1 files changed, 40 insertions, 2 deletions

diff --git a/compiler/rustc_resolve/src/check_unused.rs b/compiler/rustc_resolve/src/check_unused.rs
index b2578e4c4b4..dbf6cec788b 100644
--- a/compiler/rustc_resolve/src/check_unused.rs
+++ b/compiler/rustc_resolve/src/check_unused.rs
@@ -32,9 +32,10 @@ use rustc_ast::visit::{self, Visitor};
 use rustc_data_structures::fx::{FxHashMap, FxIndexMap};
 use rustc_data_structures::unord::UnordSet;
 use rustc_errors::{pluralize, MultiSpan};
+use rustc_hir::def::{DefKind, Res};
 use rustc_session::lint::builtin::{MACRO_USE_EXTERN_CRATE, UNUSED_EXTERN_CRATES, UNUSED_IMPORTS};
 use rustc_session::lint::BuiltinLintDiagnostics;
-use rustc_span::symbol::Ident;
+use rustc_span::symbol::{kw, Ident};
 use rustc_span::{Span, DUMMY_SP};
 
 struct UnusedImport<'a> {
@@ -58,6 +59,7 @@ struct UnusedImportCheckVisitor<'a, 'b, 'tcx> {
     base_use_tree: Option<&'a ast::UseTree>,
     base_id: ast::NodeId,
     item_span: Span,
+    base_use_is_pub: bool,
 }
 
 struct ExternCrateToLint {
@@ -110,6 +112,35 @@ impl<'a, 'b, 'tcx> UnusedImportCheckVisitor<'a, 'b, 'tcx> {
             unused: Default::default(),
         })
     }
+
+    fn check_import_as_underscore(&mut self, item: &ast::UseTree, id: ast::NodeId) {
+        match item.kind {
+            ast::UseTreeKind::Simple(Some(ident)) => {
+                if ident.name == kw::Underscore
+                    && !self
+                        .r
+                        .import_res_map
+                        .get(&id)
+                        .map(|per_ns| {
+                            per_ns.iter().filter_map(|res| res.as_ref()).any(|res| {
+                                matches!(res, Res::Def(DefKind::Trait | DefKind::TraitAlias, _))
+                            })
+                        })
+                        .unwrap_or(false)
+                {
+                    self.unused_import(self.base_id).add(id);
+                }
+            }
+            ast::UseTreeKind::Nested(ref items) => self.check_imports_as_underscore(items),
+            _ => {}
+        }
+    }
+
+    fn check_imports_as_underscore(&mut self, items: &[(ast::UseTree, ast::NodeId)]) {
+        for (item, id) in items {
+            self.check_import_as_underscore(item, *id);
+        }
+    }
 }
 
 impl<'a, 'b, 'tcx> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b, 'tcx> {
@@ -119,7 +150,8 @@ impl<'a, 'b, 'tcx> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b, 'tcx> {
             // whether they're used or not. Also ignore imports with a dummy span
             // because this means that they were generated in some fashion by the
             // compiler and we don't need to consider them.
-            ast::ItemKind::Use(..) if item.vis.kind.is_pub() || item.span.is_dummy() => return,
+            ast::ItemKind::Use(..) if item.span.is_dummy() => return,
+            ast::ItemKind::Use(..) => self.base_use_is_pub = item.vis.kind.is_pub(),
             ast::ItemKind::ExternCrate(orig_name) => {
                 self.extern_crate_items.push(ExternCrateToLint {
                     id: item.id,
@@ -146,6 +178,11 @@ impl<'a, 'b, 'tcx> Visitor<'a> for UnusedImportCheckVisitor<'a, 'b, 'tcx> {
             self.base_use_tree = Some(use_tree);
         }
 
+        if self.base_use_is_pub {
+            self.check_import_as_underscore(use_tree, id);
+            return;
+        }
+
         if let ast::UseTreeKind::Nested(ref items) = use_tree.kind {
             if items.is_empty() {
                 self.unused_import(self.base_id).add(id);
@@ -300,6 +337,7 @@ impl Resolver<'_, '_> {
             base_use_tree: None,
             base_id: ast::DUMMY_NODE_ID,
             item_span: DUMMY_SP,
+            base_use_is_pub: false,
         };
         visit::walk_crate(&mut visitor, krate);