Rollup merge of #141741 - nnethercote:overhaul-UsePath, r=petrochenkov

Overhaul `UsePath`

It currently uses `SmallVec<[Res; 3]>` which is really weird. Details in the individual commits.

r? `@petrochenkov`

1 files changed, 9 insertions, 7 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 422e79ca82f..f41627e479f 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -64,7 +64,7 @@ use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
 use rustc_session::parse::{add_feature_diagnostics, feature_err};
 use rustc_span::symbol::{Ident, Symbol, kw, sym};
 use rustc_span::{DUMMY_SP, DesugaringKind, Span};
-use smallvec::{SmallVec, smallvec};
+use smallvec::SmallVec;
 use thin_vec::ThinVec;
 use tracing::{debug, instrument, trace};
 
@@ -705,14 +705,16 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         self.resolver.get_partial_res(id).map_or(Res::Err, |pr| pr.expect_full_res())
     }
 
-    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() {
+    fn lower_import_res(&mut self, id: NodeId, span: Span) -> PerNS<Option<Res>> {
+        let per_ns = self.resolver.get_import_res(id);
+        let per_ns = per_ns.map(|res| res.map(|res| self.lower_res(res)));
+        if per_ns.is_empty() {
+            // Propagate the error to all namespaces, just to be sure.
             self.dcx().span_delayed_bug(span, "no resolution for an import");
-            return smallvec![Res::Err];
+            let err = Some(Res::Err);
+            return PerNS { type_ns: err, value_ns: err, macro_ns: err };
         }
-        res
+        per_ns
     }
 
     fn make_lang_item_qpath(