introing one-time diagnostics: only emit "lint level defined here" once

src/librustc/lint/context.rs

@@ -452,8 +452,7 @@ pub fn raw_struct_lint<'a>(sess: &'a Session,
     }
 
     if let Some(span) = def {
-        let explanation = "lint level defined here";
-        err.span_note(span, &explanation);
+        sess.diag_span_note_once(&mut err, span, "lint level defined here");
     }
 
     err

src/librustc/session/mod.rs

@@ -17,7 +17,7 @@ use middle::dependency_format;
 use session::search_paths::PathKind;
 use session::config::DebugInfoLevel;
 use ty::tls;
-use util::nodemap::{NodeMap, FnvHashMap};
+use util::nodemap::{NodeMap, FnvHashMap, FnvHashSet};
 use util::common::duration_to_secs_str;
 use mir::transform as mir_pass;
 
@@ -75,6 +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
+    /// redundantly verbose output (Issue #24690).
+    pub one_time_diagnostics: RefCell<FnvHashSet<(Span, String)>>,
     pub plugin_llvm_passes: RefCell<Vec<String>>,
     pub mir_passes: RefCell<mir_pass::Passes>,
     pub plugin_attributes: RefCell<Vec<(String, AttributeType)>>,
@@ -288,6 +292,26 @@ impl Session {
     pub fn diagnostic<'a>(&'a self) -> &'a errors::Handler {
         &self.parse_sess.span_diagnostic
     }
+
+    /// Analogous to calling `.span_note` on the given DiagnosticBuilder, but
+    /// deduplicates on span and message for this `Session`.
+    //
+    // FIXME: if the need arises for one-time diagnostics other than
+    // `span_note`, we almost certainly want to generalize this "check 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) {
+        let span_message = (span, message.to_owned());
+        let already_noted: bool = self.one_time_diagnostics.borrow()
+            .contains(&span_message);
+        if !already_noted {
+            diag_builder.span_note(span, &message);
+            self.one_time_diagnostics.borrow_mut().insert(span_message);
+        }
+    }
+
     pub fn codemap<'a>(&'a self) -> &'a codemap::CodeMap {
         self.parse_sess.codemap()
     }
@@ -561,6 +585,7 @@ pub fn build_session_(sopts: config::Options,
         working_dir: env::current_dir().unwrap(),
         lint_store: RefCell::new(lint::LintStore::new()),
         lints: RefCell::new(NodeMap()),
+        one_time_diagnostics: RefCell::new(FnvHashSet()),
         plugin_llvm_passes: RefCell::new(Vec::new()),
         mir_passes: RefCell::new(mir_pass::Passes::new()),
         plugin_attributes: RefCell::new(Vec::new()),

src/test/compile-fail/lint-group-style.rs

@@ -20,7 +20,6 @@ mod test {
 
     #[forbid(bad_style)]
     //~^ NOTE lint level defined here
-    //~^^ NOTE lint level defined here
     mod bad {
         fn CamelCase() {} //~ ERROR function `CamelCase` should have a snake case name
 
@@ -30,7 +29,6 @@ mod test {
     mod warn {
         #![warn(bad_style)]
         //~^ NOTE lint level defined here
-        //~| NOTE lint level defined here
 
         fn CamelCase() {} //~ WARN function `CamelCase` should have a snake case name
 

src/test/compile-fail/lint-unconditional-recursion.rs

@@ -10,19 +10,7 @@
 
 #![deny(unconditional_recursion)]
 //~^ NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
-//~| NOTE lint level defined here
+
 #![allow(dead_code)]
 fn foo() { //~ ERROR function cannot return without recurring
     foo(); //~ NOTE recursive call site