Deny clippy:;four_forward_slashes and fix the only occurrence

Author: joshtriplett

compiler/rustc_resolve/src/rustdoc.rs

@@ -347,7 +347,7 @@ pub fn strip_generics_from_path(path_str: &str) -> Result<Box<str>, MalformedGen
 
 /// Returns whether the first doc-comment is an inner attribute.
 ///
-//// If there are no doc-comments, return true.
+/// If there are no doc-comments, return true.
 /// FIXME(#78591): Support both inner and outer attributes on the same item.
 pub fn inner_docs(attrs: &[impl AttributeExt]) -> bool {
     attrs

src/bootstrap/src/core/build_steps/clippy.rs

@@ -395,6 +395,7 @@ impl Step for CI {
                 "clippy::correctness".into(),
                 "clippy::clone_on_ref_ptr".into(),
                 "clippy::format_in_format_args".into(),
+                "clippy::four_forward_slashes".into(),
             ],
             forbid: vec![],
         };