Auto merge of #113847 - SparrowLii:path_clone, r=cjgillot

avoid clone path prefix when lowering to hir

Found this while trying to parallelize `lower_to_hir`.

When lowering to hir, `Nested` paths in `ast` will be split and the prefix segments will be cloned. This could be omited, since the only consequence is that the prefix segments in `Path`s in hir will have the same `HirId`s, and it seems harmless.

This simplifies the process of lowering to hir and avoids re-modification of `ResolverAstLowering`.

r? `@Aaron1011`
cc #99292

2 files changed, 3 insertions, 21 deletions

diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index ab68436c093..2f58f566c81 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -551,17 +551,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 for &(ref use_tree, id) in trees {
                     let new_hir_id = self.local_def_id(id);
 
-                    let mut prefix = prefix.clone();
-
-                    // Give the segments new node-ids since they are being cloned.
-                    for seg in &mut prefix.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;
-                    }
-
                     // Each `use` import is an item and thus are owners of the
                     // names in the path. Up to this point the nested import is
                     // the current owner, since we want each desugared import to
@@ -570,6 +559,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     self.with_hir_id_owner(id, |this| {
                         let mut ident = *ident;
 
+                        // `prefix` is lowered multiple times, but in different HIR owners.
+                        // So each segment gets renewed `HirId` with the same
+                        // `ItemLocalId` and the new owner. (See `lower_node_id`)
                         let kind =
                             this.lower_use_tree(use_tree, &prefix, id, vis_span, &mut ident, attrs);
                         if let Some(attrs) = attrs {
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 9e193402feb..ac750690046 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -148,10 +148,6 @@ trait ResolverAstLoweringExt {
     fn legacy_const_generic_args(&self, expr: &Expr) -> Option<Vec<usize>>;
     fn get_partial_res(&self, id: NodeId) -> Option<PartialRes>;
     fn get_import_res(&self, id: NodeId) -> PerNS<Option<Res<NodeId>>>;
-    // Clones the resolution (if any) on 'source' and applies it
-    // to 'target'. Used when desugaring a `UseTreeKind::Nested` to
-    // multiple `UseTreeKind::Simple`s
-    fn clone_res(&mut self, source: NodeId, target: NodeId);
     fn get_label_res(&self, id: NodeId) -> Option<NodeId>;
     fn get_lifetime_res(&self, id: NodeId) -> Option<LifetimeRes>;
     fn take_extra_lifetime_params(&mut self, id: NodeId) -> Vec<(Ident, NodeId, LifetimeRes)>;
@@ -184,12 +180,6 @@ impl ResolverAstLoweringExt for ResolverAstLowering {
         None
     }
 
-    fn clone_res(&mut self, source: NodeId, target: NodeId) {
-        if let Some(res) = self.partial_res_map.get(&source) {
-            self.partial_res_map.insert(target, *res);
-        }
-    }
-
     /// Obtains resolution for a `NodeId` with a single resolution.
     fn get_partial_res(&self, id: NodeId) -> Option<PartialRes> {
         self.partial_res_map.get(&id).copied()