8747ccbcdf
`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.
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