Rollup merge of #68809 - ecstatic-morse:const-int-functions, r=oli-obk

Make more arithmetic functions unstably const

This is a smaller version of #66884 (thanks @9999years) that constifies many of the arithmetic functions on integer primitives from #53718 that were blocked on #49146.

This makes the following things unstably const.

- `feature = const_int_unchecked_arith`
  - `intrinsics::unchecked_add`
  - `intrinsics::unchecked_sub`
  - `intrinsics::unchecked_mul`
  - `intrinsics::unchecked_div`
  - `intrinsics::unchecked_rem`

- `feature = const_checked_int_methods`
  - `checked_add`
  - `checked_sub`
  - `checked_mul`
  - `checked_div` (Uses `intrinsics::unchecked_div` internally)
  - `checked_rem` (Uses `intrinsics::unchecked_rem` internally)
  - `checked_neg`
  - `checked_shl`
  - `checked_shr`
  - `checked_abs`

- `feature = const_saturating_int_methods`
  - `saturating_mul`
  - `saturating_neg`  (Uses `intrinsics::unchecked_sub` internally)
  - `saturating_abs` (Uses `intrinsics::unchecked_sub` internally)

- `feature = const_wrapping_int_methods`
  - `wrapping_div`
  - `wrapping_rem`

- `feature = const_overflowing_int_methods`
  - `overflowing_div`
  - `overflowing_rem`

- `feature = const_euclidean_int_methods`
  - `checked_div_euclid`
  - `checked_rem_euclid`
  - `wrapping_div_euclid`
  - `wrapping_rem_euclid`
  - `overflowing_div_euclid`
  - `overflowing_rem_euclid`

Exponentiation and operations on the `NonZero` types are left to a later PR.

r? @oli-obk
cc @rust-lang/wg-const-eval @rust-lang/libs

Author: Dylan-DPC

src/libcore/intrinsics.rs

@@ -1305,9 +1305,11 @@ extern "rust-intrinsic" {
 
     /// Performs an unchecked division, resulting in undefined behavior
     /// where y = 0 or x = `T::min_value()` and y = -1
+    #[rustc_const_unstable(feature = "const_int_unchecked_arith", issue = "none")]
     pub fn unchecked_div<T>(x: T, y: T) -> T;
     /// Returns the remainder of an unchecked division, resulting in
     /// undefined behavior where y = 0 or x = `T::min_value()` and y = -1
+    #[rustc_const_unstable(feature = "const_int_unchecked_arith", issue = "none")]
     pub fn unchecked_rem<T>(x: T, y: T) -> T;
 
     /// Performs an unchecked left shift, resulting in undefined behavior when
@@ -1321,14 +1323,17 @@ extern "rust-intrinsic" {
 
     /// Returns the result of an unchecked addition, resulting in
     /// undefined behavior when `x + y > T::max_value()` or `x + y < T::min_value()`.
+    #[rustc_const_unstable(feature = "const_int_unchecked_arith", issue = "none")]
     pub fn unchecked_add<T>(x: T, y: T) -> T;
 
     /// Returns the result of an unchecked subtraction, resulting in
     /// undefined behavior when `x - y > T::max_value()` or `x - y < T::min_value()`.
+    #[rustc_const_unstable(feature = "const_int_unchecked_arith", issue = "none")]
     pub fn unchecked_sub<T>(x: T, y: T) -> T;
 
     /// Returns the result of an unchecked multiplication, resulting in
     /// undefined behavior when `x * y > T::max_value()` or `x * y < T::min_value()`.
+    #[rustc_const_unstable(feature = "const_int_unchecked_arith", issue = "none")]
     pub fn unchecked_mul<T>(x: T, y: T) -> T;
 
     /// Performs rotate left.

src/libcore/lib.rs

@@ -72,6 +72,11 @@
 #![feature(concat_idents)]
 #![feature(const_alloc_layout)]
 #![feature(const_if_match)]
+#![feature(const_checked_int_methods)]
+#![feature(const_euclidean_int_methods)]
+#![feature(const_overflowing_int_methods)]
+#![feature(const_saturating_int_methods)]
+#![feature(const_int_unchecked_arith)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]
 #![feature(const_generics)]