show each object-safety violation once

different supertraits can suffer from the same object-safety violation,
leading to duplication in the error message. Avoid it.

Fixes #20692

src/librustc/middle/traits/error_reporting.rs

@@ -28,7 +28,8 @@ use middle::def_id::DefId;
 use middle::infer::InferCtxt;
 use middle::ty::{self, ToPredicate, HasTypeFlags, ToPolyTraitRef, TraitRef, Ty};
 use middle::ty::fold::TypeFoldable;
-use std::collections::HashMap;
+use util::nodemap::{FnvHashMap, FnvHashSet};
+
 use std::fmt;
 use syntax::codemap::Span;
 use rustc_front::attr::{AttributeMethods, AttrMetaMethods};
@@ -124,7 +125,7 @@ fn report_on_unimplemented<'a, 'tcx>(infcx: &InferCtxt<'a, 'tcx>,
                                                (gen.name.as_str().to_string(),
                                                 trait_ref.substs.types.get(param, i)
                                                          .to_string())
-                                              }).collect::<HashMap<String, String>>();
+                                              }).collect::<FnvHashMap<String, String>>();
                 generic_map.insert("Self".to_string(),
                                    trait_ref.self_ty().to_string());
                 let parser = Parser::new(&istring);
@@ -329,7 +330,11 @@ pub fn report_object_safety_error<'tcx>(tcx: &ty::ctxt<'tcx>,
         "the trait `{}` cannot be made into an object",
         tcx.item_path_str(trait_def_id));
 
+    let mut reported_violations = FnvHashSet();
     for violation in object_safety_violations(tcx, trait_def_id) {
+        if !reported_violations.insert(violation.clone()) {
+            continue;
+        }
         match violation {
             ObjectSafetyViolation::SizedSelf => {
                 tcx.sess.fileline_note(

src/librustc/middle/traits/object_safety.rs

@@ -27,7 +27,7 @@ use middle::ty::{self, ToPolyTraitRef, Ty};
 use std::rc::Rc;
 use syntax::ast;
 
-#[derive(Debug)]
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
 pub enum ObjectSafetyViolation<'tcx> {
     /// Self : Sized declared on the trait
     SizedSelf,

src/librustc/middle/ty/mod.rs

@@ -272,6 +272,20 @@ impl<'tcx> Method<'tcx> {
     }
 }
 
+impl<'tcx> PartialEq for Method<'tcx> {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool { self.def_id == other.def_id }
+}
+
+impl<'tcx> Eq for Method<'tcx> {}
+
+impl<'tcx> Hash for Method<'tcx> {
+    #[inline]
+    fn hash<H: Hasher>(&self, s: &mut H) {
+        self.def_id.hash(s)
+    }
+}
+
 #[derive(Clone, Copy, Debug)]
 pub struct AssociatedConst<'tcx> {
     pub name: Name,

src/test/compile-fail/issue-20692.rs

@@ -0,0 +1,23 @@
+// Copyright 2015 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.
+
+trait Array: Sized {}
+
+fn f<T: Array>(x: &T) {
+    let _ = x
+    //~^ ERROR `Array` cannot be made into an object
+    //~| NOTE the trait cannot require that `Self : Sized`
+    as
+    &Array;
+    //~^ ERROR `Array` cannot be made into an object
+    //~| NOTE the trait cannot require that `Self : Sized`
+}
+
+fn main() {}