Auto merge of #106582 - compiler-errors:better-spans-on-bad-tys, r=lcnr

Improve spans of non-WF implied bound types

Fixes #60980

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -97,21 +97,22 @@ pub(super) fn enter_wf_checking_ctxt<'tcx, F>(
     let infcx = &tcx.infer_ctxt().build();
     let ocx = ObligationCtxt::new(infcx);
 
-    let assumed_wf_types = ocx.assumed_wf_types(param_env, span, body_def_id);
-
     let mut wfcx = WfCheckingCtxt { ocx, span, body_id, param_env };
 
     if !tcx.features().trivial_bounds {
         wfcx.check_false_global_bounds()
     }
     f(&mut wfcx);
+
+    let assumed_wf_types = wfcx.ocx.assumed_wf_types(param_env, span, body_def_id);
+    let implied_bounds = infcx.implied_bounds_tys(param_env, body_id, assumed_wf_types);
+
     let errors = wfcx.select_all_or_error();
     if !errors.is_empty() {
         infcx.err_ctxt().report_fulfillment_errors(&errors, None);
         return;
     }
 
-    let implied_bounds = infcx.implied_bounds_tys(param_env, body_id, assumed_wf_types);
     let outlives_environment =
         OutlivesEnvironment::with_bounds(param_env, Some(infcx), implied_bounds);
 
@@ -1489,54 +1490,38 @@ fn check_fn_or_method<'tcx>(
     def_id: LocalDefId,
 ) {
     let tcx = wfcx.tcx();
-    let sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), sig);
+    let mut sig = tcx.liberate_late_bound_regions(def_id.to_def_id(), sig);
 
     // Normalize the input and output types one at a time, using a different
     // `WellFormedLoc` for each. We cannot call `normalize_associated_types`
     // on the entire `FnSig`, since this would use the same `WellFormedLoc`
     // for each type, preventing the HIR wf check from generating
     // a nice error message.
-    let ty::FnSig { mut inputs_and_output, c_variadic, unsafety, abi } = sig;
+    let arg_span =
-    inputs_and_output = tcx.mk_type_list(inputs_and_output.iter().enumerate().map(|(i, ty)| {
+        |idx| hir_decl.inputs.get(idx).map_or(hir_decl.output.span(), |arg: &hir::Ty<'_>| arg.span);
+
+    sig.inputs_and_output =
+        tcx.mk_type_list(sig.inputs_and_output.iter().enumerate().map(|(idx, ty)| {
             wfcx.normalize(
-            span,
+                arg_span(idx),
                 Some(WellFormedLoc::Param {
                     function: def_id,
                     // Note that the `param_idx` of the output type is
                     // one greater than the index of the last input type.
-                param_idx: i.try_into().unwrap(),
+                    param_idx: idx.try_into().unwrap(),
                 }),
                 ty,
             )
         }));
-    // Manually call `normalize_associated_types_in` on the other types
-    // in `FnSig`. This ensures that if the types of these fields
-    // ever change to include projections, we will start normalizing
-    // them automatically.
-    let sig = ty::FnSig {
-        inputs_and_output,
-        c_variadic: wfcx.normalize(span, None, c_variadic),
-        unsafety: wfcx.normalize(span, None, unsafety),
-        abi: wfcx.normalize(span, None, abi),
-    };
 
-    for (i, (&input_ty, ty)) in iter::zip(sig.inputs(), hir_decl.inputs).enumerate() {
+    for (idx, ty) in sig.inputs_and_output.iter().enumerate() {
         wfcx.register_wf_obligation(
-            ty.span,
+            arg_span(idx),
-            Some(WellFormedLoc::Param { function: def_id, param_idx: i.try_into().unwrap() }),
+            Some(WellFormedLoc::Param { function: def_id, param_idx: idx.try_into().unwrap() }),
-            input_ty.into(),
+            ty.into(),
         );
     }
 
-    wfcx.register_wf_obligation(
-        hir_decl.output.span(),
-        Some(WellFormedLoc::Param {
-            function: def_id,
-            param_idx: sig.inputs().len().try_into().unwrap(),
-        }),
-        sig.output().into(),
-    );
-
     check_where_clauses(wfcx, span, def_id);
 
     check_return_position_impl_trait_in_trait_bounds(

src/test/ui/associated-types/associated-types-for-unimpl-trait.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `Self: Get` is not satisfied
-  --> $DIR/associated-types-for-unimpl-trait.rs:10:5
+  --> $DIR/associated-types-for-unimpl-trait.rs:10:40
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |                                        ^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
    |
 help: consider further restricting `Self`
    |

src/test/ui/associated-types/associated-types-no-suitable-bound.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `T: Get` is not satisfied
-  --> $DIR/associated-types-no-suitable-bound.rs:11:5
+  --> $DIR/associated-types-no-suitable-bound.rs:11:21
    |
 LL |     fn uhoh<T>(foo: <T as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `T`
+   |                     ^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `T`
    |
 help: consider restricting type parameter `T`
    |

src/test/ui/associated-types/associated-types-no-suitable-supertrait-2.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `Self: Get` is not satisfied
-  --> $DIR/associated-types-no-suitable-supertrait-2.rs:17:5
+  --> $DIR/associated-types-no-suitable-supertrait-2.rs:17:40
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |                                        ^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
    |
 help: consider further restricting `Self`
    |

src/test/ui/associated-types/associated-types-no-suitable-supertrait.stderr

@@ -1,14 +1,14 @@
 error[E0277]: the trait bound `(T, U): Get` is not satisfied
-  --> $DIR/associated-types-no-suitable-supertrait.rs:22:5
+  --> $DIR/associated-types-no-suitable-supertrait.rs:22:40
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <(T, U) as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `(T, U)`
+   |                                        ^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `(T, U)`
 
 error[E0277]: the trait bound `Self: Get` is not satisfied
-  --> $DIR/associated-types-no-suitable-supertrait.rs:17:5
+  --> $DIR/associated-types-no-suitable-supertrait.rs:17:40
    |
 LL |     fn uhoh<U:Get>(&self, foo: U, bar: <Self as Get>::Value) {}
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |                                        ^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
    |
 help: consider further restricting `Self`
    |

src/test/ui/associated-types/associated-types-projection-to-unrelated-trait-in-method-without-default.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `Self: Get` is not satisfied
-  --> $DIR/associated-types-projection-to-unrelated-trait-in-method-without-default.rs:10:5
+  --> $DIR/associated-types-projection-to-unrelated-trait-in-method-without-default.rs:10:40
    |
 LL |     fn okay<U:Get>(&self, foo: U, bar: <Self as Get>::Value);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
+   |                                        ^^^^^^^^^^^^^^^^^^^^ the trait `Get` is not implemented for `Self`
    |
 help: consider further restricting `Self`
    |

src/test/ui/associated-types/issue-59324.rs

@@ -15,9 +15,9 @@ pub trait ThriftService<Bug: NotFoo>:
 {
     fn get_service(
     //~^ ERROR the trait bound `Bug: Foo` is not satisfied
-    //~| ERROR the trait bound `Bug: Foo` is not satisfied
         &self,
     ) -> Self::AssocType;
+    //~^ ERROR the trait bound `Bug: Foo` is not satisfied
 }
 
 fn with_factory<H>(factory: dyn ThriftService<()>) {}

src/test/ui/associated-types/issue-59324.stderr

@@ -20,7 +20,7 @@ LL | |
 LL | |
 LL | |     Service<AssocType = <Bug as Foo>::OnlyFoo>
 ...  |
-LL | |     ) -> Self::AssocType;
+LL | |
 LL | | }
    | |_^ the trait `Foo` is not implemented for `Bug`
    |
@@ -34,7 +34,6 @@ error[E0277]: the trait bound `Bug: Foo` is not satisfied
    |
 LL | /     fn get_service(
 LL | |
-LL | |
 LL | |         &self,
 LL | |     ) -> Self::AssocType;
    | |_________________________^ the trait `Foo` is not implemented for `Bug`
@@ -45,20 +44,16 @@ LL | pub trait ThriftService<Bug: NotFoo + Foo>:
    |                                     +++++
 
 error[E0277]: the trait bound `(): Foo` is not satisfied
-  --> $DIR/issue-59324.rs:23:1
+  --> $DIR/issue-59324.rs:23:29
    |
 LL | fn with_factory<H>(factory: dyn ThriftService<()>) {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `()`
+   |                             ^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `()`
 
 error[E0277]: the trait bound `Bug: Foo` is not satisfied
-  --> $DIR/issue-59324.rs:16:5
+  --> $DIR/issue-59324.rs:19:10
    |
-LL | /     fn get_service(
+LL |     ) -> Self::AssocType;
-LL | |
+   |          ^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `Bug`
-LL | |
-LL | |         &self,
-LL | |     ) -> Self::AssocType;
-   | |_________________________^ the trait `Foo` is not implemented for `Bug`
    |
 help: consider further restricting this bound
    |

src/test/ui/issues/issue-18611.stderr

@@ -1,10 +1,8 @@
 error[E0277]: the trait bound `isize: HasState` is not satisfied
-  --> $DIR/issue-18611.rs:1:1
+  --> $DIR/issue-18611.rs:1:18
    |
-LL | / fn add_state(op: <isize as HasState>::State) {
+LL | fn add_state(op: <isize as HasState>::State) {
-LL | |
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HasState` is not implemented for `isize`
-LL | | }
-   | |_^ the trait `HasState` is not implemented for `isize`
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-20831-debruijn.stderr

@@ -1,14 +1,8 @@
 error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
-  --> $DIR/issue-20831-debruijn.rs:28:5
+  --> $DIR/issue-20831-debruijn.rs:28:33
    |
-LL | /     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
+LL |     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
-LL | |         // Not obvious, but there is an implicit lifetime here -------^
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-LL | |
-LL | |         //
-...  |
-LL | |         self.sub = t;
-LL | |     }
-   | |_____^
    |
 note: first, the lifetime cannot outlive the anonymous lifetime defined here...
   --> $DIR/issue-20831-debruijn.rs:28:58
@@ -21,16 +15,10 @@ note: ...but the lifetime must also be valid for the lifetime `'a` as defined he
 LL | impl<'a> Publisher<'a> for MyStruct<'a> {
    |      ^^
 note: ...so that the types are compatible
-  --> $DIR/issue-20831-debruijn.rs:28:5
+  --> $DIR/issue-20831-debruijn.rs:28:33
    |
-LL | /     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
+LL |     fn subscribe(&mut self, t : Box<dyn Subscriber<Input=<Self as Publisher>::Output> + 'a>) {
-LL | |         // Not obvious, but there is an implicit lifetime here -------^
+   |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-LL | |
-LL | |         //
-...  |
-LL | |         self.sub = t;
-LL | |     }
-   | |_____^
    = note: expected `<MyStruct<'a> as Publisher<'_>>`
               found `<MyStruct<'_> as Publisher<'_>>`
 

src/test/ui/issues/issue-35570.stderr

@@ -1,3 +1,9 @@
+error[E0277]: the trait bound `for<'a> (): Trait2<'a>` is not satisfied
+  --> $DIR/issue-35570.rs:8:40
+   |
+LL | fn _ice(param: Box<dyn for <'a> Trait1<<() as Trait2<'a>>::Ty>>) {
+   |                                        ^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'a> Trait2<'a>` is not implemented for `()`
+
 error[E0277]: the trait bound `for<'a> (): Trait2<'a>` is not satisfied
   --> $DIR/issue-35570.rs:8:1
    |
@@ -8,12 +14,6 @@ LL | |     let _e: (usize, usize) = unsafe{mem::transmute(param)};
 LL | | }
    | |_^ the trait `for<'a> Trait2<'a>` is not implemented for `()`
 
-error[E0277]: the trait bound `for<'a> (): Trait2<'a>` is not satisfied
-  --> $DIR/issue-35570.rs:8:40
-   |
-LL | fn _ice(param: Box<dyn for <'a> Trait1<<() as Trait2<'a>>::Ty>>) {
-   |                                        ^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'a> Trait2<'a>` is not implemented for `()`
-
 error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/nll/normalization-bounds-error.stderr

@@ -1,8 +1,8 @@
 error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'d` due to conflicting requirements
-  --> $DIR/normalization-bounds-error.rs:12:1
+  --> $DIR/normalization-bounds-error.rs:12:31
    |
 LL | fn visit_seq<'d, 'a: 'd>() -> <&'a () as Visitor<'d>>::Value {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: first, the lifetime cannot outlive the lifetime `'d` as defined here...
   --> $DIR/normalization-bounds-error.rs:12:14
@@ -15,10 +15,10 @@ note: ...but the lifetime must also be valid for the lifetime `'a` as defined he
 LL | fn visit_seq<'d, 'a: 'd>() -> <&'a () as Visitor<'d>>::Value {}
    |                  ^^
 note: ...so that the types are compatible
-  --> $DIR/normalization-bounds-error.rs:12:1
+  --> $DIR/normalization-bounds-error.rs:12:31
    |
 LL | fn visit_seq<'d, 'a: 'd>() -> <&'a () as Visitor<'d>>::Value {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    = note: expected `Visitor<'d>`
               found `Visitor<'_>`
 

src/test/ui/regions/regions-implied-bounds-projection-gap-hr-1.stderr

@@ -1,3 +1,14 @@
+error[E0277]: the trait bound `for<'z> T: Trait2<'y, 'z>` is not satisfied
+  --> $DIR/regions-implied-bounds-projection-gap-hr-1.rs:21:49
+   |
+LL | fn callee<'x, 'y, T>(t: &'x dyn for<'z> Trait1< <T as Trait2<'y, 'z>>::Foo >)
+   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'z> Trait2<'y, 'z>` is not implemented for `T`
+   |
+help: consider restricting type parameter `T`
+   |
+LL | fn callee<'x, 'y, T: for<'z> Trait2<'y, 'z>>(t: &'x dyn for<'z> Trait1< <T as Trait2<'y, 'z>>::Foo >)
+   |                    ++++++++++++++++++++++++
+
 error[E0277]: the trait bound `for<'z> T: Trait2<'y, 'z>` is not satisfied
   --> $DIR/regions-implied-bounds-projection-gap-hr-1.rs:21:1
    |
@@ -13,17 +24,6 @@ help: consider restricting type parameter `T`
 LL | fn callee<'x, 'y, T: for<'z> Trait2<'y, 'z>>(t: &'x dyn for<'z> Trait1< <T as Trait2<'y, 'z>>::Foo >)
    |                    ++++++++++++++++++++++++
 
-error[E0277]: the trait bound `for<'z> T: Trait2<'y, 'z>` is not satisfied
-  --> $DIR/regions-implied-bounds-projection-gap-hr-1.rs:21:49
-   |
-LL | fn callee<'x, 'y, T>(t: &'x dyn for<'z> Trait1< <T as Trait2<'y, 'z>>::Foo >)
-   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `for<'z> Trait2<'y, 'z>` is not implemented for `T`
-   |
-help: consider restricting type parameter `T`
-   |
-LL | fn callee<'x, 'y, T: for<'z> Trait2<'y, 'z>>(t: &'x dyn for<'z> Trait1< <T as Trait2<'y, 'z>>::Foo >)
-   |                    ++++++++++++++++++++++++
-
 error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

src/test/ui/specialization/min_specialization/issue-79224.rs

@@ -15,8 +15,10 @@ impl ToString for Cow<'_, str> {
     }
 }
 
-impl<B: ?Sized> Display for Cow<'_, B> { //~ ERROR: the trait bound `B: Clone` is not satisfied [E0277]
+impl<B: ?Sized> Display for Cow<'_, B> {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { //~ ERROR: the trait bound `B: Clone` is not satisfied [E0277]
+    //~^ ERROR: the trait bound `B: Clone` is not satisfied [E0277]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        //~^ ERROR: the trait bound `B: Clone` is not satisfied [E0277]
         write!(f, "foo")
     }
 }

src/test/ui/specialization/min_specialization/issue-79224.stderr

@@ -1,12 +1,8 @@
 error[E0277]: the trait bound `B: Clone` is not satisfied
-  --> $DIR/issue-79224.rs:18:1
+  --> $DIR/issue-79224.rs:18:17
    |
-LL | / impl<B: ?Sized> Display for Cow<'_, B> {
+LL | impl<B: ?Sized> Display for Cow<'_, B> {
-LL | |     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+   |                 ^^^^^^^ the trait `Clone` is not implemented for `B`
-LL | |         write!(f, "foo")
-LL | |     }
-LL | | }
-   | |_^ the trait `Clone` is not implemented for `B`
    |
    = note: required for `B` to implement `ToOwned`
 help: consider further restricting this bound
@@ -15,12 +11,10 @@ LL | impl<B: ?Sized + std::clone::Clone> Display for Cow<'_, B> {
    |                +++++++++++++++++++
 
 error[E0277]: the trait bound `B: Clone` is not satisfied
-  --> $DIR/issue-79224.rs:19:5
+  --> $DIR/issue-79224.rs:20:12
    |
-LL | /     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+LL |     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-LL | |         write!(f, "foo")
+   |            ^^^^^ the trait `Clone` is not implemented for `B`
-LL | |     }
-   | |_____^ the trait `Clone` is not implemented for `B`
    |
    = note: required for `B` to implement `ToOwned`
 help: consider further restricting this bound

src/test/ui/traits/issue-91594.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `Foo: HasComponent<()>` is not satisfied
-  --> $DIR/issue-91594.rs:10:1
+  --> $DIR/issue-91594.rs:10:6
    |
 LL | impl HasComponent<<Foo as Component<Foo>>::Interface> for Foo {}
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HasComponent<()>` is not implemented for `Foo`
+   |      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `HasComponent<()>` is not implemented for `Foo`
    |
    = help: the trait `HasComponent<<Foo as Component<Foo>>::Interface>` is implemented for `Foo`
 note: required for `Foo` to implement `Component<Foo>`

src/test/ui/wf/issue-103573.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `<<Self as TraitC<E>>::TypeC<'a> as TraitB>::TypeB: TraitA` is not satisfied
-  --> $DIR/issue-103573.rs:18:5
+  --> $DIR/issue-103573.rs:18:18
    |
 LL |     fn g<'a>(_: &<<Self::TypeC<'a> as TraitB>::TypeB as TraitA>::TypeA);
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TraitA` is not implemented for `<<Self as TraitC<E>>::TypeC<'a> as TraitB>::TypeB`
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `TraitA` is not implemented for `<<Self as TraitC<E>>::TypeC<'a> as TraitB>::TypeB`
    |
 help: consider further restricting the associated type
    |

src/test/ui/wf/wf-foreign-fn-decl-ret.stderr

@@ -1,8 +1,8 @@
 error[E0277]: the trait bound `(): Foo` is not satisfied
-  --> $DIR/wf-foreign-fn-decl-ret.rs:11:5
+  --> $DIR/wf-foreign-fn-decl-ret.rs:11:25
    |
 LL |     pub fn lint_me() -> <() as Foo>::Assoc;
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `()`
+   |                         ^^^^^^^^^^^^^^^^^^ the trait `Foo` is not implemented for `()`
 
 error[E0277]: the trait bound `u32: Unsatisfied` is not satisfied
   --> $DIR/wf-foreign-fn-decl-ret.rs:14:32