Implement underscore lifetimes

src/librustc/diagnostics.rs

@@ -2086,4 +2086,5 @@ register_diagnostics! {
     E0566, // conflicting representation hints
     E0623, // lifetime mismatch where both parameters are anonymous regions
     E0628, // generators cannot have explicit arguments
+    E0637, // "'_" is not a valid lifetime bound
 }

src/librustc/hir/mod.rs

@@ -159,7 +159,8 @@ impl fmt::Debug for Lifetime {
 
 impl Lifetime {
     pub fn is_elided(&self) -> bool {
-        self.name == keywords::Invalid.name()
+        self.name == keywords::Invalid.name() ||
+        self.name == "'_"
     }
 
     pub fn is_static(&self) -> bool {

src/librustc/middle/resolve_lifetime.rs

@@ -1422,7 +1422,7 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             let lifetime_i = &lifetimes[i];
 
             for lifetime in lifetimes {
-                if lifetime.lifetime.is_static() {
+                if lifetime.lifetime.is_static() || lifetime.lifetime.name == "'_"  {
                     let lifetime = lifetime.lifetime;
                     let mut err = struct_span_err!(self.sess, lifetime.span, E0262,
                                   "invalid lifetime parameter name: `{}`", lifetime.name);
@@ -1452,7 +1452,13 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
             self.check_lifetime_def_for_shadowing(old_scope, &lifetime_i.lifetime);
 
             for bound in &lifetime_i.bounds {
-                if !bound.is_static() {
+                if bound.name == "'_" {
+                    let mut err = struct_span_err!(self.sess, bound.span, E0637,
+                        "invalid lifetime bound name: `{}`", bound.name);
+                    err.span_label(bound.span,
+                                   format!("{} is a reserved lifetime name", bound.name));
+                    err.emit();
+                } else if !bound.is_static() {
                     self.resolve_lifetime_ref(bound);
                 } else {
                     self.insert_lifetime(bound, Region::Static);

src/librustc_passes/ast_validation.rs

@@ -122,14 +122,6 @@ impl<'a> AstValidator<'a> {
 }
 
 impl<'a> Visitor<'a> for AstValidator<'a> {
-    fn visit_lifetime(&mut self, lt: &'a Lifetime) {
-        if lt.ident.name == "'_" {
-            self.err_handler().span_err(lt.span, &format!("invalid lifetime name `{}`", lt.ident));
-        }
-
-        visit::walk_lifetime(self, lt)
-    }
-
     fn visit_expr(&mut self, expr: &'a Expr) {
         match expr.node {
             ExprKind::While(.., Some(ident)) |

src/libsyntax/feature_gate.rs

@@ -389,6 +389,9 @@ declare_features! (
     // Copy/Clone closures (RFC 2132)
     (active, clone_closures, "1.22.0", Some(44490)),
     (active, copy_closures, "1.22.0", Some(44490)),
+
+    // allow `'_` placeholder lifetimes
+    (active, underscore_lifetimes, "1.22.0", Some(44524)),
 );
 
 declare_features! (
@@ -1572,6 +1575,14 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
         }
         visit::walk_lifetime_def(self, lifetime_def)
     }
+
+    fn visit_lifetime(&mut self, lt: &'a ast::Lifetime) {
+        if lt.ident.name == "'_" {
+            gate_feature_post!(&self, underscore_lifetimes, lt.span,
+                               "underscore lifetimes are unstable");
+        }
+        visit::walk_lifetime(self, lt)
+    }
 }
 
 pub fn get_features(span_handler: &Handler, krate_attrs: &[ast::Attribute]) -> Features {

src/test/compile-fail/E0637.rs

@@ -0,0 +1,20 @@
+// Copyright 2017 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.
+#![feature(underscore_lifetimes)]
+
+struct Foo<'a: '_>(&'a u8); //~ ERROR invalid lifetime bound name: `'_`
+fn foo<'a: '_>(_: &'a u8) {} //~ ERROR invalid lifetime bound name: `'_`
+
+struct Bar<'a>(&'a u8);
+impl<'a: '_> Bar<'a> { //~ ERROR invalid lifetime bound name: `'_`
+  fn bar() {}
+}
+
+fn main() {}

src/test/compile-fail/feature-gate-underscore-lifetimes.rs

@@ -0,0 +1,20 @@
+// Copyright 2017 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.
+
+struct Foo<'a>(&'a u8);
+
+fn foo(x: &u8) -> Foo<'_> { //~ ERROR underscore lifetimes are unstable
+    Foo(x)
+}
+
+fn main() {
+    let x = 5;
+    let _ = foo(&x);
+}

src/test/compile-fail/label-underscore.rs

@@ -8,12 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-fn _f<'_>() //~ ERROR invalid lifetime name `'_`
-    -> &'_ u8 //~ ERROR invalid lifetime name `'_`
-{
-    panic!();
-}
-
 fn main() {
     '_: loop { //~ ERROR invalid label name `'_`
         break '_ //~ ERROR invalid label name `'_`

src/test/compile-fail/underscore-lifetime-binders.rs

@@ -0,0 +1,32 @@
+// Copyright 2017 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.
+
+#![feature(underscore_lifetimes)]
+
+struct Foo<'a>(&'a u8);
+
+fn foo<'_> //~ ERROR invalid lifetime parameter name: `'_`
+(_: Foo<'_>) {}
+
+trait Meh<'a> {}
+impl<'a> Meh<'a> for u8 {}
+
+fn meh() -> Box<for<'_> Meh<'_>> //~ ERROR invalid lifetime parameter name: `'_`
+//~^ ERROR missing lifetime specifier
+//~^^ ERROR missing lifetime specifier
+{
+  Box::new(5u8)
+}
+
+fn main() {
+    let x = 5;
+    foo(Foo(&x));
+    let _ = meh();
+}

src/test/run-pass/underscore-lifetimes.rs

@@ -0,0 +1,35 @@
+// Copyright 2017 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.
+
+#![feature(underscore_lifetimes)]
+
+struct Foo<'a>(&'a u8);
+
+fn foo(x: &u8) -> Foo<'_> {
+    Foo(x)
+}
+
+fn foo2(x: &'_ u8) -> Foo<'_> {
+    Foo(x)
+}
+
+fn foo3(x: &'_ u8) -> Foo {
+    Foo(x)
+}
+
+fn foo4(_: Foo<'_>) {}
+
+fn main() {
+    let x = &5;
+    let _ = foo(x);
+    let _ = foo2(x);
+    let _ = foo3(x);
+    foo4(Foo(x));
+}