Suggest correct syntax when writing type arg instead of assoc type

When confusing an associated type with a type argument, suggest the
appropriate syntax.

Given `Iterator<isize>`, suggest `Iterator<Item = isize>`.

2 files changed, 39 insertions, 0 deletions

diff --git a/src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.rs b/src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.rs
new file mode 100644
index 00000000000..58e7718ba5b
--- /dev/null
+++ b/src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.rs
@@ -0,0 +1,8 @@
+pub trait T<X, Y> {
+    type A;
+    type B;
+    type C;
+}
+ pub struct Foo { i: Box<T<usize, usize, usize, usize, B=usize>> }
+
+ fn main() {}
diff --git a/src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.stderr b/src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.stderr
new file mode 100644
index 00000000000..053f7010421
--- /dev/null
+++ b/src/test/ui/suggestions/use-type-argument-instead-of-assoc-type.stderr
@@ -0,0 +1,31 @@
+error[E0107]: wrong number of type arguments: expected 2, found 4
+  --> $DIR/use-type-argument-instead-of-assoc-type.rs:6:42
+   |
+LL |  pub struct Foo { i: Box<T<usize, usize, usize, usize, B=usize>> }
+   |                                          ^^^^^  ^^^^^ unexpected type argument
+   |                                          |
+   |                                          unexpected type argument
+
+error[E0191]: the value of the associated types `A` (from the trait `T`), `C` (from the trait `T`) must be specified
+  --> $DIR/use-type-argument-instead-of-assoc-type.rs:6:26
+   |
+LL |     type A;
+   |     ------- `A` defined here
+LL |     type B;
+LL |     type C;
+   |     ------- `C` defined here
+LL | }
+LL |  pub struct Foo { i: Box<T<usize, usize, usize, usize, B=usize>> }
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                          |
+   |                          missing associated type `A` value
+   |                          missing associated type `C` value
+help: if you meant to assign the missing associated type, use the name
+   |
+LL |  pub struct Foo { i: Box<T<usize, usize, A = usize, C = usize, B=usize>> }
+   |                                          ^^^^^^^^^  ^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0107, E0191.
+For more information about an error, try `rustc --explain E0107`.