Add explanation to type mismatch involving type params and assoc types

src/librustc/traits/mod.rs

@@ -655,11 +655,11 @@ pub struct VtableTraitAliasData<'tcx, N> {
 }
 
 /// Creates predicate obligations from the generic bounds.
-pub fn predicates_for_generics<'tcx>(cause: ObligationCause<'tcx>,
-                                     param_env: ty::ParamEnv<'tcx>,
-                                     generic_bounds: &ty::InstantiatedPredicates<'tcx>)
-                                     -> PredicateObligations<'tcx>
-{
+pub fn predicates_for_generics<'tcx>(
+    cause: ObligationCause<'tcx>,
+    param_env: ty::ParamEnv<'tcx>,
+    generic_bounds: &ty::InstantiatedPredicates<'tcx>,
+) -> PredicateObligations<'tcx> {
     util::predicates_for_generics(cause, 0, param_env, generic_bounds)
 }
 

src/librustc/traits/util.rs

@@ -513,20 +513,19 @@ pub fn impl_trait_ref_and_oblig<'a, 'tcx>(
 }
 
 /// See [`super::obligations_for_generics`].
-pub fn predicates_for_generics<'tcx>(cause: ObligationCause<'tcx>,
-                                     recursion_depth: usize,
-                                     param_env: ty::ParamEnv<'tcx>,
-                                     generic_bounds: &ty::InstantiatedPredicates<'tcx>)
-                                     -> Vec<PredicateObligation<'tcx>>
-{
-    debug!("predicates_for_generics(generic_bounds={:?})",
-           generic_bounds);
-
-    generic_bounds.predicates.iter().map(|predicate| {
-        Obligation { cause: cause.clone(),
-                     recursion_depth,
-                     param_env,
-                     predicate: predicate.clone() }
+pub fn predicates_for_generics<'tcx>(
+    cause: ObligationCause<'tcx>,
+    recursion_depth: usize,
+    param_env: ty::ParamEnv<'tcx>,
+    generic_bounds: &ty::InstantiatedPredicates<'tcx>,
+) -> Vec<PredicateObligation<'tcx>> {
+    debug!("predicates_for_generics(generic_bounds={:?})", generic_bounds);
+
+    generic_bounds.predicates.iter().map(|predicate| Obligation {
+        cause: cause.clone(),
+        recursion_depth,
+        param_env,
+        predicate: predicate.clone(),
     }).collect()
 }
 

src/librustc/ty/error.rs

@@ -275,10 +275,10 @@ impl<'tcx> TyCtxt<'tcx> {
                                  `.await`ing on both of them");
                     }
                 }
-                if let (ty::Infer(ty::IntVar(_)), ty::Float(_)) =
-                       (&values.found.sty, &values.expected.sty) // Issue #53280
-                {
-                    if let Ok(snippet) = self.sess.source_map().span_to_snippet(sp) {
+                match (&values.expected.sty, &values.found.sty) {
+                    (ty::Float(_), ty::Infer(ty::IntVar(_))) => if let Ok( // Issue #53280
+                        snippet,
+                    ) = self.sess.source_map().span_to_snippet(sp) {
                         if snippet.chars().all(|c| c.is_digit(10) || c == '-' || c == '_') {
                             db.span_suggestion(
                                 sp,
@@ -287,8 +287,96 @@ impl<'tcx> TyCtxt<'tcx> {
                                 Applicability::MachineApplicable
                             );
                         }
+                    },
+                    (ty::Param(_), ty::Param(_)) => {
+                        db.note("a type parameter was expected, but a different one was found; \
+                                 you might be missing a type parameter or trait bound");
+                        db.note("for more information, visit \
+                                 https://doc.rust-lang.org/book/ch10-02-traits.html\
+                                 #traits-as-parameters");
+                    }
+                    (ty::Projection(_), ty::Projection(_)) => {
+                        db.note("an associated type was expected, but a different one was found");
+                    }
+                    (ty::Param(_), ty::Projection(_)) | (ty::Projection(_), ty::Param(_)) => {
+                        db.note("you might be missing a type parameter or trait bound");
+                    }
+                    (ty::Param(_), _) | (_, ty::Param(_)) => {
+                        db.help("type parameters must be constrained to match other types");
+                        if self.sess.teach(&db.get_code().unwrap()) {
+                            db.help("given a type parameter `T` and a method `foo`:
+```
+trait Trait<T> { fn foo(&self) -> T; }
+```
+the only ways to implement method `foo` are:
+- constrain `T` with an explicit type:
+```
+impl Trait<String> for X {
+    fn foo(&self) -> String { String::new() }
+}
+```
+- add a trait bound to `T` and call a method on that trait that returns `Self`:
+```
+impl<T: std::default::Default> Trait<T> for X {
+    fn foo(&self) -> T { <T as std::default::Default>::default() }
+}
+```
+- change `foo` to return an argument of type `T`:
+```
+impl<T> Trait<T> for X {
+    fn foo(&self, x: T) -> T { x }
+}
+```");
+                        }
+                        db.note("for more information, visit \
+                                 https://doc.rust-lang.org/book/ch10-02-traits.html\
+                                 #traits-as-parameters");
+                    }
+                    (ty::Projection(_), _) => {
+                        db.note(&format!(
+                            "consider constraining the associated type `{}` to `{}` or calling a \
+                             method that returns `{}`",
+                            values.expected,
+                            values.found,
+                            values.expected,
+                        ));
+                        if self.sess.teach(&db.get_code().unwrap()) {
+                            db.help("given an associated type `T` and a method `foo`:
+```
+trait Trait {
+    type T;
+    fn foo(&self) -> Self::T;
+}
+```
+the only way of implementing method `foo` is to constrain `T` with an explicit associated type:
+```
+impl Trait for X {
+    type T = String;
+    fn foo(&self) -> Self::T { String::new() }
+}
+```");
+                        }
+                        db.note("for more information, visit \
+                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html");
+                    }
+                    (_, ty::Projection(_)) => {
+                        db.note(&format!(
+                            "consider constraining the associated type `{}` to `{}`",
+                            values.found,
+                            values.expected,
+                        ));
+                        db.note("for more information, visit \
+                                 https://doc.rust-lang.org/book/ch19-03-advanced-traits.html");
                     }
+                    _ => {}
                 }
+                debug!(
+                    "note_and_explain_type_err expected={:?} ({:?}) found={:?} ({:?})",
+                    values.expected,
+                    values.expected.sty,
+                    values.found,
+                    values.found.sty,
+                );
             },
             CyclicTy(ty) => {
                 // Watch out for various cases of cyclic types and try to explain.

src/test/ui/associated-const/associated-const-generic-obligations.stderr

@@ -9,6 +9,8 @@ LL |     const FROM: &'static str = "foo";
    |
    = note: expected type `<T as Foo>::Out`
               found type `&'static str`
+   = note: consider constraining the associated type `<T as Foo>::Out` to `&'static str` or calling a method that returns `<T as Foo>::Out`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-eq-3.stderr

@@ -6,6 +6,8 @@ LL |     let _: Bar = x.boo();
    |
    = note: expected type `Bar`
               found type `<I as Foo>::A`
+   = note: consider constraining the associated type `<I as Foo>::A` to `Bar`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0271]: type mismatch resolving `<isize as Foo>::A == Bar`
   --> $DIR/associated-types-eq-3.rs:38:5

src/test/ui/associated-types/associated-types-issue-20346.stderr

@@ -9,6 +9,8 @@ LL |     is_iterator_of::<Option<T>, _>(&adapter);
    |
    = note: expected type `T`
               found type `std::option::Option<T>`
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/associated-types/associated-types-multiple-types-one-trait.stderr

@@ -9,6 +9,8 @@ LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
    |
    = note: expected type `<T as Foo>::Y`
               found type `i32`
+   = note: consider constraining the associated type `<T as Foo>::Y` to `i32` or calling a method that returns `<T as Foo>::Y`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0271]: type mismatch resolving `<T as Foo>::X == u32`
   --> $DIR/associated-types-multiple-types-one-trait.rs:18:5
@@ -21,6 +23,8 @@ LL | fn want_x<T:Foo<X=u32>>(t: &T) { }
    |
    = note: expected type `<T as Foo>::X`
               found type `u32`
+   = note: consider constraining the associated type `<T as Foo>::X` to `u32` or calling a method that returns `<T as Foo>::X`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to 2 previous errors
 

src/test/ui/compare-method/reordered-type-param.stderr

@@ -9,6 +9,8 @@ LL |   fn b<F:Clone,G>(&self, _x: G) -> G { panic!() }
    |
    = note: expected type `fn(&E, F) -> F`
               found type `fn(&E, G) -> G`
+   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/hrtb/issue-62203-hrtb-ice.stderr

@@ -6,6 +6,8 @@ LL |     let v = Unit2.m(
    |
    = note: expected type `Unit4`
               found type `<_ as Ty<'_>>::V`
+   = note: consider constraining the associated type `<_ as Ty<'_>>::V` to `Unit4`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0271]: type mismatch resolving `<[closure@$DIR/issue-62203-hrtb-ice.rs:42:17: 42:39] as std::ops::FnOnce<((&u8,),)>>::Output == Unit3`
   --> $DIR/issue-62203-hrtb-ice.rs:38:19

src/test/ui/impl-trait/bound-normalization-fail.stderr

@@ -14,6 +14,8 @@ LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
    |
    = note: expected type `()`
               found type `<T as impl_trait::Trait>::Assoc`
+   = note: consider constraining the associated type `<T as impl_trait::Trait>::Assoc` to `()`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
    = note: the return type of a function must have a statically known size
 
 error: `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
@@ -30,6 +32,8 @@ LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
    |
    = note: expected type `()`
               found type `<T as lifetimes::Trait<'static>>::Assoc`
+   = note: consider constraining the associated type `<T as lifetimes::Trait<'static>>::Assoc` to `()`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
    = note: the return type of a function must have a statically known size
 
 error: aborting due to 3 previous errors

src/test/ui/impl-trait/equality2.stderr

@@ -15,6 +15,8 @@ LL |     let _: i32 = Leak::leak(hide(0_i32));
    |
    = note: expected type `i32`
               found type `<impl Foo as Leak>::T`
+   = note: consider constraining the associated type `<impl Foo as Leak>::T` to `i32`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/equality2.rs:38:10

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

@@ -9,6 +9,8 @@ LL |     fn foo<B: Debug>(&self, a: &impl Debug, b: &B) { }
    |
    = note: expected type `fn(&(), &B, &impl Debug)`
               found type `fn(&(), &impl Debug, &B)`
+   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/universal-mismatched-type.stderr

@@ -8,6 +8,8 @@ LL |     x
    |
    = note: expected type `std::string::String`
               found type `impl Debug`
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/impl-trait/universal-two-impl-traits.stderr

@@ -6,6 +6,8 @@ LL |     a = y;
    |
    = note: expected type `impl Debug` (type parameter)
               found type `impl Debug` (type parameter)
+   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-13853.stderr

@@ -9,6 +9,8 @@ LL |         self.iter()
    |
    = note: expected type `I`
               found type `std::slice::Iter<'_, N>`
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error[E0599]: no method named `iter` found for type `&G` in the current scope
   --> $DIR/issue-13853.rs:27:23

src/test/ui/issues/issue-20225.stderr

@@ -6,6 +6,8 @@ LL |   extern "rust-call" fn call(&self, (_,): (T,)) {}
    |
    = note: expected type `extern "rust-call" fn(&Foo, (&'a T,))`
               found type `extern "rust-call" fn(&Foo, (T,))`
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error[E0053]: method `call_mut` has an incompatible type for trait
   --> $DIR/issue-20225.rs:12:3
@@ -15,6 +17,8 @@ LL |   extern "rust-call" fn call_mut(&mut self, (_,): (T,)) {}
    |
    = note: expected type `extern "rust-call" fn(&mut Foo, (&'a T,))`
               found type `extern "rust-call" fn(&mut Foo, (T,))`
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error[E0053]: method `call_once` has an incompatible type for trait
   --> $DIR/issue-20225.rs:20:3
@@ -24,6 +28,8 @@ LL |   extern "rust-call" fn call_once(self, (_,): (T,)) {}
    |
    = note: expected type `extern "rust-call" fn(Foo, (&'a T,))`
               found type `extern "rust-call" fn(Foo, (T,))`
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to 3 previous errors
 

src/test/ui/issues/issue-24204.stderr

@@ -9,6 +9,7 @@ LL | fn test<T: Trait<B=i32>>(b: i32) -> T where T::A: MultiDispatch<i32> { T::n
    |
    = note: expected type `<<T as Trait>::A as MultiDispatch<i32>>::O`
               found type `T`
+   = note: you might be missing a type parameter or trait bound
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-2951.stderr

@@ -6,6 +6,8 @@ LL |     xx = y;
    |
    = note: expected type `T`
               found type `U`
+   = note: a type parameter was expected, but a different one was found; you might be missing a type parameter or trait bound
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/issues/issue-32323.stderr

@@ -8,6 +8,8 @@ LL | pub fn f<'a, T: Tr<'a>>() -> <T as Tr<'a>>::Out {}
    |
    = note: expected type `<T as Tr<'a>>::Out`
               found type `()`
+   = note: consider constraining the associated type `<T as Tr<'a>>::Out` to `()` or calling a method that returns `<T as Tr<'a>>::Out`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to previous error
 

src/test/ui/mismatched_types/issue-35030.stderr

@@ -6,6 +6,8 @@ LL |         Some(true)
    |
    = note: expected type `bool` (type parameter)
               found type `bool` (bool)
+   = help: type parameters must be constrained to match other types
+   = note: for more information, visit https://doc.rust-lang.org/book/ch10-02-traits.html#traits-as-parameters
 
 error: aborting due to previous error
 

src/test/ui/specialization/specialization-default-projection.stderr

@@ -9,6 +9,8 @@ LL |     ()
    |
    = note: expected type `<T as Foo>::Assoc`
               found type `()`
+   = note: consider constraining the associated type `<T as Foo>::Assoc` to `()` or calling a method that returns `<T as Foo>::Assoc`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/specialization-default-projection.rs:28:5
@@ -23,6 +25,8 @@ LL |     generic::<()>()
    |
    = note: expected type `()`
               found type `<() as Foo>::Assoc`
+   = note: consider constraining the associated type `<() as Foo>::Assoc` to `()`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to 2 previous errors
 

src/test/ui/specialization/specialization-default-types.stderr

@@ -8,6 +8,8 @@ LL |         Box::new(self)
    |
    = note: expected type `<T as Example>::Output`
               found type `std::boxed::Box<T>`
+   = note: consider constraining the associated type `<T as Example>::Output` to `std::boxed::Box<T>` or calling a method that returns `<T as Example>::Output`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0308]: mismatched types
   --> $DIR/specialization-default-types.rs:25:5
@@ -19,6 +21,8 @@ LL |     Example::generate(t)
    |
    = note: expected type `std::boxed::Box<T>`
               found type `<T as Example>::Output`
+   = note: consider constraining the associated type `<T as Example>::Output` to `std::boxed::Box<T>`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error: aborting due to 2 previous errors