Rollup merge of #69373 - tspiteri:const_int_conversion, r=oli-obk

Stabilize const for integer {to,from}_{be,le,ne}_bytes methods

All of these functions can be implemented simply and naturally as const functions, e.g. `u32::from_le_bytes` can be implemented as
```rust
(bytes[0] as u32)
    | (bytes[1] as u32) << 8
    | (bytes[2] as u32) << 16
    | (bytes[3] as u32) << 24
```
So stabilizing the constness will not expose that internally they are implemented using transmute which is not const in stable.

Author: Centril

src/libcore/lib.rs

@@ -130,7 +130,6 @@
 #![feature(rtm_target_feature)]
 #![feature(f16c_target_feature)]
 #![feature(hexagon_target_feature)]
-#![feature(const_int_conversion)]
 #![feature(const_transmute)]
 #![feature(structural_match)]
 #![feature(abi_unadjusted)]

src/libcore/num/mod.rs

@@ -2154,7 +2154,7 @@ let bytes = ", $swap_op, stringify!($SelfT), ".to_be_bytes();
 assert_eq!(bytes, ", $be_bytes, ");
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn to_be_bytes(self) -> [u8; mem::size_of::<Self>()] {
                 self.to_be().to_ne_bytes()
@@ -2174,7 +2174,7 @@ let bytes = ", $swap_op, stringify!($SelfT), ".to_le_bytes();
 assert_eq!(bytes, ", $le_bytes, ");
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn to_le_bytes(self) -> [u8; mem::size_of::<Self>()] {
                 self.to_le().to_ne_bytes()
@@ -2209,12 +2209,20 @@ assert_eq!(
 );
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
+            // SAFETY: const sound because integers are plain old datatypes so we can always
+            // transmute them to arrays of bytes
+            #[allow_internal_unstable(const_fn_union)]
             #[inline]
             pub const fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
+                #[repr(C)]
+                union Bytes {
+                    val: $SelfT,
+                    bytes: [u8; mem::size_of::<$SelfT>()],
+                }
                 // SAFETY: integers are plain old datatypes so we can always transmute them to
                 // arrays of bytes
-                unsafe { mem::transmute(self) }
+                unsafe { Bytes { val: self }.bytes }
             }
         }
 
@@ -2243,7 +2251,7 @@ fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
 }
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn from_be_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
                 Self::from_be(Self::from_ne_bytes(bytes))
@@ -2276,7 +2284,7 @@ fn read_le_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
 }
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn from_le_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
                 Self::from_le(Self::from_ne_bytes(bytes))
@@ -2319,11 +2327,19 @@ fn read_ne_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
 }
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
+            // SAFETY: const sound because integers are plain old datatypes so we can always
+            // transmute to them
+            #[allow_internal_unstable(const_fn_union)]
             #[inline]
             pub const fn from_ne_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
+                #[repr(C)]
+                union Bytes {
+                    val: $SelfT,
+                    bytes: [u8; mem::size_of::<$SelfT>()],
+                }
                 // SAFETY: integers are plain old datatypes so we can always transmute to them
-                unsafe { mem::transmute(bytes) }
+                unsafe { Bytes { bytes }.val }
             }
         }
 
@@ -4099,7 +4115,7 @@ let bytes = ", $swap_op, stringify!($SelfT), ".to_be_bytes();
 assert_eq!(bytes, ", $be_bytes, ");
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn to_be_bytes(self) -> [u8; mem::size_of::<Self>()] {
                 self.to_be().to_ne_bytes()
@@ -4119,7 +4135,7 @@ let bytes = ", $swap_op, stringify!($SelfT), ".to_le_bytes();
 assert_eq!(bytes, ", $le_bytes, ");
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn to_le_bytes(self) -> [u8; mem::size_of::<Self>()] {
                 self.to_le().to_ne_bytes()
@@ -4154,12 +4170,20 @@ assert_eq!(
 );
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
+            // SAFETY: const sound because integers are plain old datatypes so we can always
+            // transmute them to arrays of bytes
+            #[allow_internal_unstable(const_fn_union)]
             #[inline]
             pub const fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
+                #[repr(C)]
+                union Bytes {
+                    val: $SelfT,
+                    bytes: [u8; mem::size_of::<$SelfT>()],
+                }
                 // SAFETY: integers are plain old datatypes so we can always transmute them to
                 // arrays of bytes
-                unsafe { mem::transmute(self) }
+                unsafe { Bytes { val: self }.bytes }
             }
         }
 
@@ -4188,7 +4212,7 @@ fn read_be_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
 }
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn from_be_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
                 Self::from_be(Self::from_ne_bytes(bytes))
@@ -4221,7 +4245,7 @@ fn read_le_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
 }
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
             #[inline]
             pub const fn from_le_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
                 Self::from_le(Self::from_ne_bytes(bytes))
@@ -4264,11 +4288,19 @@ fn read_ne_", stringify!($SelfT), "(input: &mut &[u8]) -> ", stringify!($SelfT),
 }
 ```"),
             #[stable(feature = "int_to_from_bytes", since = "1.32.0")]
-            #[rustc_const_unstable(feature = "const_int_conversion", issue = "53718")]
+            #[rustc_const_stable(feature = "const_int_conversion", since = "1.43.0")]
+            // SAFETY: const sound because integers are plain old datatypes so we can always
+            // transmute to them
+            #[allow_internal_unstable(const_fn_union)]
             #[inline]
             pub const fn from_ne_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
+                #[repr(C)]
+                union Bytes {
+                    val: $SelfT,
+                    bytes: [u8; mem::size_of::<$SelfT>()],
+                }
                 // SAFETY: integers are plain old datatypes so we can always transmute to them
-                unsafe { mem::transmute(bytes) }
+                unsafe { Bytes { bytes }.val }
             }
         }
 

src/test/ui/consts/const-int-conversion-rpass.rs

@@ -1,7 +1,5 @@
 // run-pass
 
-#![feature(const_int_conversion)]
-
 const REVERSE: u32 = 0x12345678_u32.reverse_bits();
 const FROM_BE_BYTES: i32 = i32::from_be_bytes([0x12, 0x34, 0x56, 0x78]);
 const FROM_LE_BYTES: i32 = i32::from_le_bytes([0x12, 0x34, 0x56, 0x78]);