Auto merge of #67268 - estebank:assoc-types, r=oli-obk

Tweak errors for missing associated types and type parameters

* On `dyn Trait` missing associated types, provide a structured suggestion for them
* On missing type parameters, provide structured suggestion for them
* Point at trait definition when missing required type parameter
* Tweak output of E0658
* Tweak wording of E0719
* Account for `Trait1 + Trait2` case

Fix #66380, fix #60595. CC #63711.

1 files changed, 51 insertions, 0 deletions

diff --git a/src/librustc_error_codes/error_codes/E0222.md b/src/librustc_error_codes/error_codes/E0222.md
new file mode 100644
index 00000000000..66b6c4d712b
--- /dev/null
+++ b/src/librustc_error_codes/error_codes/E0222.md
@@ -0,0 +1,51 @@
+An attempt was made to constrain an associated type.
+For example:
+
+```compile_fail,E0222
+pub trait Vehicle {
+    type Color;
+}
+
+pub trait Box {
+    type Color;
+}
+
+pub trait BoxCar : Box + Vehicle {}
+
+fn dent_object<COLOR>(c: dyn BoxCar<Color=COLOR>) {} // Invalid constraint
+```
+
+In this example, `BoxCar` has two super-traits: `Vehicle` and `Box`. Both of
+these traits define an associated type `Color`. `BoxCar` inherits two types
+with that name from both super-traits. Because of this, we need to use the
+fully qualified path syntax to refer to the appropriate `Color` associated
+type, either `<BoxCar as Vehicle>::Color` or `<BoxCar as Box>::Color`, but this
+syntax is not allowed to be used in a function signature.
+
+In order to encode this kind of constraint, a `where` clause and a new type
+parameter are needed:
+
+```
+pub trait Vehicle {
+    type Color;
+}
+
+pub trait Box {
+    type Color;
+}
+
+pub trait BoxCar : Box + Vehicle {}
+
+// Introduce a new `CAR` type parameter
+fn foo<CAR, COLOR>(
+    c: CAR,
+) where
+    // Bind the type parameter `CAR` to the trait `BoxCar`
+    CAR: BoxCar,
+    // Further restrict `<BoxCar as Vehicle>::Color` to be the same as the
+    // type parameter `COLOR`
+    CAR: Vehicle<Color = COLOR>,
+    // We can also simultaneously restrict the other trait's associated type
+    CAR: Box<Color = COLOR>
+{}
+```