Suggest return type

compiler/rustc_typeck/src/check/compare_method.rs

@@ -329,7 +329,33 @@ fn compare_predicate_entailment<'tcx>(
                     );
                 }
                 TypeError::ArgumentMutability(i) | TypeError::ArgumentSorts(_, i) => {
-                    if let Some(trait_ty) = trait_sig.inputs().get(*i) {
+                    if trait_sig.inputs().len() == *i {
+                        // Suggestion to change output type. We do not suggest in `async` functions
+                        // to avoid complex logic or incorrect output.
+                        let impl_m_hir_id =
+                            tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
+                        match tcx.hir().expect_impl_item(impl_m_hir_id).kind {
+                            ImplItemKind::Fn(ref sig, _)
+                                if sig.header.asyncness == hir::IsAsync::NotAsync =>
+                            {
+                                let (span, sugg) = match sig.decl.output {
+                                    hir::FnRetTy::DefaultReturn(sp) => {
+                                        (sp, format!(" -> {} ", trait_sig.output()))
+                                    }
+                                    hir::FnRetTy::Return(hir_ty) => {
+                                        (hir_ty.span, trait_sig.output().to_string())
+                                    }
+                                };
+                                diag.span_suggestion(
+                                    span,
+                                    "change the output type to match the trait",
+                                    sugg,
+                                    Applicability::MachineApplicable,
+                                );
+                            }
+                            _ => {}
+                        };
+                    } else if let Some(trait_ty) = trait_sig.inputs().get(*i) {
                         diag.span_suggestion(
                             impl_err_span,
                             "change the parameter type to match the trait",

src/test/ui/associated-types/defaults-specialization.stderr

@@ -15,7 +15,10 @@ LL |     fn make() -> Self::Ty {
    |                  -------- type in trait
 ...
 LL |     fn make() -> u8 { 0 }
-   |                  ^^ expected associated type, found `u8`
+   |                  ^^
+   |                  |
+   |                  expected associated type, found `u8`
+   |                  help: change the output type to match the trait: `<A<T> as Tr>::Ty`
    |
    = note: expected fn pointer `fn() -> <A<T> as Tr>::Ty`
               found fn pointer `fn() -> u8`
@@ -30,7 +33,10 @@ LL |     default type Ty = bool;
    |     ----------------------- expected this associated type
 LL | 
 LL |     fn make() -> bool { true }
-   |                  ^^^^ expected associated type, found `bool`
+   |                  ^^^^
+   |                  |
+   |                  expected associated type, found `bool`
+   |                  help: change the output type to match the trait: `<B<T> as Tr>::Ty`
    |
    = note: expected fn pointer `fn() -> <B<T> as Tr>::Ty`
               found fn pointer `fn() -> bool`

src/test/ui/compare-method/bad-self-type.rs

@@ -15,9 +15,12 @@ impl Future for MyFuture {
 
 trait T {
     fn foo(self);
+    fn bar(self) -> Option<()>;
 }
 
 impl T for MyFuture {
     fn foo(self: Box<Self>) {}
     //~^ ERROR method `foo` has an incompatible type for trait
+    fn bar(self) {}
+    //~^ ERROR method `bar` has an incompatible type for trait
 }

src/test/ui/compare-method/bad-self-type.stderr

@@ -11,7 +11,7 @@ LL |     fn poll(self, _: &mut Context<'_>) -> Poll<()> {
               found fn pointer `fn(MyFuture, &mut Context<'_>) -> Poll<_>`
 
 error[E0053]: method `foo` has an incompatible type for trait
-  --> $DIR/bad-self-type.rs:21:18
+  --> $DIR/bad-self-type.rs:22:18
    |
 LL |     fn foo(self);
    |            ---- type in trait
@@ -25,6 +25,21 @@ LL |     fn foo(self: Box<Self>) {}
    = note: expected fn pointer `fn(MyFuture)`
               found fn pointer `fn(Box<MyFuture>)`
 
-error: aborting due to 2 previous errors
+error[E0053]: method `bar` has an incompatible type for trait
+  --> $DIR/bad-self-type.rs:24:18
+   |
+LL |     fn bar(self) -> Option<()>;
+   |                     ---------- type in trait
+...
+LL |     fn bar(self) {}
+   |                  ^
+   |                  |
+   |                  expected enum `Option`, found `()`
+   |                  help: change the output type to match the trait: `-> Option<()>`
+   |
+   = note: expected fn pointer `fn(MyFuture) -> Option<()>`
+              found fn pointer `fn(MyFuture)`
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0053`.

src/test/ui/issues/issue-21332.stderr

@@ -2,7 +2,10 @@ error[E0053]: method `next` has an incompatible type for trait
   --> $DIR/issue-21332.rs:5:27
    |
 LL |     fn next(&mut self) -> Result<i32, i32> { Ok(7) }
-   |                           ^^^^^^^^^^^^^^^^ expected enum `Option`, found enum `Result`
+   |                           ^^^^^^^^^^^^^^^^
+   |                           |
+   |                           expected enum `Option`, found enum `Result`
+   |                           help: change the output type to match the trait: `Option<i32>`
    |
    = note: expected fn pointer `fn(&mut S) -> Option<i32>`
               found fn pointer `fn(&mut S) -> Result<i32, i32>`

src/test/ui/issues/issue-35869.stderr

@@ -50,7 +50,10 @@ LL |     fn qux() -> u8;
    |                 -- type in trait
 ...
 LL |     fn qux() -> u16 { 5u16 }
-   |                 ^^^ expected `u8`, found `u16`
+   |                 ^^^
+   |                 |
+   |                 expected `u8`, found `u16`
+   |                 help: change the output type to match the trait: `u8`
    |
    = note: expected fn pointer `fn() -> u8`
               found fn pointer `fn() -> u16`

src/test/ui/wrong-mul-method-signature.stderr

@@ -26,7 +26,10 @@ error[E0053]: method `mul` has an incompatible type for trait
   --> $DIR/wrong-mul-method-signature.rs:52:29
    |
 LL |     fn mul(self, s: f64) -> f64 {
-   |                             ^^^ expected `i32`, found `f64`
+   |                             ^^^
+   |                             |
+   |                             expected `i32`, found `f64`
+   |                             help: change the output type to match the trait: `i32`
    |
    = note: expected fn pointer `fn(Vec3, _) -> i32`
               found fn pointer `fn(Vec3, _) -> f64`