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, 7 insertions, 4 deletions

diff --git a/src/tools/clippy/clippy_utils/src/paths.rs b/src/tools/clippy/clippy_utils/src/paths.rs
index 9d7f3086b05..f37a609497e 100644
--- a/src/tools/clippy/clippy_utils/src/paths.rs
+++ b/src/tools/clippy/clippy_utils/src/paths.rs
@@ -306,10 +306,13 @@ fn local_item_child_by_name(tcx: TyCtxt<'_>, local_id: LocalDefId, ns: PathNS, n
             let item = tcx.hir_item(item_id);
             if let ItemKind::Use(path, UseKind::Single(ident)) = item.kind {
                 if ident.name == name {
-                    path.res
-                        .iter()
-                        .find(|res| ns.matches(res.ns()))
-                        .and_then(Res::opt_def_id)
+                    let opt_def_id = |ns: Option<Res>| ns.and_then(|res| res.opt_def_id());
+                    match ns {
+                        PathNS::Type => opt_def_id(path.res.type_ns),
+                        PathNS::Value => opt_def_id(path.res.value_ns),
+                        PathNS::Macro => opt_def_id(path.res.macro_ns),
+                        PathNS::Arbitrary => unreachable!(),
+                    }
                 } else {
                     None
                 }