Make error apply only to impl/trait mismatch

Author: estebank

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

@@ -26,21 +26,14 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
                         if let (
                             ValuePairs::Types(sub_expected_found),
                             ValuePairs::Types(sup_expected_found),
-                        ) = (&sub_trace.values, &sup_trace.values) {
+                            CompareImplMethodObligation { trait_item_def_id, .. },
+                        ) = (&sub_trace.values, &sup_trace.values, &sub_trace.cause.code) {
                             if sup_expected_found == sub_expected_found {
-                                let sp = var_origin.span();
-                                let impl_sp = if let CompareImplMethodObligation {
-                                    trait_item_def_id, ..
-                                } = &sub_trace.cause.code {
-                                    Some(self.tcx().def_span(*trait_item_def_id))
-                                } else {
-                                    None
-                                };
                                 self.emit_err(
-                                    sp,
+                                    var_origin.span(),
                                     sub_expected_found.expected,
                                     sub_expected_found.found,
-                                    impl_sp,
+                                    self.tcx().def_span(*trait_item_def_id),
                                 );
                                 return Some(ErrorReported);
                             }
@@ -53,16 +46,14 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
         None
     }
 
-    fn emit_err(&self, sp: Span, expected: Ty<'tcx>, found: Ty<'tcx>, impl_sp: Option<Span>) {
+    fn emit_err(&self, sp: Span, expected: Ty<'tcx>, found: Ty<'tcx>, impl_sp: Span) {
         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));
-        if let Some(span) = impl_sp {
-            err.span_label(span, &format!("expected {:?}", expected));
-        }
+        err.span_label(impl_sp, &format!("expected {:?}", expected));
         err.emit();
     }
 }

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 signature doesn't match `trait` item signature
+//~^ ERROR cannot infer an appropriate lifetime for lifetime parameter `'lw`
 
 pub fn main() { }

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

@@ -89,16 +89,27 @@ 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 signature doesn't match `trait` item signature
+error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'lw` due to conflicting requirements
   --> $DIR/reject-specialized-drops-8142.rs:54:1
    |
 LL | impl<'lw>         Drop for W<'lw,'lw>     { fn drop(&mut self) { } } // REJECT
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found W<'_, '_>
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: first, the lifetime cannot outlive the lifetime `'l1` as defined on the struct at 17:10...
+  --> $DIR/reject-specialized-drops-8142.rs:17:10
+   |
+LL | struct W<'l1, 'l2> { x: &'l1 i8, y: &'l2 u8 }
+   |          ^^^
+note: ...but the lifetime must also be valid for the lifetime `'l2` as defined on the struct at 17:15...
+  --> $DIR/reject-specialized-drops-8142.rs:17:15
    |
-   = note: expected `W<'l1, 'l2>`
-              found `W<'_, '_>`
+LL | struct W<'l1, 'l2> { x: &'l1 i8, y: &'l2 u8 }
+   |               ^^^
+   = note: ...so that the types are compatible:
+           expected W<'l1, 'l2>
+              found W<'_, '_>
 
 error: aborting due to 8 previous errors
 
-Some errors have detailed explanations: E0308, E0366, E0367.
+Some errors have detailed explanations: E0308, E0366, E0367, E0495.
 For more information about an error, try `rustc --explain E0308`.