Extract out error message generation.

Author: frewsxcv

src/librustc/traits/error_reporting.rs

@@ -21,7 +21,6 @@ use super::{
     SelectionContext,
     SelectionError,
     ObjectSafetyViolation,
-    MethodViolationCode,
 };
 
 use fmt_macros::{Parser, Piece, Position};
@@ -679,38 +678,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
             if !reported_violations.insert(violation.clone()) {
                 continue;
             }
-            let buf;
-            let note = match violation {
-                ObjectSafetyViolation::SizedSelf => {
-                    "the trait cannot require that `Self : Sized`"
-                }
-
-                ObjectSafetyViolation::SupertraitSelf => {
-                    "the trait cannot use `Self` as a type parameter \
-                         in the supertrait listing"
-                }
-
-                ObjectSafetyViolation::Method(name,
-                                              MethodViolationCode::StaticMethod) => {
-                    buf = format!("method `{}` has no receiver", name);
-                    &buf
-                }
-
-                ObjectSafetyViolation::Method(name,
-                                              MethodViolationCode::ReferencesSelf) => {
-                    buf = format!("method `{}` references the `Self` type \
-                                       in its arguments or return type",
-                                  name);
-                    &buf
-                }
-
-                ObjectSafetyViolation::Method(name,
-                                              MethodViolationCode::Generic) => {
-                    buf = format!("method `{}` has generic type parameters", name);
-                    &buf
-                }
-            };
-            err.note(note);
+            err.note(&violation.error_msg());
         }
         err
     }

src/librustc/traits/object_safety.rs

@@ -23,6 +23,7 @@ use hir::def_id::DefId;
 use traits;
 use ty::{self, Ty, TyCtxt, TypeFoldable};
 use ty::subst::Substs;
+use std::borrow::Cow;
 use syntax::ast;
 
 #[derive(Clone, Debug, PartialEq, Eq, Hash)]
@@ -38,6 +39,25 @@ pub enum ObjectSafetyViolation {
     Method(ast::Name, MethodViolationCode),
 }
 
+impl ObjectSafetyViolation {
+    pub fn error_msg(&self) -> Cow<'static, str> {
+        match *self {
+            ObjectSafetyViolation::SizedSelf =>
+                "the trait cannot require that `Self : Sized`".into(),
+            ObjectSafetyViolation::SupertraitSelf =>
+                "the trait cannot use `Self` as a type parameter \
+                 in the supertrait listing".into(),
+            ObjectSafetyViolation::Method(name, MethodViolationCode::StaticMethod) =>
+                format!("method `{}` has no receiver", name).into(),
+            ObjectSafetyViolation::Method(name, MethodViolationCode::ReferencesSelf) =>
+                format!("method `{}` references the `Self` type \
+                         in its arguments or return type", name).into(),
+            ObjectSafetyViolation::Method(name, MethodViolationCode::Generic) =>
+                format!("method `{}` has generic type parameters", name).into(),
+        }
+    }
+}
+
 /// Reasons a method might not be object-safe.
 #[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
 pub enum MethodViolationCode {