Add test to ensure that the missing_doc_code_examples is not triggered on foreign trait implementations

Author: GuillaumeGomez

src/test/rustdoc-ui/doc-without-codeblock.rs

@@ -11,3 +11,12 @@ pub mod foo {
     //~^ ERROR missing code example in this documentation
     pub fn bar() {}
 }
+
+// This impl is here to ensure the lint isn't emitted for foreign traits implementations.
+impl std::ops::Neg for Foo {
+    type Output = Self;
+
+    fn neg(self) -> Self::Output {
+        Self
+    }
+}

src/test/rustdoc-ui/doc-without-codeblock.stderr

@@ -6,7 +6,7 @@ LL | |
 LL | | /// Some docs.
 LL | |
 ...  |
-LL | |     pub fn bar() {}
+LL | |     }
 LL | | }
    | |_^
    |