Implement RFC 2128 (use_nested_groups)

This commit adds support for nested groups inside `use` declarations,
such as `use foo::{bar, sub::{baz::Foo, *}};`.

1 files changed, 26 insertions, 0 deletions

diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 89d1a3699e8..8507c9653db 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -428,6 +428,9 @@ declare_features! (
 
     // In-band lifetime bindings (e.g. `fn foo(x: &'a u8) -> &'a u8`)
     (active, in_band_lifetimes, "1.23.0", Some(44524)),
+
+    // Nested groups in `use` (RFC 2128)
+    (active, use_nested_groups, "1.23.0", Some(44494)),
 );
 
 declare_features! (
@@ -1661,6 +1664,29 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         visit::walk_path(self, path);
     }
 
+    fn visit_use_tree(&mut self, use_tree: &'a ast::UseTree, id: NodeId, nested: bool) {
+        if nested {
+            match use_tree.kind {
+                ast::UseTreeKind::Simple(_) => {
+                    if use_tree.prefix.segments.len() != 1 {
+                        gate_feature_post!(&self, use_nested_groups, use_tree.span,
+                                           "paths in `use` groups are experimental");
+                    }
+                }
+                ast::UseTreeKind::Glob => {
+                    gate_feature_post!(&self, use_nested_groups, use_tree.span,
+                                       "glob imports in `use` groups are experimental");
+                }
+                ast::UseTreeKind::Nested(_) => {
+                    gate_feature_post!(&self, use_nested_groups, use_tree.span,
+                                       "nested groups in `use` are experimental");
+                }
+            }
+        }
+
+        visit::walk_use_tree(self, use_tree, id);
+    }
+
     fn visit_vis(&mut self, vis: &'a ast::Visibility) {
         if let ast::Visibility::Crate(span, ast::CrateSugar::JustCrate) = *vis {
             gate_feature_post!(&self, crate_visibility_modifier, span,