deduplicate one-time diagnostics on lint ID as well as span and message

Some lint-level attributes (like `bad-style`, or, more dramatically,
`warnings`) can affect more than one lint; it seems fairer to point out
the attribute once for each distinct lint affected. Also, a UI test is
added. This remains in the matter of #24690.

src/librustc/lint/context.rs

@@ -452,7 +452,7 @@ pub fn raw_struct_lint<'a>(sess: &'a Session,
     }
 
     if let Some(span) = def {
-        sess.diag_span_note_once(&mut err, span, "lint level defined here");
+        sess.diag_span_note_once(&mut err, lint, span, "lint level defined here");
     }
 
     err

src/librustc/session/mod.rs

@@ -75,10 +75,10 @@ pub struct Session {
     pub working_dir: PathBuf,
     pub lint_store: RefCell<lint::LintStore>,
     pub lints: RefCell<NodeMap<Vec<(lint::LintId, Span, String)>>>,
-    /// Set of (span, message) tuples tracking lint (sub)diagnostics that have
-    /// been set once, but should not be set again, in order to avoid
+    /// Set of (LintId, span, message) tuples tracking lint (sub)diagnostics
+    /// that have been set once, but should not be set again, in order to avoid
     /// redundantly verbose output (Issue #24690).
-    pub one_time_diagnostics: RefCell<FnvHashSet<(Span, String)>>,
+    pub one_time_diagnostics: RefCell<FnvHashSet<(lint::LintId, Span, String)>>,
     pub plugin_llvm_passes: RefCell<Vec<String>>,
     pub mir_passes: RefCell<mir_pass::Passes>,
     pub plugin_attributes: RefCell<Vec<(String, AttributeType)>>,
@@ -294,25 +294,26 @@ impl Session {
     }
 
     /// Analogous to calling `.span_note` on the given DiagnosticBuilder, but
-    /// deduplicates on span and message for this `Session` if we're not
-    /// outputting in JSON mode.
+    /// deduplicates on lint ID, span, and message for this `Session` if we're
+    /// not outputting in JSON mode.
     //
     // FIXME: if the need arises for one-time diagnostics other than
     // `span_note`, we almost certainly want to generalize this
     // "check/insert-into the one-time diagnostics map, then set message if
     // it's not already there" code to accomodate all of them
     pub fn diag_span_note_once<'a, 'b>(&'a self,
                                        diag_builder: &'b mut DiagnosticBuilder<'a>,
-                                       span: Span, message: &str) {
+                                       lint: &'static lint::Lint, span: Span, message: &str) {
         match self.opts.error_format {
             // when outputting JSON for tool consumption, the tool might want
             // the duplicates
             config::ErrorOutputType::Json => {
                 diag_builder.span_note(span, &message);
             },
             _ => {
-                let span_message = (span, message.to_owned());
-                let fresh = self.one_time_diagnostics.borrow_mut().insert(span_message);
+                let lint_id = lint::LintId::of(lint);
+                let id_span_message = (lint_id, span, message.to_owned());
+                let fresh = self.one_time_diagnostics.borrow_mut().insert(id_span_message);
                 if fresh {
                     diag_builder.span_note(span, &message);
                 }

src/test/ui/span/issue-24690.rs

@@ -0,0 +1,22 @@
+// 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.
+
+//! A test to ensure that helpful `note` messages aren't emitted more often
+//! than necessary.
+
+// Although there are three errors, we should only get two "lint level defined
+// here" notes pointing at the `warnings` span, one for each error type.
+#![deny(warnings)]
+
+fn main() {
+    let theTwo = 2;
+    let theOtherTwo = 2;
+    println!("{}", theTwo);
+}

src/test/ui/span/issue-24690.stderr

@@ -0,0 +1,32 @@
+error: unused variable: `theOtherTwo`
+  --> $DIR/issue-24690.rs:20:9
+   |
+20 |     let theOtherTwo = 2;
+   |         ^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/issue-24690.rs:16:9
+   |
+16 | #![deny(warnings)]
+   |         ^^^^^^^^
+
+error: variable `theTwo` should have a snake case name such as `the_two`
+  --> $DIR/issue-24690.rs:19:9
+   |
+19 |     let theTwo = 2;
+   |         ^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/issue-24690.rs:16:9
+   |
+16 | #![deny(warnings)]
+   |         ^^^^^^^^
+
+error: variable `theOtherTwo` should have a snake case name such as `the_other_two`
+  --> $DIR/issue-24690.rs:20:9
+   |
+20 |     let theOtherTwo = 2;
+   |         ^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+