+                        } else {
+                            db.emit();
+                        }
                     }
                 }
             }

src/test/ui/lint/not_found.rs

@@ -0,0 +1,21 @@
+// Copyright 2014–2017 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.
+
+// this tests the `unknown_lint` lint, especially the suggestions
+
+// the suggestion only appears if a lint with the lowercase name exists
+#[allow(FOO_BAR)]
+// the suggestion appears on all-uppercase names
+#[warn(DEAD_CODE)]
+// the suggestion appears also on mixed-case names
+#[deny(Warnings)]
+fn main() {
+    unimplemented!();
+}

src/test/ui/lint/not_found.stderr

@@ -0,0 +1,20 @@
+warning: unknown lint: `FOO_BAR`
+  --> $DIR/not_found.rs:14:9
+   |
+14 | #[allow(FOO_BAR)]
+   |         ^^^^^^^
+   |
+   = note: #[warn(unknown_lints)] on by default
+
+warning: unknown lint: `DEAD_CODE`
+  --> $DIR/not_found.rs:16:8
+   |
+16 | #[warn(DEAD_CODE)]
+   |        ^^^^^^^^^ help: lowercase the lint name: `dead_code`
+
+warning: unknown lint: `Warnings`
+  --> $DIR/not_found.rs:18:8
+   |
+18 | #[deny(Warnings)]
+   |        ^^^^^^^^ help: lowercase the lint name: `warnings`
+