AST: Make renames in imports closer to the source

Fix `unused_import_braces` lint false positive on `use prefix::{self as rename}`

7 files changed, 36 insertions, 26 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 7ced6424824..fdb32486b5d 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -1880,18 +1880,29 @@ pub type Variant = Spanned<Variant_>;
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum UseTreeKind {
-    Simple(Ident),
-    Glob,
+    Simple(Option<Ident>),
     Nested(Vec<(UseTree, NodeId)>),
+    Glob,
 }
 
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub struct UseTree {
-    pub kind: UseTreeKind,
     pub prefix: Path,
+    pub kind: UseTreeKind,
     pub span: Span,
 }
 
+impl UseTree {
+    pub fn ident(&self) -> Ident {
+        match self.kind {
+            UseTreeKind::Simple(Some(rename)) => rename,
+            UseTreeKind::Simple(None) =>
+                self.prefix.segments.last().expect("empty prefix in a simple import").identifier,
+            _ => panic!("`UseTree::ident` can only be used on a simple import"),
+        }
+    }
+}
+
 /// Distinguishes between Attributes that decorate items and Attributes that
 /// are contained as statements within items. These two cases need to be
 /// distinguished for pretty-printing.
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index b88e064e7e5..97f784dd617 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -294,7 +294,7 @@ pub trait AstBuilder {
                 vis: ast::Visibility, vp: P<ast::UseTree>) -> P<ast::Item>;
     fn item_use_simple(&self, sp: Span, vis: ast::Visibility, path: ast::Path) -> P<ast::Item>;
     fn item_use_simple_(&self, sp: Span, vis: ast::Visibility,
-                        ident: ast::Ident, path: ast::Path) -> P<ast::Item>;
+                        ident: Option<ast::Ident>, path: ast::Path) -> P<ast::Item>;
     fn item_use_list(&self, sp: Span, vis: ast::Visibility,
                      path: Vec<ast::Ident>, imports: &[ast::Ident]) -> P<ast::Item>;
     fn item_use_glob(&self, sp: Span,
@@ -1159,16 +1159,15 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
     }
 
     fn item_use_simple(&self, sp: Span, vis: ast::Visibility, path: ast::Path) -> P<ast::Item> {
-        let last = path.segments.last().unwrap().identifier;
-        self.item_use_simple_(sp, vis, last, path)
+        self.item_use_simple_(sp, vis, None, path)
     }
 
     fn item_use_simple_(&self, sp: Span, vis: ast::Visibility,
-                        ident: ast::Ident, path: ast::Path) -> P<ast::Item> {
+                        rename: Option<ast::Ident>, path: ast::Path) -> P<ast::Item> {
         self.item_use(sp, vis, P(ast::UseTree {
             span: sp,
             prefix: path,
-            kind: ast::UseTreeKind::Simple(ident),
+            kind: ast::UseTreeKind::Simple(rename),
         }))
     }
 
@@ -1178,7 +1177,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
             (ast::UseTree {
                 span: sp,
                 prefix: self.path(sp, vec![*id]),
-                kind: ast::UseTreeKind::Simple(*id),
+                kind: ast::UseTreeKind::Simple(None),
             }, ast::DUMMY_NODE_ID)
         }).collect();
 
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index 1a65fb7639a..146f580e04e 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -323,7 +323,8 @@ pub fn noop_fold_use_tree<T: Folder>(use_tree: UseTree, fld: &mut T) -> UseTree
         span: fld.new_span(use_tree.span),
         prefix: fld.fold_path(use_tree.prefix),
         kind: match use_tree.kind {
-            UseTreeKind::Simple(ident) => UseTreeKind::Simple(fld.fold_ident(ident)),
+            UseTreeKind::Simple(rename) =>
+                UseTreeKind::Simple(rename.map(|ident| fld.fold_ident(ident))),
             UseTreeKind::Glob => UseTreeKind::Glob,
             UseTreeKind::Nested(items) => UseTreeKind::Nested(items.move_map(|(tree, id)| {
                 (fld.fold_use_tree(tree), fld.new_id(id))
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index d1234d25764..7a432930490 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -7033,9 +7033,7 @@ impl<'a> Parser<'a> {
                 }
             } else {
                 // `use path::foo;` or `use path::foo as bar;`
-                let rename = self.parse_rename()?.
-                                  unwrap_or(prefix.segments.last().unwrap().identifier);
-                UseTreeKind::Simple(rename)
+                UseTreeKind::Simple(self.parse_rename()?)
             }
         };
 
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index 097c0fd16d0..18a758ba968 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -2949,13 +2949,12 @@ impl<'a> State<'a> {
 
     pub fn print_use_tree(&mut self, tree: &ast::UseTree) -> io::Result<()> {
         match tree.kind {
-            ast::UseTreeKind::Simple(ref ident) => {
+            ast::UseTreeKind::Simple(rename) => {
                 self.print_path(&tree.prefix, false, 0, true)?;
-
-                if tree.prefix.segments.last().unwrap().identifier.name != ident.name {
+                if let Some(rename) = rename {
                     self.s.space()?;
                     self.word_space("as")?;
-                    self.print_ident(*ident)?;
+                    self.print_ident(rename)?;
                 }
             }
             ast::UseTreeKind::Glob => {
diff --git a/src/libsyntax/test.rs b/src/libsyntax/test.rs
index 39306229c82..9edfa767d31 100644
--- a/src/libsyntax/test.rs
+++ b/src/libsyntax/test.rs
@@ -78,7 +78,7 @@ pub fn modify_for_testing(sess: &ParseSess,
                           span_diagnostic: &errors::Handler,
                           features: &Features) -> ast::Crate {
     // Check for #[reexport_test_harness_main = "some_name"] which
-    // creates a `use some_name = __test::main;`. This needs to be
+    // creates a `use __test::main as some_name;`. This needs to be
     // unconditional, so that the attribute is still marked as used in
     // non-test builds.
     let reexport_test_harness_main =
@@ -240,7 +240,8 @@ fn mk_reexport_mod(cx: &mut TestCtxt,
                                   cx.ext_cx.path(DUMMY_SP, vec![super_, r]))
     }).chain(tested_submods.into_iter().map(|(r, sym)| {
         let path = cx.ext_cx.path(DUMMY_SP, vec![super_, r, sym]);
-        cx.ext_cx.item_use_simple_(DUMMY_SP, dummy_spanned(ast::VisibilityKind::Public), r, path)
+        cx.ext_cx.item_use_simple_(DUMMY_SP, dummy_spanned(ast::VisibilityKind::Public),
+                                   Some(r), path)
     })).collect();
 
     let reexport_mod = ast::Mod {
@@ -502,7 +503,7 @@ fn mk_std(cx: &TestCtxt) -> P<ast::Item> {
         (ast::ItemKind::Use(P(ast::UseTree {
             span: DUMMY_SP,
             prefix: path_node(vec![id_test]),
-            kind: ast::UseTreeKind::Simple(id_test),
+            kind: ast::UseTreeKind::Simple(None),
         })),
          ast::VisibilityKind::Public, keywords::Invalid.ident())
     } else {
@@ -590,13 +591,13 @@ fn mk_test_module(cx: &mut TestCtxt) -> (P<ast::Item>, Option<P<ast::Item>>) {
         tokens: None,
     })).pop().unwrap();
     let reexport = cx.reexport_test_harness_main.map(|s| {
-        // building `use <ident> = __test::main`
-        let reexport_ident = Ident::with_empty_ctxt(s);
+        // building `use __test::main as <ident>;`
+        let rename = Ident::with_empty_ctxt(s);
 
         let use_path = ast::UseTree {
             span: DUMMY_SP,
             prefix: path_node(vec![mod_ident, Ident::from_str("main")]),
-            kind: ast::UseTreeKind::Simple(reexport_ident),
+            kind: ast::UseTreeKind::Simple(Some(rename)),
         };
 
         expander.fold_item(P(ast::Item {
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 3bf9bfab245..bbf1fe124f1 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -354,10 +354,11 @@ 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(ident) => {
-            visitor.visit_ident(use_tree.span, ident);
+        UseTreeKind::Simple(rename) => {
+            if let Some(rename) = rename {
+                visitor.visit_ident(use_tree.span, rename);
+            }
         }
         UseTreeKind::Glob => {},
         UseTreeKind::Nested(ref use_trees) => {