rustc_ast_lowering: Stop lowering imports into multiple items

compiler/rustc_ast/src/ast.rs

@@ -2517,10 +2517,7 @@ pub struct Variant {
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum UseTreeKind {
     /// `use prefix` or `use prefix as rename`
-    ///
-    /// The extra `NodeId`s are for HIR lowering, when additional statements are created for each
-    /// namespace.
-    Simple(Option<Ident>, NodeId, NodeId),
+    Simple(Option<Ident>),
     /// `use prefix::{...}`
     Nested(Vec<(UseTree, NodeId)>),
     /// `use prefix::*`
@@ -2539,8 +2536,8 @@ pub struct UseTree {
 impl UseTree {
     pub fn ident(&self) -> Ident {
         match self.kind {
-            UseTreeKind::Simple(Some(rename), ..) => rename,
-            UseTreeKind::Simple(None, ..) => {
+            UseTreeKind::Simple(Some(rename)) => rename,
+            UseTreeKind::Simple(None) => {
                 self.prefix.segments.last().expect("empty prefix in a simple import").ident
             }
             _ => panic!("`UseTree::ident` can only be used on a simple import"),

compiler/rustc_ast/src/mut_visit.rs

@@ -410,11 +410,7 @@ pub fn noop_visit_use_tree<T: MutVisitor>(use_tree: &mut UseTree, vis: &mut T) {
     let UseTree { prefix, kind, span } = use_tree;
     vis.visit_path(prefix);
     match kind {
-        UseTreeKind::Simple(rename, id1, id2) => {
-            visit_opt(rename, |rename| vis.visit_ident(rename));
-            vis.visit_id(id1);
-            vis.visit_id(id2);
-        }
+        UseTreeKind::Simple(rename) => visit_opt(rename, |rename| vis.visit_ident(rename)),
         UseTreeKind::Nested(items) => {
             for (tree, id) in items {
                 vis.visit_use_tree(tree);

compiler/rustc_ast/src/visit.rs

@@ -439,7 +439,7 @@ pub fn walk_path<'a, V: Visitor<'a>>(visitor: &mut V, path: &'a Path) {
 pub fn walk_use_tree<'a, V: Visitor<'a>>(visitor: &mut V, use_tree: &'a UseTree, id: NodeId) {
     visitor.visit_path(&use_tree.prefix, id);
     match &use_tree.kind {
-        UseTreeKind::Simple(rename, ..) => {
+        UseTreeKind::Simple(rename) => {
             // The extra IDs are handled during HIR lowering.
             if let &Some(rename) = rename {
                 visitor.visit_ident(rename);

compiler/rustc_ast_lowering/src/item.rs

@@ -19,7 +19,6 @@ use rustc_span::symbol::{kw, sym, Ident};
 use rustc_span::{Span, Symbol};
 use rustc_target::spec::abi;
 use smallvec::{smallvec, SmallVec};
-use std::iter;
 use thin_vec::ThinVec;
 
 pub(super) struct ItemLowerer<'a, 'hir> {
@@ -179,36 +178,22 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let mut node_ids =
             smallvec![hir::ItemId { owner_id: hir::OwnerId { def_id: self.local_def_id(i.id) } }];
         if let ItemKind::Use(use_tree) = &i.kind {
-            self.lower_item_id_use_tree(use_tree, i.id, &mut node_ids);
+            self.lower_item_id_use_tree(use_tree, &mut node_ids);
         }
         node_ids
     }
 
-    fn lower_item_id_use_tree(
-        &mut self,
-        tree: &UseTree,
-        base_id: NodeId,
-        vec: &mut SmallVec<[hir::ItemId; 1]>,
-    ) {
+    fn lower_item_id_use_tree(&mut self, tree: &UseTree, vec: &mut SmallVec<[hir::ItemId; 1]>) {
         match &tree.kind {
             UseTreeKind::Nested(nested_vec) => {
                 for &(ref nested, id) in nested_vec {
                     vec.push(hir::ItemId {
                         owner_id: hir::OwnerId { def_id: self.local_def_id(id) },
                     });
-                    self.lower_item_id_use_tree(nested, id, vec);
-                }
-            }
-            UseTreeKind::Glob => {}
-            UseTreeKind::Simple(_, id1, id2) => {
-                for (_, id) in
-                    iter::zip(self.expect_full_res_from_use(base_id).skip(1), [*id1, *id2])
-                {
-                    vec.push(hir::ItemId {
-                        owner_id: hir::OwnerId { def_id: self.local_def_id(id) },
-                    });
+                    self.lower_item_id_use_tree(nested, vec);
                 }
             }
+            UseTreeKind::Simple(..) | UseTreeKind::Glob => {}
         }
     }
 
@@ -489,7 +474,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let segments = prefix.segments.iter().chain(path.segments.iter()).cloned().collect();
 
         match tree.kind {
-            UseTreeKind::Simple(rename, id1, id2) => {
+            UseTreeKind::Simple(rename) => {
                 *ident = tree.ident();
 
                 // First, apply the prefix to the path.
@@ -505,66 +490,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     }
                 }
 
-                let mut resolutions = self.expect_full_res_from_use(id).fuse();
-                // We want to return *something* from this function, so hold onto the first item
-                // for later.
-                let ret_res = self.lower_res(resolutions.next().unwrap_or(Res::Err));
-
-                // Here, we are looping over namespaces, if they exist for the definition
-                // being imported. We only handle type and value namespaces because we
-                // won't be dealing with macros in the rest of the compiler.
-                // Essentially a single `use` which imports two names is desugared into
-                // two imports.
-                for new_node_id in [id1, id2] {
-                    let new_id = self.local_def_id(new_node_id);
-                    let Some(res) = resolutions.next() else {
-                        debug_assert!(self.children.iter().find(|(id, _)| id == &new_id).is_none());
-                        // Associate an HirId to both ids even if there is no resolution.
-                        self.children.push((
-                            new_id,
-                            hir::MaybeOwner::NonOwner(hir::HirId::make_owner(new_id))),
-                        );
-                        continue;
-                    };
-                    let ident = *ident;
-                    let mut path = path.clone();
-                    for seg in &mut path.segments {
-                        // Give the cloned segment the same resolution information
-                        // as the old one (this is needed for stability checking).
-                        let new_id = self.next_node_id();
-                        self.resolver.clone_res(seg.id, new_id);
-                        seg.id = new_id;
-                    }
-                    let span = path.span;
-
-                    self.with_hir_id_owner(new_node_id, |this| {
-                        let res = this.lower_res(res);
-                        let path = this.lower_path_extra(res, &path, ParamMode::Explicit);
-                        let kind = hir::ItemKind::Use(path, hir::UseKind::Single);
-                        if let Some(attrs) = attrs {
-                            this.attrs.insert(hir::ItemLocalId::new(0), attrs);
-                        }
-
-                        let item = hir::Item {
-                            owner_id: hir::OwnerId { def_id: new_id },
-                            ident: this.lower_ident(ident),
-                            kind,
-                            vis_span,
-                            span: this.lower_span(span),
-                        };
-                        hir::OwnerNode::Item(this.arena.alloc(item))
-                    });
-                }
-
-                let path = self.lower_path_extra(ret_res, &path, ParamMode::Explicit);
+                let res =
+                    self.expect_full_res_from_use(id).map(|res| self.lower_res(res)).collect();
+                let path = self.lower_use_path(res, &path, ParamMode::Explicit);
                 hir::ItemKind::Use(path, hir::UseKind::Single)
             }
             UseTreeKind::Glob => {
-                let path = self.lower_path(
-                    id,
-                    &Path { segments, span: path.span, tokens: None },
-                    ParamMode::Explicit,
-                );
+                let res = self.expect_full_res(id);
+                let res = smallvec![self.lower_res(res)];
+                let path = Path { segments, span: path.span, tokens: None };
+                let path = self.lower_use_path(res, &path, ParamMode::Explicit);
                 hir::ItemKind::Use(path, hir::UseKind::Glob)
             }
             UseTreeKind::Nested(ref trees) => {
@@ -634,9 +569,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     });
                 }
 
-                let res = self.expect_full_res_from_use(id).next().unwrap_or(Res::Err);
-                let res = self.lower_res(res);
-                let path = self.lower_path_extra(res, &prefix, ParamMode::Explicit);
+                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);
                 hir::ItemKind::Use(path, hir::UseKind::ListStem)
             }
         }

compiler/rustc_ast_lowering/src/path.rs

@@ -12,7 +12,7 @@ use rustc_hir::GenericArg;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{BytePos, Span, DUMMY_SP};
 
-use smallvec::smallvec;
+use smallvec::{smallvec, SmallVec};
 
 impl<'a, 'hir> LoweringContext<'a, 'hir> {
     #[instrument(level = "trace", skip(self))]
@@ -144,13 +144,13 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         );
     }
 
-    pub(crate) fn lower_path_extra(
+    pub(crate) fn lower_use_path(
         &mut self,
-        res: Res,
+        res: SmallVec<[Res; 3]>,
         p: &Path,
         param_mode: ParamMode,
-    ) -> &'hir hir::Path<'hir> {
-        self.arena.alloc(hir::Path {
+    ) -> &'hir hir::UsePath<'hir> {
+        self.arena.alloc(hir::UsePath {
             res,
             segments: self.arena.alloc_from_iter(p.segments.iter().map(|segment| {
                 self.lower_path_segment(
@@ -165,17 +165,6 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         })
     }
 
-    pub(crate) fn lower_path(
-        &mut self,
-        id: NodeId,
-        p: &Path,
-        param_mode: ParamMode,
-    ) -> &'hir hir::Path<'hir> {
-        let res = self.expect_full_res(id);
-        let res = self.lower_res(res);
-        self.lower_path_extra(res, p, param_mode)
-    }
-
     pub(crate) fn lower_path_segment(
         &mut self,
         path_span: Span,

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -663,7 +663,7 @@ impl<'a> State<'a> {
 
     fn print_use_tree(&mut self, tree: &ast::UseTree) {
         match &tree.kind {
-            ast::UseTreeKind::Simple(rename, ..) => {
+            ast::UseTreeKind::Simple(rename) => {
                 self.print_path(&tree.prefix, false, 0);
                 if let &Some(rename) = rename {
                     self.nbsp();

compiler/rustc_builtin_macros/src/assert/context.rs

@@ -106,7 +106,7 @@ impl<'cx, 'a> Context<'cx, 'a> {
             (
                 UseTree {
                     prefix: this.cx.path(this.span, vec![Ident::with_dummy_span(sym)]),
-                    kind: UseTreeKind::Simple(None, DUMMY_NODE_ID, DUMMY_NODE_ID),
+                    kind: UseTreeKind::Simple(None),
                     span: this.span,
                 },
                 DUMMY_NODE_ID,

compiler/rustc_data_structures/src/stable_hasher.rs

@@ -399,7 +399,7 @@ where
     }
 }
 
-impl<A, CTX> HashStable<CTX> for SmallVec<[A; 1]>
+impl<A, const N: usize, CTX> HashStable<CTX> for SmallVec<[A; N]>
 where
     A: HashStable<CTX>,
 {

compiler/rustc_hir/src/arena.rs

@@ -39,6 +39,7 @@ macro_rules! arena_types {
             [] param: rustc_hir::Param<'tcx>,
             [] pat: rustc_hir::Pat<'tcx>,
             [] path: rustc_hir::Path<'tcx>,
+            [] use_path: rustc_hir::UsePath<'tcx>,
             [] path_segment: rustc_hir::PathSegment<'tcx>,
             [] poly_trait_ref: rustc_hir::PolyTraitRef<'tcx>,
             [] qpath: rustc_hir::QPath<'tcx>,

compiler/rustc_hir/src/hir.rs

@@ -183,14 +183,17 @@ impl Lifetime {
 /// `std::cmp::PartialEq`. It's represented as a sequence of identifiers,
 /// along with a bunch of supporting information.
 #[derive(Debug, HashStable_Generic)]
-pub struct Path<'hir> {
+pub struct Path<'hir, R = Res> {
     pub span: Span,
     /// The resolution for the path.
-    pub res: Res,
+    pub res: R,
     /// The segments in the path: the things separated by `::`.
     pub segments: &'hir [PathSegment<'hir>],
 }
 
+/// Up to three resolutions for type, value and macro namespaces.
+pub type UsePath<'hir> = Path<'hir, SmallVec<[Res; 3]>>;
+
 impl Path<'_> {
     pub fn is_global(&self) -> bool {
         !self.segments.is_empty() && self.segments[0].ident.name == kw::PathRoot
@@ -3068,7 +3071,7 @@ pub enum ItemKind<'hir> {
     /// or just
     ///
     /// `use foo::bar::baz;` (with `as baz` implicitly on the right).
-    Use(&'hir Path<'hir>, UseKind),
+    Use(&'hir UsePath<'hir>, UseKind),
 
     /// A `static` item.
     Static(&'hir Ty<'hir>, Mutability, BodyId),

compiler/rustc_hir/src/intravisit.rs

@@ -367,7 +367,7 @@ pub trait Visitor<'v>: Sized {
     fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v FnDecl<'v>, b: BodyId, _: Span, id: HirId) {
         walk_fn(self, fk, fd, b, id)
     }
-    fn visit_use(&mut self, path: &'v Path<'v>, hir_id: HirId) {
+    fn visit_use(&mut self, path: &'v UsePath<'v>, hir_id: HirId) {
         walk_use(self, path, hir_id)
     }
     fn visit_trait_item(&mut self, ti: &'v TraitItem<'v>) {
@@ -422,7 +422,7 @@ pub trait Visitor<'v>: Sized {
     fn visit_qpath(&mut self, qpath: &'v QPath<'v>, id: HirId, _span: Span) {
         walk_qpath(self, qpath, id)
     }
-    fn visit_path(&mut self, path: &'v Path<'v>, _id: HirId) {
+    fn visit_path(&mut self, path: &Path<'v>, _id: HirId) {
         walk_path(self, path)
     }
     fn visit_path_segment(&mut self, path_segment: &'v PathSegment<'v>) {
@@ -938,9 +938,12 @@ pub fn walk_fn_kind<'v, V: Visitor<'v>>(visitor: &mut V, function_kind: FnKind<'
     }
 }
 
-pub fn walk_use<'v, V: Visitor<'v>>(visitor: &mut V, path: &'v Path<'v>, hir_id: HirId) {
+pub fn walk_use<'v, V: Visitor<'v>>(visitor: &mut V, path: &'v UsePath<'v>, hir_id: HirId) {
     visitor.visit_id(hir_id);
-    visitor.visit_path(path, hir_id);
+    let UsePath { segments, ref res, span } = *path;
+    for &res in res {
+        visitor.visit_path(&Path { segments, res, span }, hir_id);
+    }
 }
 
 pub fn walk_trait_item<'v, V: Visitor<'v>>(visitor: &mut V, trait_item: &'v TraitItem<'v>) {
@@ -1126,7 +1129,7 @@ pub fn walk_qpath<'v, V: Visitor<'v>>(visitor: &mut V, qpath: &'v QPath<'v>, id:
     }
 }
 
-pub fn walk_path<'v, V: Visitor<'v>>(visitor: &mut V, path: &'v Path<'v>) {
+pub fn walk_path<'v, V: Visitor<'v>>(visitor: &mut V, path: &Path<'v>) {
     for segment in path.segments {
         visitor.visit_path_segment(segment);
     }

compiler/rustc_hir_analysis/src/collect/lifetimes.rs

@@ -814,7 +814,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
         }
     }
 
-    fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
+    fn visit_path(&mut self, path: &hir::Path<'tcx>, _: hir::HirId) {
         for (i, segment) in path.segments.iter().enumerate() {
             let depth = path.segments.len() - i - 1;
             if let Some(ref args) = segment.args {

compiler/rustc_hir_pretty/src/lib.rs

@@ -1591,7 +1591,7 @@ impl<'a> State<'a> {
         self.print_ident(Ident::with_dummy_span(name))
     }
 
-    pub fn print_path(&mut self, path: &hir::Path<'_>, colons_before_params: bool) {
+    pub fn print_path<R>(&mut self, path: &hir::Path<'_, R>, colons_before_params: bool) {
         self.maybe_print_comment(path.span.lo());
 
         for (i, segment) in path.segments.iter().enumerate() {

compiler/rustc_lint/src/internal.rs

@@ -117,7 +117,7 @@ impl<'tcx> LateLintPass<'tcx> for TyTyKind {
     fn check_path(
         &mut self,
         cx: &LateContext<'tcx>,
-        path: &'tcx rustc_hir::Path<'tcx>,
+        path: &rustc_hir::Path<'tcx>,
         _: rustc_hir::HirId,
     ) {
         if let Some(segment) = path.segments.iter().nth_back(1)

compiler/rustc_lint/src/late.rs

@@ -292,7 +292,7 @@ impl<'tcx, T: LateLintPass<'tcx>> hir_visit::Visitor<'tcx> for LateContextAndPas
         hir_visit::walk_lifetime(self, lt);
     }
 
-    fn visit_path(&mut self, p: &'tcx hir::Path<'tcx>, id: hir::HirId) {
+    fn visit_path(&mut self, p: &hir::Path<'tcx>, id: hir::HirId) {
         lint_callback!(self, check_path, p, id);
         hir_visit::walk_path(self, p);
     }

compiler/rustc_lint/src/passes.rs

@@ -44,7 +44,7 @@ macro_rules! late_lint_methods {
             fn check_struct_def(a: &$hir hir::VariantData<$hir>);
             fn check_field_def(a: &$hir hir::FieldDef<$hir>);
             fn check_variant(a: &$hir hir::Variant<$hir>);
-            fn check_path(a: &$hir hir::Path<$hir>, b: hir::HirId);
+            fn check_path(a: &hir::Path<$hir>, b: hir::HirId);
             fn check_attribute(a: &$hir ast::Attribute);
 
             /// Called when entering a syntax node that can have lint attributes such

compiler/rustc_lint/src/unused.rs

@@ -1264,7 +1264,7 @@ impl UnusedImportBraces {
 
             // Trigger the lint if the nested item is a non-self single item
             let node_name = match items[0].0.kind {
-                ast::UseTreeKind::Simple(rename, ..) => {
+                ast::UseTreeKind::Simple(rename) => {
                     let orig_ident = items[0].0.prefix.segments.last().unwrap().ident;
                     if orig_ident.name == kw::SelfLower {
                         return;