Auto merge of #49251 - nikomatsakis:issue-15872-elision-impl-header, r=cramertj

support elision in impl headers

You can now do things like:

```
impl MyTrait<'_> for &u32 { ... }
```

Each `'_` or elided lifetime is a fresh parameter. `'_` and elision are still not permitted in associated type values. (Plausibly we could support that if there is a single input lifetime.) The original lifetime elision RFC was a bit unclear on this point: [as documented here, I think this is the correct interpretation, both because it fits existing impls and it's most analogous to the behavior in fns](https://github.com/rust-lang/rust/issues/15872#issuecomment-338700138).

We do not support elision with deprecated forms:

```
impl MyTrait for std::cell::Ref<u32> { } // ERROR
```

Builds on the in-band lifetime stuff.

r? @cramertj

Fixes #15872

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

@@ -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/in-band-lifetimes/impl/assoc-type.rs

@@ -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.stderr

@@ -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/dyn-trait.rs

@@ -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.stderr

@@ -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/path-elided.rs

@@ -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.stderr

@@ -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-underscore.rs

@@ -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/ref-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();
+}

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

@@ -0,0 +1,21 @@
+// 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<'a> { }
+
+impl MyTrait for u32 {
+    //~^ ERROR missing lifetime specifier
+}
+
+fn main() {}

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

@@ -0,0 +1,9 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/trait-elided.rs:17:6
+   |
+LL | impl MyTrait for u32 {
+   |      ^^^^^^^ 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/trait-underscore.rs

@@ -0,0 +1,48 @@
+// 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`.
+//
+// run-pass
+
+#![allow(warnings)]
+
+#![feature(in_band_lifetimes)]
+#![feature(underscore_lifetimes)]
+
+trait MyTrait<'a> { }
+
+// This is equivalent to `MyTrait<'a> for &'b i32`, which is proven by
+// the code below.
+impl MyTrait<'_> for &i32 {
+}
+
+// When called, T will be `&'x i32` for some `'x`, so since we can
+// prove that `&'x i32: for<'a> MyTrait<'a>, then we know that the
+// lifetime parameter above is disconnected.
+fn impls_my_trait<T: for<'a> MyTrait<'a>>() { }
+
+fn impls_my_trait_val<T: for<'a> MyTrait<'a>>(_: T) {
+    impls_my_trait::<T>();
+}
+
+fn random_where_clause()
+where for<'a, 'b> &'a i32: MyTrait<'b> { }
+
+fn main() {
+    let x = 22;
+    let f = &x;
+    impls_my_trait_val(f);
+
+    impls_my_trait::<&'static i32>();
+
+    random_where_clause();
+}

src/test/ui/underscore-lifetime/dyn-trait-underscore-in-struct.rs

@@ -0,0 +1,26 @@
+// Copyright 2015 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.
+
+// Check that the `'_` in `dyn Trait + '_` acts like ordinary elision,
+// and not like an object lifetime default.
+//
+// cc #48468
+
+#![feature(dyn_trait)]
+#![feature(underscore_lifetimes)]
+
+use std::fmt::Debug;
+
+struct Foo {
+    x: Box<dyn Debug + '_>, //~ ERROR missing lifetime specifier
+    //~^ ERROR E0228
+}
+
+fn main() { }

src/test/ui/underscore-lifetime/dyn-trait-underscore-in-struct.stderr

@@ -0,0 +1,16 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/dyn-trait-underscore-in-struct.rs:22:24
+   |
+LL |     x: Box<dyn Debug + '_>, //~ ERROR missing lifetime specifier
+   |                        ^^ expected lifetime parameter
+
+error[E0228]: the lifetime bound for this object type cannot be deduced from context; please supply an explicit bound
+  --> $DIR/dyn-trait-underscore-in-struct.rs:22:12
+   |
+LL |     x: Box<dyn Debug + '_>, //~ ERROR missing lifetime specifier
+   |            ^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0106, E0228.
+For more information about an error, try `rustc --explain E0106`.