Stabilize const_fn_transmute

compiler/rustc_ast/src/lib.rs

@@ -10,7 +10,7 @@
 )]
 #![feature(box_syntax)]
 #![feature(box_patterns)]
-#![feature(const_fn_transmute)]
+#![cfg_attr(bootstrap, feature(const_fn_transmute))]
 #![feature(crate_visibility_modifier)]
 #![feature(iter_zip)]
 #![feature(label_break_value)]

compiler/rustc_feature/src/accepted.rs

@@ -290,6 +290,8 @@ macro_rules! declare_features {
     /// Allows bindings in the subpattern of a binding pattern.
     /// For example, you can write `x @ Some(y)`.
     (accepted, bindings_after_at, "1.54.0", Some(65490), None),
+    /// Allows calling `transmute` in const fn
+    (accepted, const_fn_transmute, "1.56.0", Some(53605), None),
 
     // -------------------------------------------------------------------------
     // feature-group-end: accepted features

compiler/rustc_feature/src/active.rs

@@ -565,9 +565,6 @@ pub fn set(&self, features: &mut Features, span: Span) {
     /// Lazily evaluate constants. This allows constants to depend on type parameters.
     (incomplete, lazy_normalization_consts, "1.46.0", Some(72219), None),
 
-    /// Allows calling `transmute` in const fn
-    (active, const_fn_transmute, "1.46.0", Some(53605), None),
-
     /// Allows `if let` guard in match arms.
     (incomplete, if_let_guard, "1.47.0", Some(51114), None),
 

compiler/rustc_mir/src/transform/check_consts/check.rs

@@ -876,15 +876,6 @@ fn visit_terminator(&mut self, terminator: &Terminator<'tcx>, location: Location
 
                 let is_intrinsic = tcx.fn_sig(callee).abi() == RustIntrinsic;
 
-                // HACK: This is to "unstabilize" the `transmute` intrinsic
-                // within const fns. `transmute` is allowed in all other const contexts.
-                // This won't really scale to more intrinsics or functions. Let's allow const
-                // transmutes in const fn before we add more hacks to this.
-                if is_intrinsic && tcx.item_name(callee) == sym::transmute {
-                    self.check_op(ops::Transmute);
-                    return;
-                }
-
                 if !tcx.is_const_fn_raw(callee) {
                     let mut permitted = false;
 

compiler/rustc_mir/src/transform/check_consts/ops.rs

@@ -501,29 +501,6 @@ fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<
     }
 }
 
-#[derive(Debug)]
-pub struct Transmute;
-impl NonConstOp for Transmute {
-    fn status_in_item(&self, ccx: &ConstCx<'_, '_>) -> Status {
-        if ccx.const_kind() != hir::ConstContext::ConstFn {
-            Status::Allowed
-        } else {
-            Status::Unstable(sym::const_fn_transmute)
-        }
-    }
-
-    fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> DiagnosticBuilder<'tcx> {
-        let mut err = feature_err(
-            &ccx.tcx.sess.parse_sess,
-            sym::const_fn_transmute,
-            span,
-            &format!("`transmute` is not allowed in {}s", ccx.const_kind()),
-        );
-        err.note("`transmute` is only allowed in constants and statics for now");
-        err
-    }
-}
-
 #[derive(Debug)]
 pub struct UnionAccess;
 impl NonConstOp for UnionAccess {

library/core/src/intrinsics.rs

@@ -911,6 +911,9 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
     /// cause [undefined behavior][ub] with this function. `transmute` should be
     /// the absolute last resort.
     ///
+    /// Transmuting pointers to integers in a `const` context is [undefined behavior][ub].
+    /// Any attempt to use the resulting value for integer operations will abort const-evaluation.
+    ///
     /// The [nomicon](../../nomicon/transmutes.html) has additional
     /// documentation.
     ///
@@ -1128,8 +1131,6 @@ pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    // NOTE: While this makes the intrinsic const stable, we have some custom code in const fn
-    // checks that prevent its use within `const fn`.
     #[rustc_const_stable(feature = "const_transmute", since = "1.46.0")]
     #[rustc_diagnostic_item = "transmute"]
     pub fn transmute<T, U>(e: T) -> U;

library/core/src/lib.rs

@@ -159,7 +159,7 @@
 #![feature(rtm_target_feature)]
 #![feature(f16c_target_feature)]
 #![feature(hexagon_target_feature)]
-#![feature(const_fn_transmute)]
+#![cfg_attr(bootstrap, feature(const_fn_transmute))]
 #![feature(abi_unadjusted)]
 #![feature(adx_target_feature)]
 #![feature(associated_type_bounds)]

library/core/src/num/int_macros.rs

@@ -2096,7 +2096,7 @@ pub const fn is_negative(self) -> bool { self < 0 }
         #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
         // SAFETY: const sound because integers are plain old datatypes so we can always
         // transmute them to arrays of bytes
-        #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+        #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
         #[inline]
         pub const fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
             // SAFETY: integers are plain old datatypes so we can always transmute them to
@@ -2202,7 +2202,7 @@ pub const fn is_negative(self) -> bool { self < 0 }
         #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
         // SAFETY: const sound because integers are plain old datatypes so we can always
         // transmute to them
-        #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+        #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
         #[inline]
         pub const fn from_ne_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
             // SAFETY: integers are plain old datatypes so we can always transmute to them

library/core/src/num/uint_macros.rs

@@ -1926,7 +1926,7 @@ pub const fn wrapping_next_power_of_two(self) -> Self {
         #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
         // SAFETY: const sound because integers are plain old datatypes so we can always
         // transmute them to arrays of bytes
-        #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+        #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
         #[inline]
         pub const fn to_ne_bytes(self) -> [u8; mem::size_of::<Self>()] {
             // SAFETY: integers are plain old datatypes so we can always transmute them to
@@ -2032,7 +2032,7 @@ pub const fn wrapping_next_power_of_two(self) -> Self {
         #[rustc_const_stable(feature = "const_int_conversion", since = "1.44.0")]
         // SAFETY: const sound because integers are plain old datatypes so we can always
         // transmute to them
-        #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+        #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
         #[inline]
         pub const fn from_ne_bytes(bytes: [u8; mem::size_of::<Self>()]) -> Self {
             // SAFETY: integers are plain old datatypes so we can always transmute to them

library/core/src/str/converts.rs

@@ -157,7 +157,7 @@ pub fn from_utf8_mut(v: &mut [u8]) -> Result<&mut str, Utf8Error> {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_stable(feature = "const_str_from_utf8_unchecked", since = "1.55.0")]
-#[rustc_allow_const_fn_unstable(const_fn_transmute)]
+#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
 pub const unsafe fn from_utf8_unchecked(v: &[u8]) -> &str {
     // SAFETY: the caller must guarantee that the bytes `v` are valid UTF-8.
     // Also relies on `&str` and `&[u8]` having the same layout.

library/core/src/str/mod.rs

@@ -231,7 +231,7 @@ pub fn is_char_boundary(&self, index: usize) -> bool {
     #[rustc_const_stable(feature = "str_as_bytes", since = "1.39.0")]
     #[inline(always)]
     #[allow(unused_attributes)]
-    #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
     pub const fn as_bytes(&self) -> &[u8] {
         // SAFETY: const sound because we transmute two types with the same layout
         unsafe { mem::transmute(self) }

library/std/src/net/ip.rs

@@ -1087,7 +1087,7 @@ impl Ipv6Addr {
     ///
     /// let addr = Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff);
     /// ```
-    #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
     #[rustc_const_stable(feature = "const_ipv6", since = "1.32.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]
@@ -1149,7 +1149,7 @@ pub const fn new(a: u16, b: u16, c: u16, d: u16, e: u16, f: u16, g: u16, h: u16)
     /// assert_eq!(Ipv6Addr::new(0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff).segments(),
     ///            [0, 0, 0, 0, 0, 0xffff, 0xc00a, 0x2ff]);
     /// ```
-    #[rustc_allow_const_fn_unstable(const_fn_transmute)]
+    #[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_fn_transmute))]
     #[rustc_const_stable(feature = "const_ipv6", since = "1.50.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
     #[inline]