Rollup merge of #74712 - strom-und-spiele:E0271-cleanup, r=Mark-Simulacrum

Update E0271 explanation

When reading the explanation, I got confused by it. I hope the wording is clearer now.

src/librustc_error_codes/error_codes/E0271.md

@@ -5,25 +5,6 @@ Erroneous code example:
 ```compile_fail,E0271
 trait Trait { type AssociatedType; }
 
-fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
-    println!("in foo");
-}
-
-impl Trait for i8 { type AssociatedType = &'static str; }
-
-foo(3_i8);
-```
-
-This is because of a type mismatch between the associated type of some
-trait (e.g., `T::Bar`, where `T` implements `trait Quux { type Bar; }`)
-and another type `U` that is required to be equal to `T::Bar`, but is not.
-Examples follow.
-
-Here is that same example again, with some explanatory comments:
-
-```compile_fail,E0271
-trait Trait { type AssociatedType; }
-
 fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
 //                    ~~~~~~~~ ~~~~~~~~~~~~~~~~~~
 //                        |            |
@@ -56,11 +37,9 @@ foo(3_i8);
 // therefore the type-checker complains with this error code.
 ```
 
-To avoid those issues, you have to make the types match correctly.
-So we can fix the previous examples like this:
-
+The issue can be resolved by changing the associated type:
+1) in the `foo` implementation:
 ```
-// Basic Example:
 trait Trait { type AssociatedType; }
 
 fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
@@ -70,13 +49,17 @@ fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
 impl Trait for i8 { type AssociatedType = &'static str; }
 
 foo(3_i8);
-
-// For-Loop Example:
-let vs = vec![1, 2, 3, 4];
-for v in &vs {
-    match v {
-        &1 => {}
-        _ => {}
-    }
-}
+```
+
+2) in the `Trait` implementation for `i8`:
+```
+trait Trait { type AssociatedType; }
+
+fn foo<T>(t: T) where T: Trait<AssociatedType = u32> {
+    println!("in foo");
+}
+
+impl Trait for i8 { type AssociatedType = u32; }
+
+foo(3_i8);
 ```