Auto merge of #23691 - richo:dedup-typeorigin-mergable, r=eddyb

I've started on refactoring the error handling code to avoid the need to reparse generated errors in `span_*`, but would rather land this incrementally as one monolithic PR (and have un-fond memories of merge conflicts from various other monoliths)

r? @eddyb

src/librustc/middle/infer/error_reporting.rs

@@ -357,23 +357,9 @@ impl<'a, 'tcx> ErrorReporting<'tcx> for InferCtxt<'a, 'tcx> {
             }
         };
 
-        let message_root_str = match trace.origin {
-            infer::Misc(_) => "mismatched types",
-            infer::MethodCompatCheck(_) => "method not compatible with trait",
-            infer::ExprAssignable(_) => "mismatched types",
-            infer::RelateTraitRefs(_) => "mismatched traits",
-            infer::RelateSelfType(_) => "mismatched types",
-            infer::RelateOutputImplTypes(_) => "mismatched types",
-            infer::MatchExpressionArm(_, _) => "match arms have incompatible types",
-            infer::IfExpression(_) => "if and else have incompatible types",
-            infer::IfExpressionWithNoElse(_) => "if may be missing an else clause",
-            infer::RangeExpression(_) => "start and end of range have incompatible types",
-            infer::EquatePredicate(_) => "equality predicate not satisfied",
-        };
-
         span_err!(self.tcx.sess, trace.origin.span(), E0308,
             "{}: {} ({})",
-                 message_root_str,
+                 trace.origin,
                  expected_found_str,
                  ty::type_err_to_str(self.tcx, terr));
 
@@ -1495,38 +1481,38 @@ impl<'a, 'tcx> ErrorReportingHelpers<'tcx> for InferCtxt<'a, 'tcx> {
             infer::Subtype(ref trace) => {
                 let desc = match trace.origin {
                     infer::Misc(_) => {
-                        format!("types are compatible")
+                        "types are compatible"
                     }
                     infer::MethodCompatCheck(_) => {
-                        format!("method type is compatible with trait")
+                        "method type is compatible with trait"
                     }
                     infer::ExprAssignable(_) => {
-                        format!("expression is assignable")
+                        "expression is assignable"
                     }
                     infer::RelateTraitRefs(_) => {
-                        format!("traits are compatible")
+                        "traits are compatible"
                     }
                     infer::RelateSelfType(_) => {
-                        format!("self type matches impl self type")
+                        "self type matches impl self type"
                     }
                     infer::RelateOutputImplTypes(_) => {
-                        format!("trait type parameters matches those \
-                                 specified on the impl")
+                        "trait type parameters matches those \
+                                 specified on the impl"
                     }
                     infer::MatchExpressionArm(_, _) => {
-                        format!("match arms have compatible types")
+                        "match arms have compatible types"
                     }
                     infer::IfExpression(_) => {
-                        format!("if and else have compatible types")
+                        "if and else have compatible types"
                     }
                     infer::IfExpressionWithNoElse(_) => {
-                        format!("if may be missing an else clause")
+                        "if may be missing an else clause"
                     }
                     infer::RangeExpression(_) => {
-                        format!("start and end of range have compatible types")
+                        "start and end of range have compatible types"
                     }
                     infer::EquatePredicate(_) => {
-                        format!("equality where clause is satisfied")
+                        "equality where clause is satisfied"
                     }
                 };
 
@@ -1666,8 +1652,8 @@ impl<'a, 'tcx> ErrorReportingHelpers<'tcx> for InferCtxt<'a, 'tcx> {
             infer::RelateRegionParamBound(span) => {
                 self.tcx.sess.span_note(
                     span,
-                    &format!("...so that the declared lifetime parameter bounds \
-                                are satisfied"));
+                    "...so that the declared lifetime parameter bounds \
+                                are satisfied");
             }
             infer::SafeDestructor(span) => {
                 self.tcx.sess.span_note(

src/librustc/middle/infer/mod.rs

@@ -29,6 +29,7 @@ use middle::ty::replace_late_bound_regions;
 use middle::ty::{self, Ty};
 use middle::ty_fold::{TypeFolder, TypeFoldable};
 use std::cell::{RefCell};
+use std::fmt;
 use std::rc::Rc;
 use syntax::ast;
 use syntax::codemap;
@@ -128,6 +129,30 @@ pub enum TypeOrigin {
     EquatePredicate(Span),
 }
 
+impl TypeOrigin {
+    fn as_str(&self) -> &'static str {
+        match self {
+            &TypeOrigin::Misc(_) |
+            &TypeOrigin::RelateSelfType(_) |
+            &TypeOrigin::RelateOutputImplTypes(_) |
+            &TypeOrigin::ExprAssignable(_) => "mismatched types",
+            &TypeOrigin::RelateTraitRefs(_) => "mismatched traits",
+            &TypeOrigin::MethodCompatCheck(_) => "method not compatible with trait",
+            &TypeOrigin::MatchExpressionArm(_, _) => "match arms have incompatible types",
+            &TypeOrigin::IfExpression(_) => "if and else have incompatible types",
+            &TypeOrigin::IfExpressionWithNoElse(_) => "if may be missing an else clause",
+            &TypeOrigin::RangeExpression(_) => "start and end of range have incompatible types",
+            &TypeOrigin::EquatePredicate(_) => "equality predicate not satisfied",
+        }
+    }
+}
+
+impl fmt::Display for TypeOrigin {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(),fmt::Error> {
+        fmt::Display::fmt(self.as_str(), f)
+    }
+}
+
 /// See `error_reporting.rs` for more details
 #[derive(Clone, Debug)]
 pub enum ValuePairs<'tcx> {