From 8f68bdc38024c471f925ce8b6572ec41546229e1 Mon Sep 17 00:00:00 2001
From: woppopo <woppopo@protonmail.com>
Date: Sat, 4 Dec 2021 21:57:39 +0900
Subject: [PATCH] Make `Borrow` and `BorrowMut` impls `const`

---
 library/core/src/array/mod.rs |  6 ++++--
 library/core/src/borrow.rs    | 15 ++++++++++-----
 2 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index b27c36baf37c5..e89659c41eae8 100644
--- a/library/core/src/array/mod.rs
+++ b/library/core/src/array/mod.rs
@@ -149,14 +149,16 @@ impl<T, const N: usize> AsMut<[T]> for [T; N] {
 }
 
 #[stable(feature = "array_borrow", since = "1.4.0")]
-impl<T, const N: usize> Borrow<[T]> for [T; N] {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T, const N: usize> const Borrow<[T]> for [T; N] {
     fn borrow(&self) -> &[T] {
         self
     }
 }
 
 #[stable(feature = "array_borrow", since = "1.4.0")]
-impl<T, const N: usize> BorrowMut<[T]> for [T; N] {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T, const N: usize> const BorrowMut<[T]> for [T; N] {
     fn borrow_mut(&mut self) -> &mut [T] {
         self
     }
diff --git a/library/core/src/borrow.rs b/library/core/src/borrow.rs
index f28be20aaa1e6..58eabecf3f091 100644
--- a/library/core/src/borrow.rs
+++ b/library/core/src/borrow.rs
@@ -205,7 +205,8 @@ pub trait BorrowMut<Borrowed: ?Sized>: Borrow<Borrowed> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Borrow<T> for T {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T: ?Sized> const Borrow<T> for T {
     #[rustc_diagnostic_item = "noop_method_borrow"]
     fn borrow(&self) -> &T {
         self
@@ -213,28 +214,32 @@ impl<T: ?Sized> Borrow<T> for T {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> BorrowMut<T> for T {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T: ?Sized> const BorrowMut<T> for T {
     fn borrow_mut(&mut self) -> &mut T {
         self
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Borrow<T> for &T {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T: ?Sized> const Borrow<T> for &T {
     fn borrow(&self) -> &T {
         &**self
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> Borrow<T> for &mut T {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T: ?Sized> const Borrow<T> for &mut T {
     fn borrow(&self) -> &T {
         &**self
     }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized> BorrowMut<T> for &mut T {
+#[rustc_const_unstable(feature = "const_borrow", issue = "91522")]
+impl<T: ?Sized> const BorrowMut<T> for &mut T {
     fn borrow_mut(&mut self) -> &mut T {
         &mut **self
     }