review comments

src/librustc/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -1,5 +1,7 @@
 //! Error Reporting for `impl` items that do not match the obligations from their `trait`.
 
+use syntax_pos::Span;
+use crate::ty::Ty;
 use crate::infer::{ValuePairs, Subtype};
 use crate::infer::error_reporting::nice_region_error::NiceRegionError;
 use crate::infer::lexical_region_resolve::RegionResolutionError;
@@ -25,21 +27,11 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                         ) = (&sub_trace.values, &sup_trace.values) {
                             if sup_expected_found == sub_expected_found {
                                 let sp = var_origin.span();
-                                let mut err = self.tcx().sess.struct_span_err(
+                                self.emit_err(
                                     sp,
-                                    "`impl` item doesn't match `trait` item"
-                                );
-                                err.note(&format!(
-                                    "expected: {:?}\n   found: {:?}",
                                     sub_expected_found.expected,
                                     sub_expected_found.found,
-                                ));
-                                err.span_label(sp, &format!(
-                                    "found {:?}",
-                                    sub_expected_found.found,
-                                ));
-                                // FIXME: recover the `FnPtr`'s `HirId`/`Node` to point to it.
-                                err.emit();
+                                );
                                 return Some(ErrorReported);
                             }
                         }
@@ -50,4 +42,15 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         }
         None
     }
+
+    fn emit_err(&self, sp: Span, expected: Ty<'tcx>, found: Ty<'tcx>) {
+        let mut err = self.tcx().sess.struct_span_err(
+            sp,
+            "`impl` item signature doesn't match `trait` item signature",
+        );
+        err.note(&format!("expected: {:?}\n   found: {:?}", expected, found));
+        err.span_label(sp, &format!("found {:?}", found));
+        // FIXME: recover the `FnPtr`'s `HirId`/`Node` to point to it.
+        err.emit();
+    }
 }

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.rs

@@ -9,6 +9,6 @@ impl Deref for Struct {
         unimplemented!();
     }
 }
-//~^^^^ ERROR `impl` item doesn't match `trait` item
+//~^^^^ ERROR `impl` item signature doesn't match `trait` item signature
 
 fn main() {}

src/test/ui/in-band-lifetimes/mismatched_trait_impl-2.stderr

@@ -1,4 +1,4 @@
-error: `impl` item doesn't match `trait` item
+error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/mismatched_trait_impl-2.rs:8:5
    |
 LL |     fn deref(&self) -> &dyn Trait {

src/test/ui/in-band-lifetimes/mismatched_trait_impl.rs

@@ -6,7 +6,7 @@ trait Get {
 }
 
 impl Get for i32 {
-    fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 { //~ ERROR `impl` item doesn't match `trait`
+    fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 { //~ ERROR `impl` item signature doesn't match
         x //~ ERROR lifetime mismatch
     }
 }

src/test/ui/in-band-lifetimes/mismatched_trait_impl.stderr

@@ -1,4 +1,4 @@
-error: `impl` item doesn't match `trait` item
+error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/mismatched_trait_impl.rs:9:5
    |
 LL |     fn foo(&self, x: &u32, y: &'a u32) -> &'a u32 {

src/test/ui/lifetimes/lifetime-mismatch-between-trait-and-impl.rs

@@ -4,7 +4,7 @@ trait Foo {
 
 impl Foo for () {
     fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
-    //~^ ERROR `impl` item doesn't match `trait` item
+    //~^ ERROR `impl` item signature doesn't match `trait` item signature
         if x > y { x } else { y }
     }
 }

src/test/ui/lifetimes/lifetime-mismatch-between-trait-and-impl.stderr

@@ -1,4 +1,4 @@
-error: `impl` item doesn't match `trait` item
+error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/lifetime-mismatch-between-trait-and-impl.rs:6:5
    |
 LL |     fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {

src/test/ui/reject-specialized-drops-8142.rs

@@ -52,6 +52,6 @@ impl<One>         Drop for V<One,One>     { fn drop(&mut self) { } } // REJECT
 //~^ ERROR Implementations of Drop cannot be specialized
 
 impl<'lw>         Drop for W<'lw,'lw>     { fn drop(&mut self) { } } // REJECT
-//~^ ERROR `impl` item doesn't match `trait` item
+//~^ ERROR `impl` item signature doesn't match `trait` item signature
 
 pub fn main() { }

src/test/ui/reject-specialized-drops-8142.stderr

@@ -89,7 +89,7 @@ note: Use same sequence of generic type and region parameters that is on the str
 LL | struct V<Tva, Tvb> { x: *const Tva, y: *const Tvb }
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: `impl` item doesn't match `trait` item
+error: `impl` item signature doesn't match `trait` item signature
   --> $DIR/reject-specialized-drops-8142.rs:54:1
    |
 LL | impl<'lw>         Drop for W<'lw,'lw>     { fn drop(&mut self) { } } // REJECT