Add long error explanation for E0271

Author: GuillaumeGomez

Diff for: src/librustc/diagnostics.rs

@@ -329,6 +329,57 @@ loop. Without a loop to break out of or continue in, no sensible action can be
 taken.
 "##,
 
+E0271: r##"
+This is because of a type mismatch, that happens to involve a constraint that
+forces the associated type of some particular trait to be equivalent to some
+other type. Examples:
+
+```
+// example 1:
+let vs = vec![1, 2, 3, 4];
+
+for v in &vs {
+    match v {
+        1 => {}
+        _ => {}
+    }
+}
+
+//example 2:
+trait Trait { type AssociatedType; }
+impl Trait for i8 { type AssociatedType = &'static str; }
+fn foo<T>(t: T) where T: Trait<AssociatedType=u32> {
+    println!("in foo");
+}
+
+foo(3i8);
+```
+
+To avoid those issues, you have to make the types match correctly. So we can
+fix the previous examples like this:
+
+```
+// example 1:
+let vs = vec![1, 2, 3, 4];
+
+for v in &vs {
+    match v {
+        &1 => {}
+        _ => {}
+    }
+}
+
+//example 2:
+trait Trait { type AssociatedType; }
+impl Trait for i8 { type AssociatedType = &'static str; }
+fn foo<T>(t: T) where T: Trait<AssociatedType = &'static str> {
+    println!("in foo");
+}
+
+foo(3i8);
+```
+"##,
+
 E0296: r##"
 This error indicates that the given recursion limit could not be parsed. Ensure
 that the value provided is a positive integer between quotes, like so: