Don't trigger unused_qualifications on global paths

Author: jieyouxu

compiler/rustc_resolve/src/late.rs

@@ -4636,6 +4636,13 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
     }
 
     fn lint_unused_qualifications(&mut self, path: &[Segment], ns: Namespace, finalize: Finalize) {
+        // Don't lint on global paths because the user explicitly wrote out the full path.
+        if let Some(seg) = path.first()
+            && seg.ident.name == kw::PathRoot
+        {
+            return;
+        }
+
         if path.iter().any(|seg| seg.ident.span.from_expansion()) {
             return;
         }

tests/ui/lint/unused-qualifications-global-paths.rs

@@ -0,0 +1,12 @@
+// Checks that `unused_qualifications` don't fire on explicit global paths.
+// Issue: <https://github.com/rust-lang/rust/issues/122374>.
+
+//@ check-pass
+
+#![deny(unused_qualifications)]
+
+pub fn bar() -> u64 {
+    ::std::default::Default::default()
+}
+
+fn main() {}