Implement import renaming with _ (RFC 2166)

src/libsyntax/feature_gate.rs

@@ -455,6 +455,9 @@ declare_features! (
 
     // Parentheses in patterns
     (active, pattern_parentheses, "1.26.0", None, None),
+
+    // `use path as _;` and `extern crate c as _;`
+    (active, underscore_imports, "1.26.0", Some(48216), None),
 );
 
 declare_features! (
@@ -1436,9 +1439,24 @@ 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 ident.name == "_" {
+                gate_feature_post!(&self, underscore_imports, use_tree.span,
+                                   "renaming imports with `_` is unstable");
+            }
+        }
+
+        visit::walk_use_tree(self, use_tree, id);
+    }
+
     fn visit_item(&mut self, i: &'a ast::Item) {
         match i.node {
             ast::ItemKind::ExternCrate(_) => {
+                if i.ident.name == "_" {
+                    gate_feature_post!(&self, underscore_imports, i.span,
+                                       "renaming extern crates with `_` is unstable");
+                }
                 if let Some(attr) = attr::find_by_name(&i.attrs[..], "macro_reexport") {
                     gate_feature_post!(&self, macro_reexport, attr.span,
                                        "macros re-exports are experimental \

src/libsyntax/parse/parser.rs

@@ -7040,7 +7040,11 @@ impl<'a> Parser<'a> {
 
     fn parse_rename(&mut self) -> PResult<'a, Option<Ident>> {
         if self.eat_keyword(keywords::As) {
-            self.parse_ident().map(Some)
+            if self.eat(&token::Underscore) {
+                Ok(Some(Ident::with_empty_ctxt(Symbol::gensym("_"))))
+            } else {
+                self.parse_ident().map(Some)
+            }
         } else {
             Ok(None)
         }

src/test/ui/feature-gate-underscore-imports.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern crate std as _; //~ ERROR renaming extern crates with `_` is unstable
+use std::vec as _; //~ ERROR renaming imports with `_` is unstable
+
+fn main() {}

src/test/ui/feature-gate-underscore-imports.stderr

@@ -0,0 +1,19 @@
+error[E0658]: renaming extern crates with `_` is unstable (see issue #48216)
+  --> $DIR/feature-gate-underscore-imports.rs:11:1
+   |
+LL | extern crate std as _; //~ ERROR renaming extern crates with `_` is unstable
+   | ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: add #![feature(underscore_imports)] to the crate attributes to enable
+
+error[E0658]: renaming imports with `_` is unstable (see issue #48216)
+  --> $DIR/feature-gate-underscore-imports.rs:12:5
+   |
+LL | use std::vec as _; //~ ERROR renaming imports with `_` is unstable
+   |     ^^^^^^^^^^^^^
+   |
+   = help: add #![feature(underscore_imports)] to the crate attributes to enable
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

src/test/ui/rfc-2166-underscore-imports/basic.rs

@@ -0,0 +1,67 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// must-compile-successfully
+
+#![feature(underscore_imports)]
+#![warn(unused_imports, unused_extern_crates)]
+
+struct S;
+
+mod m {
+    pub trait Tr1 {
+        fn tr1_is_in_scope(&self) {}
+    }
+    pub trait Tr2 {
+        fn tr2_is_in_scope(&self) {}
+    }
+
+    impl Tr1 for ::S {}
+    impl Tr2 for ::S {}
+}
+
+mod unused {
+    use m::Tr1 as _; //~ WARN unused import
+    use S as _; //~ WARN unused import
+    extern crate core as _; //~ WARN unused extern crate
+}
+
+mod outer {
+    mod middle {
+        pub use m::Tr1 as _;
+        pub use m::Tr2 as _; // OK, no name conflict
+        struct Tr1; // OK, no name conflict
+        fn check() {
+            // Both traits are in scope
+            ::S.tr1_is_in_scope();
+            ::S.tr2_is_in_scope();
+        }
+
+        mod inner {
+            // `_` imports are fetched by glob imports
+            use super::*;
+            fn check() {
+                // Both traits are in scope
+                ::S.tr1_is_in_scope();
+                ::S.tr2_is_in_scope();
+            }
+        }
+    }
+
+    // `_` imports are fetched by glob imports
+    use self::middle::*;
+    fn check() {
+        // Both traits are in scope
+        ::S.tr1_is_in_scope();
+        ::S.tr2_is_in_scope();
+    }
+}
+
+fn main() {}

src/test/ui/rfc-2166-underscore-imports/basic.stderr

@@ -0,0 +1,30 @@
+warning: unused import: `m::Tr1 as _`
+  --> $DIR/basic.rs:31:9
+   |
+LL |     use m::Tr1 as _; //~ WARN unused import
+   |         ^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/basic.rs:14:9
+   |
+LL | #![warn(unused_imports, unused_extern_crates)]
+   |         ^^^^^^^^^^^^^^
+
+warning: unused import: `S as _`
+  --> $DIR/basic.rs:32:9
+   |
+LL |     use S as _; //~ WARN unused import
+   |         ^^^^^^
+
+warning: unused extern crate
+  --> $DIR/basic.rs:33:5
+   |
+LL |     extern crate core as _; //~ WARN unused extern crate
+   |     ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: lint level defined here
+  --> $DIR/basic.rs:14:25
+   |
+LL | #![warn(unused_imports, unused_extern_crates)]
+   |                         ^^^^^^^^^^^^^^^^^^^^
+