only emit "lint level defined here" the first time

We introduce a new `one_time_diagnostics` field on
`rustc::session::Session` to hold a hashset of diagnostic messages we've
set once but don't want to see again (as uniquified by span and message
text), "lint level defined here" being the motivating example dealt with
here. It is the responsibility of the caller setting a diagnostic to
decide whether to add to `one_time_diagnostics`; there are other
situations where we likely do want it to be possible for a note to
appear twice on the same span with the same message (e.g., "prior
assignment occurs here").

This is in the matter of rust-lang#24690.

Author: zackmdavis

src/librustc/lint/context.rs

@@ -462,7 +462,13 @@ 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);
+        let span_explanation = (span, explanation.to_owned());
+        let already_noted: bool = sess.one_time_diagnostics.borrow()
+            .contains(&span_explanation);
+        if !already_noted {
+            err.span_note(span, explanation);
+            sess.one_time_diagnostics.borrow_mut().insert(span_explanation);
+        }
     }
 
     err

src/librustc/session/mod.rs

@@ -17,7 +17,7 @@ use middle::dependency_format;
 use session::search_paths::PathKind;
 use session::config::{DebugInfoLevel, PanicStrategy};
 use ty::tls;
-use util::nodemap::{NodeMap, FnvHashMap};
+use util::nodemap::{NodeMap, FnvHashMap, FnvHashSet};
 use mir::transform as mir_pass;
 
 use syntax::ast::{NodeId, NodeIdAssigner, Name};
@@ -70,6 +70,10 @@ 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.
+    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)>>,
@@ -523,6 +527,7 @@ 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-no-drop-on-repr-extern.rs

@@ -16,7 +16,6 @@
 #![feature(unsafe_no_drop_flag)]
 #![deny(drop_with_repr_extern)]
 //~^ NOTE lint level defined here
-//~| NOTE lint level defined here
 
 #[repr(C)] struct As { x: Box<i8> }
 #[repr(C)] enum Ae { Ae(Box<i8>), _None }

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