Make some of MaybeUninit's methods const

compiler/rustc_mir/src/interpret/intrinsics.rs

@@ -407,6 +407,30 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             sym::transmute => {
                 self.copy_op_transmute(args[0], dest)?;
             }
+            sym::assert_inhabited | sym::assert_zero_valid | sym::assert_uninit_valid => {
+                let ty = instance.substs.type_at(0);
+                let layout = self.layout_of(ty)?;
+
+                if layout.abi.is_uninhabited() {
+                    throw_ub_format!("attempted to instantiate uninhabited type `{}`", ty);
+                }
+                if intrinsic_name == sym::assert_zero_valid
+                    && !layout.might_permit_raw_init(self, /*zero:*/ true).unwrap()
+                {
+                    throw_ub_format!(
+                        "attempted to zero-initialize type `{}`, which is invalid",
+                        ty
+                    );
+                }
+                if intrinsic_name == sym::assert_uninit_valid
+                    && !layout.might_permit_raw_init(self, /*zero:*/ false).unwrap()
+                {
+                    throw_ub_format!(
+                        "attempted to leave type `{}` uninitialized, which is invalid",
+                        ty
+                    );
+                }
+            }
             sym::simd_insert => {
                 let index = u64::from(self.read_scalar(args[1])?.to_u32()?);
                 let elem = args[2];

library/core/src/intrinsics.rs

@@ -815,6 +815,7 @@ extern "rust-intrinsic" {
     /// This will statically either panic, or do nothing.
     ///
     /// This intrinsic does not have a stable counterpart.
+    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
     pub fn assert_inhabited<T>();
 
     /// A guard for unsafe functions that cannot ever be executed if `T` does not permit

library/core/src/lib.rs

@@ -100,6 +100,7 @@
 #![feature(const_type_name)]
 #![feature(const_likely)]
 #![feature(const_unreachable_unchecked)]
+#![feature(const_maybe_assume_init)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

library/core/src/mem/maybe_uninit.rs

@@ -314,8 +314,10 @@ impl<T> MaybeUninit<T> {
     /// let data = read(&mut buf);
     /// ```
     #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
+    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
+    #[rustc_allow_const_fn_unstable(const_maybe_assume_init)]
     #[inline(always)]
-    pub fn uninit_array<const LEN: usize>() -> [Self; LEN] {
+    pub const fn uninit_array<const LEN: usize>() -> [Self; LEN] {
         // SAFETY: An uninitialized `[MaybeUninit<_>; LEN]` is valid.
         unsafe { MaybeUninit::<[MaybeUninit<T>; LEN]>::uninit().assume_init() }
     }
@@ -503,9 +505,10 @@ impl<T> MaybeUninit<T> {
     /// // `x` had not been initialized yet, so this last line caused undefined behavior. ⚠️
     /// ```
     #[stable(feature = "maybe_uninit", since = "1.36.0")]
+    #[rustc_const_unstable(feature = "const_maybe_assume_init", issue = "none")]
     #[inline(always)]
     #[rustc_diagnostic_item = "assume_init"]
-    pub unsafe fn assume_init(self) -> T {
+    pub const unsafe fn assume_init(self) -> T {
         // SAFETY: the caller must guarantee that `self` is initialized.
         // This also means that `self` must be a `value` variant.
         unsafe {