ast_lowering: Make sure all imports have non-empty resolution lists

HIR visitor visits import paths once per resolution, so if some import has an empty resolution list, like in case of import list stems, its path stays unvisited.

Author: petrochenkov

compiler/rustc_ast_lowering/src/item.rs

@@ -496,8 +496,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     }
                 }
 
-                let res =
-                    self.expect_full_res_from_use(id).map(|res| self.lower_res(res)).collect();
+                let res = self.lower_import_res(id);
                 let path = self.lower_use_path(res, &path, ParamMode::Explicit);
                 hir::ItemKind::Use(path, hir::UseKind::Single)
             }
@@ -567,8 +566,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     });
                 }
 
-                let res =
-                    self.expect_full_res_from_use(id).map(|res| self.lower_res(res)).collect();
+                let res = self.lower_import_res(id);
                 let path = self.lower_use_path(res, &prefix, ParamMode::Explicit);
                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
             }

compiler/rustc_ast_lowering/src/lib.rs

@@ -63,7 +63,7 @@ use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
 use rustc_session::parse::{add_feature_diagnostics, feature_err};
 use rustc_span::symbol::{kw, sym, Ident, Symbol};
 use rustc_span::{DesugaringKind, Span, DUMMY_SP};
-use smallvec::SmallVec;
+use smallvec::{smallvec, SmallVec};
 use std::collections::hash_map::Entry;
 use thin_vec::ThinVec;
 
@@ -749,8 +749,10 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.resolver.get_partial_res(id).map_or(Res::Err, |pr| pr.expect_full_res())
     }
 
-    fn expect_full_res_from_use(&mut self, id: NodeId) -> impl Iterator<Item = Res<NodeId>> {
-        self.resolver.get_import_res(id).present_items()
+    fn lower_import_res(&mut self, id: NodeId) -> SmallVec<[Res; 3]> {
+        let res = self.resolver.get_import_res(id).present_items();
+        let res: SmallVec<_> = res.map(|res| self.lower_res(res)).collect();
+        if !res.is_empty() { res } else { smallvec![Res::Err] }
     }
 
     fn make_lang_item_qpath(&mut self, lang_item: hir::LangItem, span: Span) -> hir::QPath<'hir> {

compiler/rustc_ast_lowering/src/path.rs

@@ -156,6 +156,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         p: &Path,
         param_mode: ParamMode,
     ) -> &'hir hir::UsePath<'hir> {
+        assert!((1..=3).contains(&res.len()));
         self.arena.alloc(hir::UsePath {
             res,
             segments: self.arena.alloc_from_iter(p.segments.iter().map(|segment| {