Auto merge of #47562 - Centril:feature/core_convert_id, r=oli-obk

Add the identity function as core::convert::identity

## New notes

This implements rust-lang/rfcs#2306 (see https://github.com/rust-lang/rust/issues/53500).

## Old notes (ignore this in new reviews)

Adds the identity function `fn id<T>(x: T) -> T { x }` to core::convert and the prelude.
Some motivations for why this is useful are explained in the doc tests.
Another is that using the identity function instead of `{ x }` or `|x| x` makes it clear that you intended to use an identity conversion on purpose.

The reasoning:
+ behind adding this to `convert` and not `mem` is that this is an identity *conversion*.
+ for adding this to the prelude is that it should be easy enough to use that the ease of writing your own identity function or using a closure `|x| x` doesn't overtake that.

I've separated this out into two feature gates so that the addition to the prelude can be considered and stabilized separately.

cc @bluss

src/test/ui/rfc-2306/convert-id-const-no-gate.rs

@@ -0,0 +1,17 @@
+// 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.
+
+// This test should fail since identity is not stable as a const fn yet.
+
+#![feature(convert_id)]
+
+fn main() {
+    const _FOO: u8 = ::std::convert::identity(42u8);
+}

src/test/ui/rfc-2306/convert-id-const-no-gate.stderr

@@ -0,0 +1,10 @@
+error: `std::convert::identity` is not yet stable as a const fn
+  --> $DIR/convert-id-const-no-gate.rs:16:22
+   |
+LL |     const _FOO: u8 = ::std::convert::identity(42u8);
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: in Nightly builds, add `#![feature(const_convert_id)]` to the crate attributes to enable
+
+error: aborting due to previous error
+

src/test/ui/rfc-2306/convert-id-const-with-gate.rs

@@ -0,0 +1,20 @@
+// 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.
+
+// This test should pass since we've opted into 'identity' as an
+// unstable const fn.
+
+// compile-pass
+
+#![feature(convert_id, const_convert_id)]
+
+fn main() {
+    const _FOO: u8 = ::std::convert::identity(42u8);
+}