Add some docs + Fix rebase

Author: petrochenkov

src/libsyntax/ast.rs

@@ -1877,13 +1877,19 @@ pub struct Variant_ {
 
 pub type Variant = Spanned<Variant_>;
 
+/// Part of `use` item to the right of its prefix.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum UseTreeKind {
+    /// `use prefix` or `use prefix as rename`
     Simple(Option<Ident>),
+    /// `use prefix::{...}`
     Nested(Vec<(UseTree, NodeId)>),
+    /// `use prefix::*`
     Glob,
 }
 
+/// A tree of paths sharing common prefixes.
+/// Used in `use` items both at top-level and inside of braces in import groups.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct UseTree {
     pub prefix: Path,

src/libsyntax/feature_gate.rs

@@ -1438,7 +1438,7 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
     }
 
     fn visit_use_tree(&mut self, use_tree: &'a ast::UseTree, id: NodeId, _nested: bool) {
-        if let ast::UseTreeKind::Simple(ident) = use_tree.kind {
+        if let ast::UseTreeKind::Simple(Some(ident)) = use_tree.kind {
             if ident.name == "_" {
                 gate_feature_post!(&self, underscore_imports, use_tree.span,
                                    "renaming imports with `_` is unstable");