permit '_ and &T in impl headers

Deprecated forms of elision are not supported.

Author: nikomatsakis

src/librustc/hir/intravisit.rs

@@ -420,7 +420,10 @@ pub fn walk_lifetime<'v, V: Visitor<'v>>(visitor: &mut V, lifetime: &'v Lifetime
         LifetimeName::Name(name) => {
             visitor.visit_name(lifetime.span, name);
         }
-        LifetimeName::Static | LifetimeName::Implicit | LifetimeName::Underscore => {}
+        LifetimeName::Fresh(_) |
+        LifetimeName::Static |
+        LifetimeName::Implicit |
+        LifetimeName::Underscore => {}
     }
 }
 

src/librustc/hir/lowering.rs

@@ -209,6 +209,21 @@ pub enum LifetimeName {
     /// User typed `'_`.
     Underscore,
 
+    /// Synthetic name generated when user elided a lifetime in an impl header,
+    /// e.g. the lifetimes in cases like these:
+    ///
+    ///     impl Foo for &u32
+    ///     impl Foo<'_> for u32
+    ///
+    /// in that case, we rewrite to
+    ///
+    ///     impl<'f> Foo for &'f u32
+    ///     impl<'f> Foo<'f> for u32
+    ///
+    /// where `'f` is something like `Fresh(0)`. The indices are
+    /// unique per impl, but not necessarily continuous.
+    Fresh(usize),
+
     /// User wrote `'static`
     Static,
 
@@ -221,7 +236,7 @@ impl LifetimeName {
         use self::LifetimeName::*;
         match *self {
             Implicit => keywords::Invalid.name(),
-            Underscore => keywords::UnderscoreLifetime.name(),
+            Fresh(_) | Underscore => keywords::UnderscoreLifetime.name(),
             Static => keywords::StaticLifetime.name(),
             Name(name) => name,
         }
@@ -242,7 +257,13 @@ impl Lifetime {
         use self::LifetimeName::*;
         match self.name {
             Implicit | Underscore => true,
-            Static | Name(_) => false,
+
+            // It might seem surprising that `Fresh(_)` counts as
+            // *not* elided -- but this is because, as far as the code
+            // in the compiler is concerned -- `Fresh(_)` variants act
+            // equivalently to "some fresh name". They correspond to
+            // early-bound regions on an impl, in other words.
+            Fresh(_) | Static | Name(_) => false,
         }
     }
 

src/librustc/hir/mod.rs

@@ -145,6 +145,7 @@ impl<'a> HashStable<StableHashingContext<'a>> for hir::ImplItemId {
 impl_stable_hash_for!(enum hir::LifetimeName {
     Implicit,
     Underscore,
+    Fresh(index),
     Static,
     Name(name)
 });

src/librustc/ich/impls_hir.rs

@@ -582,7 +582,7 @@ impl<'a, 'tcx> Visitor<'tcx> for LifetimeContext<'a, 'tcx> {
                         // cc #48468
                         self.resolve_elided_lifetimes(slice::from_ref(lifetime), false)
                     }
-                    LifetimeName::Static | LifetimeName::Name(_) => {
+                    LifetimeName::Fresh(_) | LifetimeName::Static | LifetimeName::Name(_) => {
                         // If the user wrote an explicit name, use that.
                         self.visit_lifetime(lifetime);
                     }
@@ -2086,7 +2086,8 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                         );
                         err.emit();
                     }
-                    hir::LifetimeName::Implicit | hir::LifetimeName::Name(_) => {}
+                    hir::LifetimeName::Fresh(_) | hir::LifetimeName::Implicit |
+                    hir::LifetimeName::Name(_) => {}
                 }
             }
 
@@ -2138,7 +2139,8 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                             ))
                             .emit();
                     }
-                    hir::LifetimeName::Implicit | hir::LifetimeName::Name(_) => {
+                    hir::LifetimeName::Fresh(_) | hir::LifetimeName::Implicit |
+                    hir::LifetimeName::Name(_) => {
                         self.resolve_lifetime_ref(bound);
                     }
                 }

src/librustc/middle/resolve_lifetime.rs

@@ -0,0 +1,22 @@
+// 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.
+
+#![allow(warnings)]
+#![feature(underscore_lifetimes)]
+
+trait MyTrait<'a> { }
+
+impl<'a> MyTrait<'a> for &u32 { }
+//~^ ERROR missing lifetime specifier
+
+impl<'a> MyTrait<'_> for &'a f32 { }
+//~^ ERROR missing lifetime specifier
+
+fn main() {}

src/test/ui/feature-gate-in_band_lifetimes-impl.rs

@@ -0,0 +1,15 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/feature-gate-in_band_lifetimes-impl.rs:16:26
+   |
+LL | impl<'a> MyTrait<'a> for &u32 { }
+   |                          ^ expected lifetime parameter
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/feature-gate-in_band_lifetimes-impl.rs:19:18
+   |
+LL | impl<'a> MyTrait<'_> for &'a f32 { }
+   |                  ^^ expected lifetime parameter
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/feature-gate-in_band_lifetimes-impl.stderr

@@ -0,0 +1,38 @@
+// 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.
+
+// Test that we do not yet support elision in associated types, even
+// when there is just one name we could take from the impl header.
+
+#![allow(warnings)]
+
+#![feature(in_band_lifetimes)]
+#![feature(underscore_lifetimes)]
+
+trait MyTrait {
+    type Output;
+}
+
+impl MyTrait for &i32 {
+    type Output = &i32;
+    //~^ ERROR missing lifetime specifier
+}
+
+impl MyTrait for &u32 {
+    type Output = &'_ i32;
+    //~^ ERROR missing lifetime specifier
+}
+
+// This is what you have to do:
+impl MyTrait for &'a f32 {
+    type Output = &'a f32;
+}
+
+fn main() { }

src/test/ui/in-band-lifetimes/impl/assoc-type.rs

@@ -0,0 +1,15 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/assoc-type.rs:24:19
+   |
+LL |     type Output = &i32;
+   |                   ^ expected lifetime parameter
+
+error[E0106]: missing lifetime specifier
+  --> $DIR/assoc-type.rs:29:20
+   |
+LL |     type Output = &'_ i32;
+   |                    ^^ expected lifetime parameter
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/in-band-lifetimes/impl/assoc-type.stderr

@@ -0,0 +1,45 @@
+// 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.
+
+// Test that `impl MyTrait<'_> for &i32` is equivalent to `impl<'a,
+// 'b> MyTrait<'a> for &'b i32`.
+
+#![allow(warnings)]
+
+#![feature(dyn_trait)]
+#![feature(in_band_lifetimes)]
+#![feature(underscore_lifetimes)]
+
+use std::fmt::Debug;
+
+// Equivalent to `Box<dyn Debug + 'static>`:
+trait StaticTrait { }
+impl StaticTrait for Box<dyn Debug> { }
+
+// Equivalent to `Box<dyn Debug + 'static>`:
+trait NotStaticTrait { }
+impl NotStaticTrait for Box<dyn Debug + '_> { }
+
+fn static_val<T: StaticTrait>(_: T) {
+}
+
+fn with_dyn_debug_static<'a>(x: Box<dyn Debug + 'a>) {
+    static_val(x); //~ ERROR cannot infer
+}
+
+fn not_static_val<T: NotStaticTrait>(_: T) {
+}
+
+fn with_dyn_debug_not_static<'a>(x: Box<dyn Debug + 'a>) {
+    not_static_val(x); // OK
+}
+
+fn main() {
+}

src/test/ui/in-band-lifetimes/impl/dyn-trait.rs

@@ -0,0 +1,22 @@
+error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
+  --> $DIR/dyn-trait.rs:34:16
+   |
+LL |     static_val(x); //~ ERROR cannot infer
+   |                ^
+   |
+note: first, the lifetime cannot outlive the lifetime 'a as defined on the function body at 33:1...
+  --> $DIR/dyn-trait.rs:33:1
+   |
+LL | fn with_dyn_debug_static<'a>(x: Box<dyn Debug + 'a>) {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   = note: ...so that the expression is assignable:
+           expected std::boxed::Box<std::fmt::Debug>
+              found std::boxed::Box<std::fmt::Debug + 'a>
+   = note: but, the lifetime must be valid for the static lifetime...
+   = note: ...so that the types are compatible:
+           expected StaticTrait
+              found StaticTrait
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0495`.

src/test/ui/in-band-lifetimes/impl/dyn-trait.stderr

@@ -0,0 +1,23 @@
+// 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.
+#![allow(warnings)]
+
+#![feature(in_band_lifetimes)]
+#![feature(underscore_lifetimes)]
+
+trait MyTrait { }
+
+struct Foo<'a> { x: &'a u32 }
+
+impl MyTrait for Foo {
+    //~^ ERROR missing lifetime specifier
+}
+
+fn main() {}

src/test/ui/in-band-lifetimes/impl/path-elided.rs

@@ -0,0 +1,9 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/path-elided.rs:19:18
+   |
+LL | impl MyTrait for Foo {
+   |                  ^^^ expected lifetime parameter
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0106`.

src/test/ui/in-band-lifetimes/impl/path-elided.stderr

@@ -0,0 +1,47 @@
+// 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.
+
+// Test that `impl MyTrait for Foo<'_>` works.
+
+// run-pass
+
+#![allow(warnings)]
+
+#![feature(in_band_lifetimes)]
+#![feature(underscore_lifetimes)]
+
+trait MyTrait { }
+
+struct Foo<'a> { x: &'a u32 }
+
+impl MyTrait for Foo<'_> {
+}
+
+fn impls_my_trait<T: MyTrait>() { }
+
+fn impls_my_trait_val<T: MyTrait>(_: T) {
+    impls_my_trait::<T>();
+}
+
+fn random_where_clause()
+where for<'a> Foo<'a>: MyTrait { }
+
+fn main() {
+    let x = 22;
+    let f = Foo { x: &x };
+
+    // This type is `Foo<'x>` for a local lifetime `'x`; so the impl
+    // must apply to any lifetime to apply to this.
+    impls_my_trait_val(f);
+
+    impls_my_trait::<Foo<'static>>();
+
+    random_where_clause();
+}

src/test/ui/in-band-lifetimes/impl/path-underscore.rs

@@ -0,0 +1,43 @@
+// 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.
+
+// Test that `impl MyTrait for &i32` works and is equivalent to any lifetime.
+
+// run-pass
+
+#![allow(warnings)]
+
+#![feature(in_band_lifetimes)]
+#![feature(underscore_lifetimes)]
+
+trait MyTrait { }
+
+impl MyTrait for &i32 {
+}
+
+fn impls_my_trait<T: MyTrait>() { }
+
+fn impls_my_trait_val<T: MyTrait>(_: T) {
+    impls_my_trait::<T>();
+}
+
+fn random_where_clause()
+where for<'a> &'a i32: MyTrait { }
+
+fn main() {
+    let x = 22;
+    let f = &x;
+
+    impls_my_trait_val(f);
+
+    impls_my_trait::<&'static i32>();
+
+    random_where_clause();
+}