Suggest restricting type param when it doesn't satisfy projection

When encountering a projection that isn't satisfied by a type parameter,
suggest constraining the type parameter.

5 files changed, 129 insertions, 3 deletions

diff --git a/src/test/ui/generic-associated-types/construct_with_other_type.stderr b/src/test/ui/generic-associated-types/construct_with_other_type.stderr
index bad746f7ef1..b9468b3330b 100644
--- a/src/test/ui/generic-associated-types/construct_with_other_type.stderr
+++ b/src/test/ui/generic-associated-types/construct_with_other_type.stderr
@@ -2,11 +2,16 @@ error[E0271]: type mismatch resolving `for<'a> <<T as Baz>::Baa<'a> as std::ops:
   --> $DIR/construct_with_other_type.rs:19:9
    |
 LL | impl<T> Baz for T where T: Foo {
-   |         ^^^ expected type parameter `T`, found associated type
+   |      -  ^^^ expected type parameter `T`, found associated type
+   |      |
+   |      this type parameter
    |
    = note: expected associated type `<T as Foo>::Bar<'_, 'static>`
               found associated type `<<T as Baz>::Quux<'_> as Foo>::Bar<'_, 'static>`
-   = note: you might be missing a type parameter or trait bound
+help: consider further restricting this bound
+   |
+LL | impl<T> Baz for T where T: Foo + Baz<Quux = T> {
+   |                                ^^^^^^^^^^^^^^^
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/generic-associated-types/missing-bounds.fixed b/src/test/ui/generic-associated-types/missing-bounds.fixed
new file mode 100644
index 00000000000..c2e619d1a7e
--- /dev/null
+++ b/src/test/ui/generic-associated-types/missing-bounds.fixed
@@ -0,0 +1,35 @@
+// run-rustfix
+
+use std::ops::Add;
+
+struct A<B>(B);
+
+impl<B> Add for A<B> where B: Add + std::ops::Add<Output = B> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        A(self.0 + rhs.0) //~ ERROR mismatched types
+    }
+}
+
+struct C<B>(B);
+
+impl<B: Add + std::ops::Add<Output = B>> Add for C<B> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0) //~ ERROR mismatched types
+    }
+}
+
+struct D<B>(B);
+
+impl<B: std::ops::Add<Output = B>> Add for D<B> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B`
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/generic-associated-types/missing-bounds.rs b/src/test/ui/generic-associated-types/missing-bounds.rs
new file mode 100644
index 00000000000..1852ef62fe6
--- /dev/null
+++ b/src/test/ui/generic-associated-types/missing-bounds.rs
@@ -0,0 +1,35 @@
+// run-rustfix
+
+use std::ops::Add;
+
+struct A<B>(B);
+
+impl<B> Add for A<B> where B: Add {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        A(self.0 + rhs.0) //~ ERROR mismatched types
+    }
+}
+
+struct C<B>(B);
+
+impl<B: Add> Add for C<B> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0) //~ ERROR mismatched types
+    }
+}
+
+struct D<B>(B);
+
+impl<B> Add for D<B> {
+    type Output = Self;
+
+    fn add(self, rhs: Self) -> Self {
+        Self(self.0 + rhs.0) //~ ERROR cannot add `B` to `B`
+    }
+}
+
+fn main() {}
diff --git a/src/test/ui/generic-associated-types/missing-bounds.stderr b/src/test/ui/generic-associated-types/missing-bounds.stderr
new file mode 100644
index 00000000000..630ceac093e
--- /dev/null
+++ b/src/test/ui/generic-associated-types/missing-bounds.stderr
@@ -0,0 +1,49 @@
+error[E0308]: mismatched types
+  --> $DIR/missing-bounds.rs:11:11
+   |
+LL | impl<B> Add for A<B> where B: Add {
+   |      - this type parameter
+...
+LL |         A(self.0 + rhs.0)
+   |           ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type
+   |
+   = note: expected type parameter `B`
+             found associated type `<B as std::ops::Add>::Output`
+help: consider further restricting this bound
+   |
+LL | impl<B> Add for A<B> where B: Add + std::ops::Add<Output = B> {
+   |                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0308]: mismatched types
+  --> $DIR/missing-bounds.rs:21:14
+   |
+LL | impl<B: Add> Add for C<B> {
+   |      - this type parameter
+...
+LL |         Self(self.0 + rhs.0)
+   |              ^^^^^^^^^^^^^^ expected type parameter `B`, found associated type
+   |
+   = note: expected type parameter `B`
+             found associated type `<B as std::ops::Add>::Output`
+help: consider further restricting this bound
+   |
+LL | impl<B: Add + std::ops::Add<Output = B>> Add for C<B> {
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0369]: cannot add `B` to `B`
+  --> $DIR/missing-bounds.rs:31:21
+   |
+LL |         Self(self.0 + rhs.0)
+   |              ------ ^ ----- B
+   |              |
+   |              B
+   |
+help: consider restricting type parameter `B`
+   |
+LL | impl<B: std::ops::Add<Output = B>> Add for D<B> {
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0308, E0369.
+For more information about an error, try `rustc --explain E0308`.
diff --git a/src/test/ui/issues/issue-24204.stderr b/src/test/ui/issues/issue-24204.stderr
index d69efc86005..d5cbcf786bf 100644
--- a/src/test/ui/issues/issue-24204.stderr
+++ b/src/test/ui/issues/issue-24204.stderr
@@ -7,7 +7,9 @@ LL |     type A: MultiDispatch<Self::B, O = Self>;
    |                                    -------- required by this bound in `Trait`
 ...
 LL | fn test<T: Trait<B=i32>>(b: i32) -> T where T::A: MultiDispatch<i32> { T::new(b) }
-   |            ^^^^^^^^^^^^ expected type parameter `T`, found associated type
+   |         -  ^^^^^^^^^^^^ expected type parameter `T`, found associated type
+   |         |
+   |         this type parameter
    |
    = note: expected type parameter `T`
              found associated type `<<T as Trait>::A as MultiDispatch<i32>>::O`