hir: Stop keeping prefixes for most of use list stems

And make sure all other imports have non-empty resolution lists.

compiler/rustc_ast_lowering/src/item.rs

@@ -496,8 +496,7 @@ fn lower_use_tree(
                     }
                 }
 
-                let res =
-                    self.expect_full_res_from_use(id).map(|res| self.lower_res(res)).collect();
+                let res = self.lower_import_res(id, path.span);
                 let path = self.lower_use_path(res, &path, ParamMode::Explicit);
                 hir::ItemKind::Use(path, hir::UseKind::Single)
             }
@@ -533,7 +532,8 @@ fn lower_use_tree(
                 // for that we return the `{}` import (called the
                 // `ListStem`).
 
-                let prefix = Path { segments, span: prefix.span.to(path.span), tokens: None };
+                let span = prefix.span.to(path.span);
+                let prefix = Path { segments, span, tokens: None };
 
                 // Add all the nested `PathListItem`s to the HIR.
                 for &(ref use_tree, id) in trees {
@@ -567,9 +567,16 @@ fn lower_use_tree(
                     });
                 }
 
-                let res =
-                    self.expect_full_res_from_use(id).map(|res| self.lower_res(res)).collect();
-                let path = self.lower_use_path(res, &prefix, ParamMode::Explicit);
+                let path = if trees.is_empty() && !prefix.segments.is_empty() {
+                    // For empty lists we need to lower the prefix so it is checked for things
+                    // like stability later.
+                    let res = self.lower_import_res(id, span);
+                    self.lower_use_path(res, &prefix, ParamMode::Explicit)
+                } else {
+                    // For non-empty lists we can just drop all the data, the prefix is already
+                    // present in HIR as a part of nested imports.
+                    self.arena.alloc(hir::UsePath { res: smallvec![], segments: &[], span })
+                };
                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
             }
         }

compiler/rustc_ast_lowering/src/lib.rs

@@ -63,7 +63,7 @@
 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 +749,14 @@ fn expect_full_res(&mut self, id: NodeId) -> Res<NodeId> {
         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, span: Span) -> 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() {
+            self.dcx().span_delayed_bug(span, "no resolution for an import");
+            return smallvec![Res::Err];
+        }
+        res
     }
 
     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 @@ pub(crate) fn lower_use_path(
         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| {