Fix impl Trait suggestion

Author: estebank

src/librustc_typeck/check/compare_method.rs

@@ -777,7 +777,7 @@ fn compare_synthetic_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                         let new_generics_span = tcx
                             .sess
                             .codemap()
-                            .generate_fn_name_span(impl_m.span)?
+                            .generate_fn_name_span(impl_span)?
                             .shrink_to_hi();
                         // in case there are generics, just replace them
                         let generics_span = impl_m

src/test/ui/impl-trait/impl-generic-mismatch.stderr

@@ -21,12 +21,8 @@ LL |     fn bar(&self, _: &impl Debug) { }
    |                       ^^^^^^^^^^ expected generic parameter, found `impl Trait`
 help: try changing the `impl Trait` argument to a generic parameter
    |
-LL |     fn bar<U: Debug><U: Debug>(&self, _: &U);
-LL | }
-LL | 
-LL | impl Bar for () {
-LL |     fn bar(&self, _: &U) { }
-   |
+LL |     fn bar<U: Debug>(&self, _: &U) { }
+   |           ^^^^^^^^^^            ^
 
 error[E0643]: method `hash` has incompatible signature for trait
   --> $DIR/impl-generic-mismatch.rs:38:33