Constier maybe uninit

compiler/rustc_mir/src/const_eval/error.rs

@@ -20,6 +20,7 @@ pub enum ConstEvalErrKind {
     ModifiedGlobal,
     AssertFailure(AssertKind<ConstInt>),
     Panic { msg: Symbol, line: u32, col: u32, file: Symbol },
+    Abort(String),
 }
 
 // The errors become `MachineStop` with plain strings when being raised.
@@ -46,6 +47,7 @@ impl fmt::Display for ConstEvalErrKind {
             Panic { msg, line, col, file } => {
                 write!(f, "the evaluated program panicked at '{}', {}:{}:{}", msg, file, line, col)
             }
+            Abort(ref msg) => write!(f, "{}", msg),
         }
     }
 }

compiler/rustc_mir/src/const_eval/machine.rs

@@ -333,6 +333,10 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         Err(ConstEvalErrKind::AssertFailure(err).into())
     }
 
+    fn abort(_ecx: &mut InterpCx<'mir, 'tcx, Self>, msg: String) -> InterpResult<'tcx, !> {
+        Err(ConstEvalErrKind::Abort(msg).into())
+    }
+
     fn ptr_to_int(_mem: &Memory<'mir, 'tcx, Self>, _ptr: Pointer) -> InterpResult<'tcx, u64> {
         Err(ConstEvalErrKind::NeedsRfc("pointer-to-integer cast".to_string()).into())
     }

compiler/rustc_mir/src/interpret/intrinsics.rs

@@ -126,7 +126,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             None => match intrinsic_name {
                 sym::transmute => throw_ub_format!("transmuting to uninhabited type"),
                 sym::unreachable => throw_ub!(Unreachable),
-                sym::abort => M::abort(self)?,
+                sym::abort => M::abort(self, "the program aborted execution".to_owned())?,
                 // Unsupported diverging intrinsic.
                 _ => return Ok(false),
             },
@@ -407,6 +407,16 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             sym::transmute => {
                 self.copy_op_transmute(args[0], dest)?;
             }
+            sym::assert_inhabited => {
+                let ty = instance.substs.type_at(0);
+                let layout = self.layout_of(ty)?;
+
+                if layout.abi.is_uninhabited() {
+                    // The run-time intrinsic panics just to get a good backtrace; here we abort
+                    // since there is no problem showing a backtrace even for aborts.
+                    M::abort(self, format!("attempted to instantiate uninhabited type `{}`", ty))?;
+                }
+            }
             sym::simd_insert => {
                 let index = u64::from(self.read_scalar(args[1])?.to_u32()?);
                 let elem = args[2];

compiler/rustc_mir/src/interpret/machine.rs

@@ -176,7 +176,7 @@ pub trait Machine<'mir, 'tcx>: Sized {
     ) -> InterpResult<'tcx>;
 
     /// Called to evaluate `Abort` MIR terminator.
-    fn abort(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx, !> {
+    fn abort(_ecx: &mut InterpCx<'mir, 'tcx, Self>, _msg: String) -> InterpResult<'tcx, !> {
         throw_unsup_format!("aborting execution is not supported")
     }
 

compiler/rustc_mir/src/interpret/terminator.rs

@@ -110,7 +110,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             }
 
             Abort => {
-                M::abort(self)?;
+                M::abort(self, "the program aborted execution".to_owned())?;
             }
 
             // When we encounter Resume, we've finished unwinding

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_assert_type", 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

@@ -69,6 +69,7 @@
 #![feature(asm)]
 #![feature(cfg_target_has_atomic)]
 #![feature(const_alloc_layout)]
+#![feature(const_assert_type)]
 #![feature(const_discriminant)]
 #![feature(const_cell_into_inner)]
 #![feature(const_checked_int_methods)]
@@ -92,6 +93,7 @@
 #![feature(const_ptr_offset)]
 #![feature(const_ptr_offset_from)]
 #![feature(const_raw_ptr_comparison)]
+#![feature(const_raw_ptr_deref)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_slice_ptr_len)]
 #![feature(const_size_of_val)]
@@ -100,6 +102,7 @@
 #![feature(const_type_name)]
 #![feature(const_likely)]
 #![feature(const_unreachable_unchecked)]
+#![feature(const_maybe_uninit_assume_init)]
 #![feature(custom_inner_attributes)]
 #![feature(decl_macro)]
 #![feature(doc_cfg)]

library/core/src/mem/maybe_uninit.rs

@@ -314,8 +314,9 @@ impl<T> MaybeUninit<T> {
     /// let data = read(&mut buf);
     /// ```
     #[unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
+    #[rustc_const_unstable(feature = "maybe_uninit_uninit_array", issue = "none")]
     #[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 +504,10 @@ 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_uninit_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 {
@@ -810,8 +812,9 @@ impl<T> MaybeUninit<T> {
     ///
     /// [`assume_init_ref`]: MaybeUninit::assume_init_ref
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
     #[inline(always)]
-    pub unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
+    pub const unsafe fn slice_assume_init_ref(slice: &[Self]) -> &[T] {
         // SAFETY: casting slice to a `*const [T]` is safe since the caller guarantees that
         // `slice` is initialized, and`MaybeUninit` is guaranteed to have the same layout as `T`.
         // The pointer obtained is valid since it refers to memory owned by `slice` which is a
@@ -831,8 +834,9 @@ impl<T> MaybeUninit<T> {
     ///
     /// [`assume_init_mut`]: MaybeUninit::assume_init_mut
     #[unstable(feature = "maybe_uninit_slice", issue = "63569")]
+    #[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
     #[inline(always)]
-    pub unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
+    pub const unsafe fn slice_assume_init_mut(slice: &mut [Self]) -> &mut [T] {
         // SAFETY: similar to safety notes for `slice_get_ref`, but we have a
         // mutable reference which is also guaranteed to be valid for writes.
         unsafe { &mut *(slice as *mut [Self] as *mut [T]) }

library/core/tests/lib.rs

@@ -11,6 +11,7 @@
 #![feature(cfg_target_has_atomic)]
 #![feature(const_assume)]
 #![feature(const_cell_into_inner)]
+#![feature(const_maybe_uninit_assume_init)]
 #![feature(core_intrinsics)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]

library/core/tests/mem.rs

@@ -129,3 +129,11 @@ fn test_discriminant_send_sync() {
     is_send_sync::<Discriminant<Regular>>();
     is_send_sync::<Discriminant<NotSendSync>>();
 }
+
+#[test]
+#[cfg(not(bootstrap))]
+fn assume_init_good() {
+    const TRUE: bool = unsafe { MaybeUninit::<bool>::new(true).assume_init() };
+
+    assert!(TRUE);
+}

src/test/ui/assume-type-intrinsics.rs

@@ -0,0 +1,11 @@
+#![feature(never_type)]
+#![feature(const_maybe_uninit_assume_init)]
+
+fn main() {
+    use std::mem::MaybeUninit;
+
+    const _BAD: () = unsafe {
+        MaybeUninit::<!>::uninit().assume_init();
+        //~^ WARN: the type `!` does not permit being left uninitialized
+    };
+}

src/test/ui/assume-type-intrinsics.stderr

@@ -0,0 +1,34 @@
+error: any use of this value will cause an error
+  --> $SRC_DIR/core/src/mem/maybe_uninit.rs:LL:COL
+   |
+LL |               intrinsics::assert_inhabited::<T>();
+   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |               |
+   |               attempted to instantiate uninhabited type `!`
+   |               inside `MaybeUninit::<!>::assume_init` at $SRC_DIR/core/src/mem/maybe_uninit.rs:LL:COL
+   |               inside `_BAD` at $DIR/assume-type-intrinsics.rs:8:9
+   | 
+  ::: $DIR/assume-type-intrinsics.rs:7:5
+   |
+LL | /     const _BAD: () = unsafe {
+LL | |         MaybeUninit::<!>::uninit().assume_init();
+LL | |
+LL | |     };
+   | |______-
+   |
+   = note: `#[deny(const_err)]` on by default
+
+warning: the type `!` does not permit being left uninitialized
+  --> $DIR/assume-type-intrinsics.rs:8:9
+   |
+LL |         MaybeUninit::<!>::uninit().assume_init();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |         |
+   |         this code causes undefined behavior when executed
+   |         help: use `MaybeUninit<T>` instead, and only call `assume_init` after initialization is done
+   |
+   = note: `#[warn(invalid_value)]` on by default
+   = note: the `!` type has no valid value
+
+error: aborting due to previous error; 1 warning emitted
+