Suggest fix when encountering different mutability from impl to trait

src/librustc_typeck/check/compare_method.rs

@@ -319,6 +319,17 @@ fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                                             E0053,
                                             "method `{}` has an incompatible type for trait",
                                             trait_m.ident);
+            if let TypeError::Mutability = terr {
+                if let Some(trait_err_span) = trait_err_span {
+                    if let Ok(trait_err_str) = tcx.sess.codemap().span_to_snippet(trait_err_span) {
+                        diag.span_suggestion(
+                            impl_err_span,
+                            "consider change the type to match the mutability in trait",
+                            format!("{}", trait_err_str),
+                        );
+                    }
+                }
+            }
 
             infcx.note_type_err(&mut diag,
                                 &cause,

src/test/ui/issue-13033.stderr

@@ -9,6 +9,10 @@ LL |     fn bar(&mut self, other: &Foo) {}
    |
    = note: expected type `fn(&mut Baz, &mut dyn Foo)`
               found type `fn(&mut Baz, &dyn Foo)`
+help: consider change the type to match the mutability in trait
+   |
+LL |     fn bar(&mut self, other: &mut Foo) {}
+   |                              ^^^^^^^^
 
 error: aborting due to previous error
 

src/test/ui/mismatched_types/E0053.stderr

@@ -21,6 +21,10 @@ LL |     fn bar(&mut self) { }
    |
    = note: expected type `fn(&Bar)`
               found type `fn(&mut Bar)`
+help: consider change the type to match the mutability in trait
+   |
+LL |     fn bar(&self) { }
+   |            ^^^^^
 
 error: aborting due to 2 previous errors
 

src/test/ui/mismatched_types/trait-impl-fn-incompatibility.stderr

@@ -21,6 +21,10 @@ LL |     fn bar(&mut self, bar: &Bar) { } //~ ERROR incompatible type
    |
    = note: expected type `fn(&mut Bar, &mut Bar)`
               found type `fn(&mut Bar, &Bar)`
+help: consider change the type to match the mutability in trait
+   |
+LL |     fn bar(&mut self, bar: &mut Bar) { } //~ ERROR incompatible type
+   |                            ^^^^^^^^
 
 error: aborting due to 2 previous errors