On type mismatch involving associated type, suggest constraint

When an associated type is found when a specific type was expected, if
possible provide a structured suggestion constraining the associated
type in a bound.

```
error[E0271]: type mismatch resolving `<T as Foo>::Y == i32`
  --> $DIR/associated-types-multiple-types-one-trait.rs:13:5
   |
LL |     want_y(t);
   |     ^^^^^^ expected `i32`, found associated type
...
LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
   |                 ----- required by this bound in `want_y`
   |
   = note:         expected type `i32`
           found associated type `<T as Foo>::Y`
help: consider constraining the associated type `<T as Foo>::Y` to `i32`
   |
LL | fn have_x_want_y<T:Foo<X=u32, Y = i32>>(t: &T)
   |                             ^^^^^^^^^
```

```
error[E0308]: mismatched types
  --> $DIR/trait-with-missing-associated-type-restriction.rs:12:9
   |
LL |     qux(x.func())
   |         ^^^^^^^^ expected `usize`, found associated type
   |
   = note:         expected type `usize`
           found associated type `<impl Trait as Trait>::A`
help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
   |
LL | fn foo(x: impl Trait<A = usize>) {
   |                     ^^^^^^^^^^
```

22 files changed, 469 insertions, 66 deletions

diff --git a/src/librustc_middle/ty/error.rs b/src/librustc_middle/ty/error.rs
index 78a94b62d47..eb5a5702c45 100644
--- a/src/librustc_middle/ty/error.rs
+++ b/src/librustc_middle/ty/error.rs
@@ -4,7 +4,7 @@ use rustc_errors::{pluralize, Applicability, DiagnosticBuilder};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
 use rustc_span::symbol::sym;
-use rustc_span::Span;
+use rustc_span::{BytePos, Span};
 use rustc_target::spec::abi;
 
 use std::borrow::Cow;
@@ -401,7 +401,10 @@ impl<'tcx> TyCtxt<'tcx> {
                     (ty::Param(_), ty::Projection(_)) | (ty::Projection(_), ty::Param(_)) => {
                         db.note("you might be missing a type parameter or trait bound");
                     }
-                    (ty::Param(p), _) | (_, ty::Param(p)) => {
+                    (ty::Param(p), ty::Dynamic(..))
+                    | (ty::Dynamic(..), ty::Param(p))
+                    | (ty::Param(p), ty::Opaque(..))
+                    | (ty::Opaque(..), ty::Param(p)) => {
                         let generics = self.generics_of(body_owner_def_id);
                         let p_span = self.def_span(generics.type_param(p, self).def_id);
                         if !sp.contains(p_span) {
@@ -441,11 +444,18 @@ impl<T> Trait<T> for X {
                                  #traits-as-parameters",
                         );
                     }
+                    (ty::Param(p), _) | (_, ty::Param(p)) => {
+                        let generics = self.generics_of(body_owner_def_id);
+                        let p_span = self.def_span(generics.type_param(p, self).def_id);
+                        if !sp.contains(p_span) {
+                            db.span_label(p_span, "this type parameter");
+                        }
+                    }
                     (ty::Projection(_), _) => {
                         db.note(&format!(
                             "consider constraining the associated type `{}` to `{}` or calling a \
-                             method that returns `{}`",
-                            values.expected, values.found, values.expected,
+                             method that returns `{0}`",
+                            values.expected, values.found,
                         ));
                         if self.sess.teach(&db.get_code().unwrap()) {
                             db.help(
@@ -470,15 +480,18 @@ impl Trait for X {
                                  https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
                         );
                     }
-                    (_, ty::Projection(_)) => {
-                        db.note(&format!(
+                    (_, ty::Projection(proj_ty)) => {
+                        let msg = 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",
                         );
+                        if !self.suggest_constraint(db, &msg, body_owner_def_id, proj_ty, values) {
+                            db.help(&msg);
+                            db.note(
+                                "for more information, visit \
+                                https://doc.rust-lang.org/book/ch19-03-advanced-traits.html",
+                            );
+                        }
                     }
                     _ => {}
                 }
@@ -513,4 +526,115 @@ impl Trait for X {
             _ => {}
         }
     }
+
+    fn suggest_constraint(
+        &self,
+        db: &mut DiagnosticBuilder<'_>,
+        msg: &str,
+        body_owner_def_id: DefId,
+        proj_ty: &ty::ProjectionTy<'tcx>,
+        values: &ExpectedFound<Ty<'tcx>>,
+    ) -> bool {
+        let assoc = self.associated_item(proj_ty.item_def_id);
+        let trait_ref = proj_ty.trait_ref(*self);
+        if let Some(item) = self.hir().get_if_local(body_owner_def_id) {
+            if let Some(hir_generics) = item.generics() {
+                // Get the `DefId` for the type parameter corresponding to `A` in `<A as T>::Foo`.
+                // This will also work for `impl Trait`.
+                let def_id = if let ty::Param(param_ty) = proj_ty.self_ty().kind {
+                    let generics = self.generics_of(body_owner_def_id);
+                    generics.type_param(&param_ty, *self).def_id
+                } else {
+                    return false;
+                };
+
+                // First look in the `where` clause, as this might be
+                // `fn foo<T>(x: T) where T: Trait`.
+                for predicate in hir_generics.where_clause.predicates {
+                    if let hir::WherePredicate::BoundPredicate(pred) = predicate {
+                        if let hir::TyKind::Path(hir::QPath::Resolved(None, path)) =
+                            pred.bounded_ty.kind
+                        {
+                            if path.res.opt_def_id() == Some(def_id) {
+                                // This predicate is binding type param `A` in `<A as T>::Foo` to
+                                // something, potentially `T`.
+                            } else {
+                                continue;
+                            }
+                        } else {
+                            continue;
+                        }
+
+                        if self.constrain_associated_type_structured_suggestion(
+                            db,
+                            &trait_ref,
+                            pred.bounds,
+                            &assoc,
+                            values,
+                            msg,
+                        ) {
+                            return true;
+                        }
+                    }
+                }
+                for param in hir_generics.params {
+                    if self.hir().opt_local_def_id(param.hir_id).map(|id| id.to_def_id())
+                        == Some(def_id)
+                    {
+                        // This is type param `A` in `<A as T>::Foo`.
+                        return self.constrain_associated_type_structured_suggestion(
+                            db,
+                            &trait_ref,
+                            param.bounds,
+                            &assoc,
+                            values,
+                            msg,
+                        );
+                    }
+                }
+            }
+        }
+        false
+    }
+
+    fn constrain_associated_type_structured_suggestion(
+        &self,
+        db: &mut DiagnosticBuilder<'_>,
+        trait_ref: &ty::TraitRef<'tcx>,
+        bounds: hir::GenericBounds<'_>,
+        assoc: &ty::AssocItem,
+        values: &ExpectedFound<Ty<'tcx>>,
+        msg: &str,
+    ) -> bool {
+        for bound in bounds {
+            match bound {
+                hir::GenericBound::Trait(ptr, hir::TraitBoundModifier::None) => {
+                    // Relate the type param against `T` in `<A as T>::Foo`.
+                    if ptr.trait_ref.trait_def_id() == Some(trait_ref.def_id) {
+                        if let Ok(has_params) = self
+                            .sess
+                            .source_map()
+                            .span_to_snippet(ptr.span)
+                            .map(|snippet| snippet.ends_with('>'))
+                        {
+                            let (span, sugg) = if has_params {
+                                let pos = ptr.span.hi() - BytePos(1);
+                                let span = Span::new(pos, pos, ptr.span.ctxt());
+                                (span, format!(", {} = {}", assoc.ident, values.expected))
+                            } else {
+                                (
+                                    ptr.span.shrink_to_hi(),
+                                    format!("<{} = {}>", assoc.ident, values.expected),
+                                )
+                            };
+                            db.span_suggestion(span, msg, sugg, Applicability::MaybeIncorrect);
+                            return true;
+                        }
+                    }
+                }
+                _ => {}
+            }
+        }
+        false
+    }
 }
diff --git a/src/test/ui/associated-types/associated-types-eq-3.stderr b/src/test/ui/associated-types/associated-types-eq-3.stderr
index a8608abb4d9..dffa4780a09 100644
--- a/src/test/ui/associated-types/associated-types-eq-3.stderr
+++ b/src/test/ui/associated-types/associated-types-eq-3.stderr
@@ -8,8 +8,10 @@ LL |     let _: Bar = x.boo();
    |
    = note:       expected struct `Bar`
            found associated 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
+help: consider constraining the associated type `<I as Foo>::A` to `Bar`
+   |
+LL | fn foo2<I: Foo<A = Bar>>(x: I) {
+   |               ^^^^^^^^^
 
 error[E0271]: type mismatch resolving `<isize as Foo>::A == Bar`
   --> $DIR/associated-types-eq-3.rs:38:5
diff --git a/src/test/ui/associated-types/associated-types-issue-20346.stderr b/src/test/ui/associated-types/associated-types-issue-20346.stderr
index 8f2b760840c..db35c1af171 100644
--- a/src/test/ui/associated-types/associated-types-issue-20346.stderr
+++ b/src/test/ui/associated-types/associated-types-issue-20346.stderr
@@ -12,8 +12,6 @@ LL |     is_iterator_of::<Option<T>, _>(&adapter);
    |
    = note: expected enum `std::option::Option<T>`
               found type `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
 
diff --git a/src/test/ui/associated-types/associated-types-multiple-types-one-trait.stderr b/src/test/ui/associated-types/associated-types-multiple-types-one-trait.stderr
index 4e481411b4d..b8f20d00ff8 100644
--- a/src/test/ui/associated-types/associated-types-multiple-types-one-trait.stderr
+++ b/src/test/ui/associated-types/associated-types-multiple-types-one-trait.stderr
@@ -9,8 +9,10 @@ LL | fn want_y<T:Foo<Y=i32>>(t: &T) { }
    |
    = note:         expected type `i32`
            found associated type `<T as Foo>::Y`
-   = note: consider constraining the associated type `<T as Foo>::Y` to `i32`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<T as Foo>::Y` to `i32`
+   |
+LL | fn have_x_want_y<T:Foo<X=u32, Y = i32>>(t: &T)
+   |                             ^^^^^^^^^
 
 error[E0271]: type mismatch resolving `<T as Foo>::X == u32`
   --> $DIR/associated-types-multiple-types-one-trait.rs:18:5
@@ -23,8 +25,10 @@ LL | fn want_x<T:Foo<X=u32>>(t: &T) { }
    |
    = note:         expected type `u32`
            found associated type `<T as Foo>::X`
-   = note: consider constraining the associated type `<T as Foo>::X` to `u32`
-   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+help: consider constraining the associated type `<T as Foo>::X` to `u32`
+   |
+LL | fn have_y_want_x<T:Foo<Y=i32, X = u32>>(t: &T)
+   |                             ^^^^^^^^^
 
 error: aborting due to 2 previous errors
 
diff --git a/src/test/ui/generic-associated-types/iterable.stderr b/src/test/ui/generic-associated-types/iterable.stderr
index b1298163aab..4a839ac4bcb 100644
--- a/src/test/ui/generic-associated-types/iterable.stderr
+++ b/src/test/ui/generic-associated-types/iterable.stderr
@@ -6,7 +6,7 @@ LL |     type Item<'a> where T: 'a = <std::slice::Iter<'a, T> as Iterator>::Item
    |
    = note:    expected reference `&T`
            found associated type `<std::vec::Vec<T> as Iterable>::Item<'_>`
-   = note: consider constraining the associated type `<std::vec::Vec<T> as Iterable>::Item<'_>` to `&_`
+   = help: consider constraining the associated type `<std::vec::Vec<T> as Iterable>::Item<'_>` to `&_`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0271]: type mismatch resolving `for<'a> <<[T] as Iterable>::Iter<'a> as std::iter::Iterator>::Item == <[T] as Iterable>::Item<'a>`
@@ -17,7 +17,7 @@ LL |     type Item<'a> where T: 'a = <std::slice::Iter<'a, T> as Iterator>::Item
    |
    = note:    expected reference `&T`
            found associated type `<[T] as Iterable>::Item<'_>`
-   = note: consider constraining the associated type `<[T] as Iterable>::Item<'_>` to `&_`
+   = help: consider constraining the associated type `<[T] as Iterable>::Item<'_>` to `&_`
    = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
 
 error[E0271]: type mismatch resolving `for<'a> <<std::vec::Vec<T> as Iterable>::Iter<'a> as std::iter::Iterator>::Item == <std::vec::Vec<T> as Iterable>::Item<'a>`
diff --git a/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr b/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr
index 759c7302d13..1c7bfa65d7c 100644
--- a/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr
+++ b/src/test/ui/hrtb/issue-62203-hrtb-ice.stderr
@@ -6,7 +6,7 @@ LL |     let v = Unit2.m(
    |
    = note:       expected struct `Unit4`
            found associated type `<_ as Ty<'_>>::V`
-   = note: consider constraining the associated type `<_ as Ty<'_>>::V` to `Unit4`
+   = help: 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`
diff --git a/src/test/ui/impl-trait/bound-normalization-fail.stderr b/src/test/ui/impl-trait/bound-normalization-fail.stderr
index 314ed96fd5e..f5092044627 100644
--- a/src/test/ui/impl-trait/bound-normalization-fail.stderr
+++ b/src/test/ui/impl-trait/bound-normalization-fail.stderr
@@ -14,9 +14,11 @@ LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
    |
    = note:         expected type `()`
            found associated 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
+help: consider constraining the associated type `<T as impl_trait::Trait>::Assoc` to `()`
+   |
+LL |     fn foo_fail<T: Trait<Assoc = ()>>() -> impl FooLike<Output=T::Assoc> {
+   |                         ^^^^^^^^^^^^
 
 error: `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
   --> $DIR/bound-normalization-fail.rs:43:41
@@ -32,9 +34,11 @@ LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
    |
    = note:         expected type `()`
            found associated 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
+help: consider constraining the associated type `<T as lifetimes::Trait<'static>>::Assoc` to `()`
+   |
+LL |     fn foo2_fail<'a, T: Trait<'a, Assoc = ()>>() -> impl FooLike<Output=T::Assoc> {
+   |                                 ^^^^^^^^^^^^
 
 error: aborting due to 3 previous errors; 1 warning emitted
 
diff --git a/src/test/ui/impl-trait/equality2.stderr b/src/test/ui/impl-trait/equality2.stderr
index b882514f616..2454c218ffc 100644
--- a/src/test/ui/impl-trait/equality2.stderr
+++ b/src/test/ui/impl-trait/equality2.stderr
@@ -25,7 +25,7 @@ LL |     let _: i32 = Leak::leak(hide(0_i32));
    |
    = note:         expected type `i32`
            found associated type `<impl Foo as Leak>::T`
-   = note: consider constraining the associated type `<impl Foo as Leak>::T` to `i32`
+   = help: 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
diff --git a/src/test/ui/impl-trait/universal-mismatched-type.stderr b/src/test/ui/impl-trait/universal-mismatched-type.stderr
index 3ffa2b55712..a12b01b4d2b 100644
--- a/src/test/ui/impl-trait/universal-mismatched-type.stderr
+++ b/src/test/ui/impl-trait/universal-mismatched-type.stderr
@@ -10,8 +10,6 @@ LL |     x
    |
    = note:      expected struct `std::string::String`
            found type parameter `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
 
diff --git a/src/test/ui/issues/issue-13853.stderr b/src/test/ui/issues/issue-13853.stderr
index 2f31636f8ad..3f1b955dddb 100644
--- a/src/test/ui/issues/issue-13853.stderr
+++ b/src/test/ui/issues/issue-13853.stderr
@@ -9,8 +9,6 @@ LL |         self.iter()
    |
    = note: expected type parameter `I`
                       found struct `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 reference `&G` in the current scope
   --> $DIR/issue-13853.rs:27:23
diff --git a/src/test/ui/issues/issue-20225.stderr b/src/test/ui/issues/issue-20225.stderr
index 1c5911e05f7..3bcc50ded84 100644
--- a/src/test/ui/issues/issue-20225.stderr
+++ b/src/test/ui/issues/issue-20225.stderr
@@ -8,8 +8,6 @@ LL |   extern "rust-call" fn call(&self, (_,): (T,)) {}
    |
    = note: expected fn pointer `extern "rust-call" fn(&Foo, (&'a T,))`
               found fn pointer `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:11:3
@@ -21,8 +19,6 @@ LL |   extern "rust-call" fn call_mut(&mut self, (_,): (T,)) {}
    |
    = note: expected fn pointer `extern "rust-call" fn(&mut Foo, (&'a T,))`
               found fn pointer `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:18:3
@@ -35,8 +31,6 @@ LL |   extern "rust-call" fn call_once(self, (_,): (T,)) {}
    |
    = note: expected fn pointer `extern "rust-call" fn(Foo, (&'a T,))`
               found fn pointer `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
 
diff --git a/src/test/ui/issues/issue-69306.stderr b/src/test/ui/issues/issue-69306.stderr
index a2a42739ca8..58e85ec700d 100644
--- a/src/test/ui/issues/issue-69306.stderr
+++ b/src/test/ui/issues/issue-69306.stderr
@@ -8,8 +8,6 @@ LL |     const C: S0<u8> = Self(0);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:5:23
@@ -21,8 +19,6 @@ LL |     const C: S0<u8> = Self(0);
    |
    = note: expected struct `S0<u8>`
               found struct `S0<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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:10:14
@@ -35,8 +31,6 @@ LL |         Self(0);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:27:14
@@ -49,8 +43,6 @@ LL |         Self(0);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:33:32
@@ -62,8 +54,6 @@ LL |     const C: S1<u8, u8> = Self(0, 1);
    |
    = note: expected type parameter `T`
                         found type `{integer}`
-   = 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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:33:27
@@ -75,8 +65,6 @@ LL |     const C: S1<u8, u8> = Self(0, 1);
    |
    = note: expected struct `S1<u8, _>`
               found struct `S1<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[E0308]: mismatched types
   --> $DIR/issue-69306.rs:41:14
diff --git a/src/test/ui/mismatched_types/issue-35030.stderr b/src/test/ui/mismatched_types/issue-35030.stderr
index 6fb04ef5c99..9f4e4398984 100644
--- a/src/test/ui/mismatched_types/issue-35030.stderr
+++ b/src/test/ui/mismatched_types/issue-35030.stderr
@@ -9,8 +9,6 @@ LL |         Some(true)
    |
    = note: expected type parameter `bool` (type parameter `bool`)
                         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
 
diff --git a/src/test/ui/specialization/specialization-default-projection.stderr b/src/test/ui/specialization/specialization-default-projection.stderr
index d03aec7ab30..4344fe23bbc 100644
--- a/src/test/ui/specialization/specialization-default-projection.stderr
+++ b/src/test/ui/specialization/specialization-default-projection.stderr
@@ -25,7 +25,7 @@ LL |     generic::<()>()
    |
    = note:    expected unit type `()`
            found associated type `<() as Foo>::Assoc`
-   = note: consider constraining the associated type `<() as Foo>::Assoc` to `()`
+   = help: 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
diff --git a/src/test/ui/specialization/specialization-default-types.stderr b/src/test/ui/specialization/specialization-default-types.stderr
index 257c114252c..d66aaa96274 100644
--- a/src/test/ui/specialization/specialization-default-types.stderr
+++ b/src/test/ui/specialization/specialization-default-types.stderr
@@ -21,7 +21,7 @@ LL |     Example::generate(t)
    |
    = note:       expected struct `std::boxed::Box<T>`
            found associated type `<T as Example>::Output`
-   = note: consider constraining the associated type `<T as Example>::Output` to `std::boxed::Box<T>`
+   = help: 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
diff --git a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
index ff256eb3094..0e68e81d7ab 100644
--- a/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
+++ b/src/test/ui/suggestions/expected-boxed-future-isnt-pinned.stderr
@@ -12,8 +12,6 @@ LL |     x
    |
    = note:      expected struct `std::pin::Pin<std::boxed::Box<(dyn std::future::Future<Output = i32> + std::marker::Send + 'static)>>`
            found type parameter `F`
-   = 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[E0308]: mismatched types
   --> $DIR/expected-boxed-future-isnt-pinned.rs:18:5
diff --git a/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.fixed b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.fixed
new file mode 100644
index 00000000000..8ef7e34ab30
--- /dev/null
+++ b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.fixed
@@ -0,0 +1,43 @@
+// run-rustfix
+#![allow(unused)] // for the fixed file
+
+trait Trait<T = Self> {
+    type A;
+
+    fn func(&self) -> Self::A;
+}
+
+struct S<T>(T);
+impl<K> S<K> {
+    fn foo<'a, T: Trait + 'a>(&self, _: impl Trait, x: impl Trait<A = usize>, _: T) {
+        qux(x.func()) //~ ERROR mismatched types
+    }
+
+    fn ban<T>(x: T) where T: Trait<A = usize> {
+        qux(x.func()) //~ ERROR mismatched types
+    }
+}
+
+fn foo<'a, T: Trait + 'a>(_: impl Trait, x: impl Trait<A = usize>, _: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bar<T: Trait<A = usize>>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn foo2(x: impl Trait<i32, A = usize>) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bar2<T: Trait<i32, A = usize>>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn ban<T>(x: T) where T: Trait<A = usize> {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn qux(_: usize) {}
+
+fn main() {}
diff --git a/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.rs b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.rs
new file mode 100644
index 00000000000..7bd38d0d45d
--- /dev/null
+++ b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.rs
@@ -0,0 +1,43 @@
+// run-rustfix
+#![allow(unused)] // for the fixed file
+
+trait Trait<T = Self> {
+    type A;
+
+    fn func(&self) -> Self::A;
+}
+
+struct S<T>(T);
+impl<K> S<K> {
+    fn foo<'a, T: Trait + 'a>(&self, _: impl Trait, x: impl Trait, _: T) {
+        qux(x.func()) //~ ERROR mismatched types
+    }
+
+    fn ban<T>(x: T) where T: Trait {
+        qux(x.func()) //~ ERROR mismatched types
+    }
+}
+
+fn foo<'a, T: Trait + 'a>(_: impl Trait, x: impl Trait, _: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bar<T: Trait>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn foo2(x: impl Trait<i32>) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bar2<T: Trait<i32>>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn ban<T>(x: T) where T: Trait {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn qux(_: usize) {}
+
+fn main() {}
diff --git a/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.stderr b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.stderr
new file mode 100644
index 00000000000..f785f7b84a7
--- /dev/null
+++ b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction-fixable.stderr
@@ -0,0 +1,94 @@
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:13:13
+   |
+LL |         qux(x.func())
+   |             ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<impl Trait as Trait>::A`
+help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
+   |
+LL |     fn foo<'a, T: Trait + 'a>(&self, _: impl Trait, x: impl Trait<A = usize>, _: T) {
+   |                                                                  ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:17:13
+   |
+LL |         qux(x.func())
+   |             ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait>::A`
+help: consider constraining the associated type `<T as Trait>::A` to `usize`
+   |
+LL |     fn ban<T>(x: T) where T: Trait<A = usize> {
+   |                                   ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:22:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<impl Trait as Trait>::A`
+help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
+   |
+LL | fn foo<'a, T: Trait + 'a>(_: impl Trait, x: impl Trait<A = usize>, _: T) {
+   |                                                       ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:26:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait>::A`
+help: consider constraining the associated type `<T as Trait>::A` to `usize`
+   |
+LL | fn bar<T: Trait<A = usize>>(x: T) {
+   |                ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:30:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<impl Trait<i32> as Trait<i32>>::A`
+help: consider constraining the associated type `<impl Trait<i32> as Trait<i32>>::A` to `usize`
+   |
+LL | fn foo2(x: impl Trait<i32, A = usize>) {
+   |                          ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:34:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait<i32>>::A`
+help: consider constraining the associated type `<T as Trait<i32>>::A` to `usize`
+   |
+LL | fn bar2<T: Trait<i32, A = usize>>(x: T) {
+   |                     ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction-fixable.rs:38:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait>::A`
+help: consider constraining the associated type `<T as Trait>::A` to `usize`
+   |
+LL | fn ban<T>(x: T) where T: Trait<A = usize> {
+   |                               ^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/suggestions/trait-with-missing-associated-type-restriction.rs b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction.rs
new file mode 100644
index 00000000000..1166785d739
--- /dev/null
+++ b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction.rs
@@ -0,0 +1,41 @@
+// These are all the possible variations of this error I could think of for.
+// `trait-with-missing-associated-type-restriction-fixable.rs` contains the subset of these that
+// can be fixed with `rustfix`.
+
+trait Trait<T = Self> {
+    type A;
+
+    fn func(&self) -> Self::A;
+}
+
+fn foo(_: impl Trait, x: impl Trait) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bar<T: Trait>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn foo2(x: impl Trait<i32>) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bar2<T: Trait<i32>>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn baz<D: std::fmt::Debug, T: Trait<A = D>>(x: T) {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn bat(x: &mut dyn Trait<(), A = ()>) {
+    qux(x) //~ ERROR mismatched types
+}
+
+fn ban<T>(x: T) where T: Trait {
+    qux(x.func()) //~ ERROR mismatched types
+}
+
+fn qux(_: usize) {}
+
+fn main() {}
diff --git a/src/test/ui/suggestions/trait-with-missing-associated-type-restriction.stderr b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction.stderr
new file mode 100644
index 00000000000..08788db6ec9
--- /dev/null
+++ b/src/test/ui/suggestions/trait-with-missing-associated-type-restriction.stderr
@@ -0,0 +1,88 @@
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:12:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<impl Trait as Trait>::A`
+help: consider constraining the associated type `<impl Trait as Trait>::A` to `usize`
+   |
+LL | fn foo(_: impl Trait, x: impl Trait<A = usize>) {
+   |                                    ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:16:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait>::A`
+help: consider constraining the associated type `<T as Trait>::A` to `usize`
+   |
+LL | fn bar<T: Trait<A = usize>>(x: T) {
+   |                ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:20:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<impl Trait<i32> as Trait<i32>>::A`
+help: consider constraining the associated type `<impl Trait<i32> as Trait<i32>>::A` to `usize`
+   |
+LL | fn foo2(x: impl Trait<i32, A = usize>) {
+   |                          ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:24:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait<i32>>::A`
+help: consider constraining the associated type `<T as Trait<i32>>::A` to `usize`
+   |
+LL | fn bar2<T: Trait<i32, A = usize>>(x: T) {
+   |                     ^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:28:9
+   |
+LL | fn baz<D: std::fmt::Debug, T: Trait<A = D>>(x: T) {
+   |        - this type parameter
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found type parameter `D`
+   |
+   = note:        expected type `usize`
+           found type parameter `D`
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:32:9
+   |
+LL |     qux(x)
+   |         ^ expected `usize`, found mutable reference
+   |
+   = note:           expected type `usize`
+           found mutable reference `&mut dyn Trait<(), A = ()>`
+
+error[E0308]: mismatched types
+  --> $DIR/trait-with-missing-associated-type-restriction.rs:36:9
+   |
+LL |     qux(x.func())
+   |         ^^^^^^^^ expected `usize`, found associated type
+   |
+   = note:         expected type `usize`
+           found associated type `<T as Trait>::A`
+help: consider constraining the associated type `<T as Trait>::A` to `usize`
+   |
+LL | fn ban<T>(x: T) where T: Trait<A = usize> {
+   |                               ^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0308`.
diff --git a/src/test/ui/type-alias-enum-variants/enum-variant-generic-args.stderr b/src/test/ui/type-alias-enum-variants/enum-variant-generic-args.stderr
index 412b4dbda4f..caea791e653 100644
--- a/src/test/ui/type-alias-enum-variants/enum-variant-generic-args.stderr
+++ b/src/test/ui/type-alias-enum-variants/enum-variant-generic-args.stderr
@@ -9,8 +9,6 @@ LL |         Self::TSVariant(());
    |
    = note: expected type parameter `T`
                    found unit type `()`
-   = 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[E0109]: type arguments are not allowed for this type
   --> $DIR/enum-variant-generic-args.rs:15:27
@@ -35,8 +33,6 @@ LL |         Self::<()>::TSVariant(());
    |
    = note: expected type parameter `T`
                    found unit type `()`
-   = 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[E0109]: type arguments are not allowed for this type
   --> $DIR/enum-variant-generic-args.rs:20:16
@@ -61,8 +57,6 @@ LL |         Self::SVariant { v: () };
    |
    = note: expected type parameter `T`
                    found unit type `()`
-   = 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[E0109]: type arguments are not allowed for this type
   --> $DIR/enum-variant-generic-args.rs:28:26
@@ -81,8 +75,6 @@ LL |         Self::SVariant::<()> { v: () };
    |
    = note: expected type parameter `T`
                    found unit type `()`
-   = 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[E0109]: type arguments are not allowed for this type
   --> $DIR/enum-variant-generic-args.rs:31:16
@@ -101,8 +93,6 @@ LL |         Self::<()>::SVariant { v: () };
    |
    = note: expected type parameter `T`
                    found unit type `()`
-   = 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[E0109]: type arguments are not allowed for this type
   --> $DIR/enum-variant-generic-args.rs:34:16
@@ -127,8 +117,6 @@ LL |         Self::<()>::SVariant::<()> { v: () };
    |
    = note: expected type parameter `T`
                    found unit type `()`
-   = 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[E0109]: type arguments are not allowed for this type
   --> $DIR/enum-variant-generic-args.rs:41:26