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Stabilize some MaybeUninit behavior as const #90896

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Nov 28, 2021
Merged

Stabilize some MaybeUninit behavior as const #90896

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ad8e6bf
Stabilize some `MaybeUninit` behavior as const
jhpratt Nov 14, 2021
44b5b83
Add test for const `MaybeUninit`
jhpratt Nov 17, 2021
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2 changes: 1 addition & 1 deletion library/core/src/intrinsics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -853,7 +853,7 @@ 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")]
#[rustc_const_stable(feature = "const_assert_type", since = "1.59.0")]
pub fn assert_inhabited<T>();

/// A guard for unsafe functions that cannot ever be executed if `T` does not permit
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3 changes: 1 addition & 2 deletions library/core/src/lib.rs
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Expand Up @@ -101,7 +101,6 @@
#![feature(const_align_of_val)]
#![feature(const_alloc_layout)]
#![feature(const_arguments_as_str)]
#![feature(const_assert_type)]
#![feature(const_bigint_helper_methods)]
#![feature(const_caller_location)]
#![feature(const_cell_into_inner)]
Expand All @@ -117,7 +116,7 @@
#![feature(const_intrinsic_copy)]
#![feature(const_intrinsic_forget)]
#![feature(const_likely)]
#![feature(const_maybe_uninit_as_ptr)]
#![feature(const_maybe_uninit_as_mut_ptr)]
#![feature(const_maybe_uninit_assume_init)]
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Can this be removed now?

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No, as the method takes a mutable parameter. const_mut_refs hasn't yet been stabilized.

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This is for assume_init, which is being stabilized... is it not? I am confused.

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Whoops, when I responded I thought misread where the code was. I'll check the necessity of this.

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Removing this results in the following errors:

error: `maybe_uninit::MaybeUninit::<T>::as_mut_ptr` is not yet stable as a const fn
   --> library/core/src/mem/maybe_uninit.rs:915:19
    |
915 |             &mut *self.as_mut_ptr()
    |                   ^^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(const_maybe_uninit_as_mut_ptr)]` to the crate attributes to enable

error: `maybe_uninit::MaybeUninit::<T>::as_mut_ptr` is not yet stable as a const fn
   --> library/core/src/ptr/mod.rs:374:32
    |
374 |         copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
    |                                ^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(const_maybe_uninit_as_mut_ptr)]` to the crate attributes to enable

error: `maybe_uninit::MaybeUninit::<T>::as_mut_ptr` is not yet stable as a const fn
   --> library/core/src/ptr/mod.rs:485:17
    |
485 |         let t = t.as_mut_ptr() as *mut u8;
    |                 ^^^^^^^^^^^^^^
    |
    = help: add `#![feature(const_maybe_uninit_as_mut_ptr)]` to the crate attributes to enable

error: `maybe_uninit::MaybeUninit::<T>::as_mut_ptr` is not yet stable as a const fn
   --> library/core/src/ptr/mod.rs:511:17
    |
511 |         let t = t.as_mut_ptr() as *mut u8;
    |                 ^^^^^^^^^^^^^^
    |
    = help: add `#![feature(const_maybe_uninit_as_mut_ptr)]` to the crate attributes to enable

error: `maybe_uninit::MaybeUninit::<T>::as_mut_ptr` is not yet stable as a const fn
   --> library/core/src/ptr/mod.rs:701:34
    |
701 |         copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
    |                                  ^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(const_maybe_uninit_as_mut_ptr)]` to the crate attributes to enable

error: `maybe_uninit::MaybeUninit::<T>::as_mut_ptr` is not yet stable as a const fn
   --> library/core/src/ptr/mod.rs:793:47
    |
793 |         copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, mem::size_of::<T>());
    |                                               ^^^^^^^^^^^^^^^^
    |
    = help: add `#![feature(const_maybe_uninit_as_mut_ptr)]` to the crate attributes to enable

error: could not compile `core` due to 6 previous errors

As such I'll be leaving the attribute in.

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@RalfJung RalfJung Nov 28, 2021 •
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I was asking about the const_maybe_uninit_assume_init, not the const_maybe_uninit_as_mut_ptr. (Note the line this comment is attached to.)

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Yeah, that probably should have been explicitly stated 🙂 I'll check when I'm on my laptop and send a follow-up if necessary.

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Thanks. :)

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Removing that line results in an error on line 496 of the same file (inside MaybeUninit::write). That's for the assume_init_mut method.

So...I think this means that const stability isn't currently subject to the same checks, in that a feature can be both simultaneously stable and unstable? If that's the case we should fix that.

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Ah I see, makes sense. I opened an issue for this: #91357

#![feature(const_num_from_num)]
#![feature(const_ops)]
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11 changes: 6 additions & 5 deletions library/core/src/mem/maybe_uninit.rs
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Expand Up @@ -528,7 +528,7 @@ impl<T> MaybeUninit<T> {
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
/// until they are, it is advisable to avoid them.)
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_as_ptr", issue = "75251")]
#[rustc_const_stable(feature = "const_maybe_uninit_as_ptr", since = "1.59.0")]
#[inline(always)]
pub const fn as_ptr(&self) -> *const T {
// `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
Expand Down Expand Up @@ -567,7 +567,7 @@ impl<T> MaybeUninit<T> {
/// (Notice that the rules around references to uninitialized data are not finalized yet, but
/// until they are, it is advisable to avoid them.)
#[stable(feature = "maybe_uninit", since = "1.36.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_as_ptr", issue = "75251")]
#[rustc_const_unstable(feature = "const_maybe_uninit_as_mut_ptr", issue = "75251")]
#[inline(always)]
pub const fn as_mut_ptr(&mut self) -> *mut T {
// `MaybeUninit` and `ManuallyDrop` are both `repr(transparent)` so we can cast the pointer.
Expand Down Expand Up @@ -620,7 +620,7 @@ 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")]
#[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.59.0")]
#[inline(always)]
#[rustc_diagnostic_item = "assume_init"]
#[track_caller]
Expand Down Expand Up @@ -788,7 +788,8 @@ impl<T> MaybeUninit<T> {
/// }
/// ```
#[stable(feature = "maybe_uninit_ref", since = "1.55.0")]
#[rustc_const_unstable(feature = "const_maybe_uninit_assume_init", issue = "none")]
#[rustc_const_stable(feature = "const_maybe_uninit_assume_init", since = "1.59.0")]
#[cfg_attr(bootstrap, rustc_allow_const_fn_unstable(const_raw_ptr_deref))]
#[inline(always)]
pub const unsafe fn assume_init_ref(&self) -> &T {
// SAFETY: the caller must guarantee that `self` is initialized.
Expand Down Expand Up @@ -968,7 +969,7 @@ 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")]
#[rustc_const_unstable(feature = "maybe_uninit_slice", issue = "63569")]
#[inline(always)]
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
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1 change: 1 addition & 0 deletions library/core/tests/lib.rs
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Expand Up @@ -10,6 +10,7 @@
#![feature(const_assume)]
#![feature(const_cell_into_inner)]
#![feature(const_convert)]
#![feature(const_maybe_uninit_as_mut_ptr)]
#![feature(const_maybe_uninit_assume_init)]
#![feature(const_ptr_read)]
#![feature(const_ptr_write)]
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32 changes: 32 additions & 0 deletions library/core/tests/mem.rs
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Expand Up @@ -269,3 +269,35 @@ fn uninit_const_assume_init_read() {
const FOO: u32 = unsafe { MaybeUninit::new(42).assume_init_read() };
assert_eq!(FOO, 42);
}

#[test]
fn const_maybe_uninit() {
use std::ptr;

#[derive(Debug, PartialEq)]
struct Foo {
x: u8,
y: u8,
}

const FIELD_BY_FIELD: Foo = unsafe {
let mut val = MaybeUninit::uninit();
init_y(&mut val); // order shouldn't matter
init_x(&mut val);
val.assume_init()
};

const fn init_x(foo: &mut MaybeUninit<Foo>) {
unsafe {
*ptr::addr_of_mut!((*foo.as_mut_ptr()).x) = 1;
}
}

const fn init_y(foo: &mut MaybeUninit<Foo>) {
unsafe {
*ptr::addr_of_mut!((*foo.as_mut_ptr()).y) = 2;
}
}

assert_eq!(FIELD_BY_FIELD, Foo { x: 1, y: 2 });
}