Overhaul `UsePath`.

`UsePath` contains a `SmallVec<[Res; 3]>`. This holds up to three `Res`
results, one per namespace (type, value, or macro). `lower_import_res`
takes a `PerNS<Option<Res<NodeId>>>` result and lowers it into the
`SmallVec`. This is pretty weird. The input `PerNS` makes it clear which
`Res` belongs to which namespace, but the `SmallVec` throws that
information away.

And code that operates on the `SmallVec` tends to use iteration (or even
just grabbing the first entry!) without knowing which namespace the
`Res` belongs to. Even weirder! Also, `SmallVec` is an overly flexible
type to use here, because it can contain any number of elements (even
though it's optimized for 3 in this case).

This commit changes `UsePath` so it also contains a
`PerNS<Option<Res<HirId>>>`. This type preserves more information and is
more self-documenting. The commit also changes a lot of the use sites to
access the result for a particular namespace. E.g. if you're looking up
a trait, it will be in the `Res` for the type namespace if it's present;
it's silly to look in the `Res` for the value namespace or macro
namespace. Overall I find the new code much easier to understand.

However, some use sites still iterate. These now use `present_items`
because that filters out the `None` results.

Also, `redundant_pub_crate.rs` gets a bigger change. A
`UseKind:ListStem` item gets no `Res` results, which means the old `all`
call in `is_not_macro_export` would succeed (because `all` succeeds on
an empty iterator) and the `ListStem` would be ignored. This is what we
want, but was more by luck than design. The new code detects `ListStem`
explicitly. The commit generalizes the name of that function
accordingly.

Finally, the commit also removes the `use_path` arena, because
`PerNS<Option<Res>>` impls `Copy` (unlike `SmallVec`) and it can be
allocated in the arena shared by all `Copy` types.

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(