Auto merge of rust-lang#34364 - cynicaldevil:develop, r=GuillaumeGomez

Modified E0220 to show error messages for more general cases

This PR extends `E0220`'s description to explain more cases.
Refer to [rust-lang#34342](rust-lang#34342) for more.
r? @GuillaumeGomez

Author: bors

src/librustc_typeck/diagnostics.rs

@@ -2787,23 +2787,42 @@ You used an associated type which isn't defined in the trait.
 Erroneous code example:
 
 ```compile_fail
-trait Trait {
+trait T1 {
     type Bar;
 }
 
-type Foo = Trait<F=i32>; // error: associated type `F` not found for
-                         //        `Trait`
+type Foo = T1<F=i32>; // error: associated type `F` not found for `T1`
+
+// or:
+
+trait T2 {
+    type Bar;
+
+    // error: Baz is used but not declared
+    fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
+}
 ```
 
-Please verify you used the right trait or you didn't misspell the
+Make sure that you have defined the associated type in the trait body.
+Also, verify that you used the right trait or you didn't misspell the
 associated type name. Example:
 
 ```
-trait Trait {
+trait T1 {
     type Bar;
 }
 
-type Foo = Trait<Bar=i32>; // ok!
+type Foo = T1<Bar=i32>; // ok!
+
+// or:
+
+trait T2 {
+    type Bar;
+    type Baz; // we declare `Baz` in our trait.
+
+    // and now we can use it here:
+    fn return_bool(&self, &Self::Bar, &Self::Baz) -> bool;
+}
 ```
 "##,