Correctly handle path stability for 'use tree' items

PR #5956 started checking the stability of path segments.
However, this was not applied to 'use tree' items
(e.g. 'use some::path::{ItemOne, ItemTwo}') due to the way
that we desugar these items in HIR lowering.

This PR modifies 'use tree' lowering to preserve resolution
information, which is needed by stability checking.

2 files changed, 20 insertions, 2 deletions

diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 7da49143b46..e176364c494 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -533,7 +533,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     let ident = *ident;
                     let mut path = path.clone();
                     for seg in &mut path.segments {
-                        seg.id = self.next_node_id();
+                        // 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;
 
@@ -602,7 +606,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
 
                     // Give the segments new node-ids since they are being cloned.
                     for seg in &mut prefix.segments {
-                        seg.id = self.next_node_id();
+                        // 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
diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index b79c1cafba0..ad6b857e94d 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -155,6 +155,10 @@ 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)>;
@@ -192,6 +196,12 @@ 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()