Rollup merge of #75291 - GuillaumeGomez:cleanup-e0750, r=pickfire

Clean up E0750

r? @Dylan-DPC

src/librustc_error_codes/error_codes/E0750.md

@@ -1,4 +1,18 @@
-Negative impls cannot be default impls. A default impl supplies
+A negative impl was made default impl.
-default values for the items within to be used by other impls, whereas
+
-a negative impl declares that there are no other impls. These don't
+Erroneous code example:
-make sense to combine.
+
+```compile_fail,E0750
+# #![feature(negative_impls)]
+# #![feature(specialization)]
+trait MyTrait {
+    type Foo;
+}
+
+default impl !MyTrait for u32 {} // error!
+# fn main() {}
+```
+
+Negative impls cannot be default impls. A default impl supplies default values
+for the items within to be used by other impls, whereas a negative impl declares
+that there are no other impls. Combining it does not make sense.

src/tools/tidy/src/error_codes_check.rs

@@ -16,8 +16,7 @@ const EXEMPTED_FROM_TEST: &[&str] = &[
 ];
 
 // Some error codes don't have any tests apparently...
-const IGNORE_EXPLANATION_CHECK: &[&str] =
+const IGNORE_EXPLANATION_CHECK: &[&str] = &["E0570", "E0601", "E0602", "E0639", "E0729", "E0749"];
-    &["E0570", "E0601", "E0602", "E0639", "E0729", "E0749", "E0750"];
 
 fn check_error_code_explanation(
     f: &str,