diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs index f435f503fc160..5b7062dec7a56 100644 --- a/library/alloc/src/lib.rs +++ b/library/alloc/src/lib.rs @@ -113,7 +113,6 @@ #![feature(const_eval_select)] #![feature(const_maybe_uninit_as_mut_ptr)] #![feature(const_maybe_uninit_write)] -#![feature(const_maybe_uninit_zeroed)] #![feature(const_pin)] #![feature(const_refs_to_cell)] #![feature(const_size_of_val)] diff --git a/library/core/src/mem/maybe_uninit.rs b/library/core/src/mem/maybe_uninit.rs index d09a24b4b1d5b..a86ccaeb839c6 100644 --- a/library/core/src/mem/maybe_uninit.rs +++ b/library/core/src/mem/maybe_uninit.rs @@ -374,6 +374,9 @@ impl MaybeUninit { /// assert_eq!(x, (0, false)); /// ``` /// + /// This can be used in const contexts, such as to indicate the end of static arrays for + /// plugin registration. + /// /// *Incorrect* usage of this function: calling `x.zeroed().assume_init()` /// when `0` is not a valid bit-pattern for the type: /// @@ -387,17 +390,19 @@ impl MaybeUninit { /// // Inside a pair, we create a `NotZero` that does not have a valid discriminant. /// // This is undefined behavior. ⚠️ /// ``` - #[stable(feature = "maybe_uninit", since = "1.36.0")] - #[rustc_const_unstable(feature = "const_maybe_uninit_zeroed", issue = "91850")] - #[must_use] #[inline] + #[must_use] #[rustc_diagnostic_item = "maybe_uninit_zeroed"] + #[stable(feature = "maybe_uninit", since = "1.36.0")] + // These are OK to allow since we do not leak &mut to user-visible API + #[rustc_allow_const_fn_unstable(const_mut_refs)] + #[rustc_allow_const_fn_unstable(const_ptr_write)] + #[rustc_allow_const_fn_unstable(const_maybe_uninit_as_mut_ptr)] + #[rustc_const_stable(feature = "const_maybe_uninit_zeroed", since = "CURRENT_RUSTC_VERSION")] pub const fn zeroed() -> MaybeUninit { let mut u = MaybeUninit::::uninit(); // SAFETY: `u.as_mut_ptr()` points to allocated memory. - unsafe { - u.as_mut_ptr().write_bytes(0u8, 1); - } + unsafe { u.as_mut_ptr().write_bytes(0u8, 1) }; u } diff --git a/library/core/tests/mem.rs b/library/core/tests/mem.rs index 5c2e18745ea21..20498b16cb22b 100644 --- a/library/core/tests/mem.rs +++ b/library/core/tests/mem.rs @@ -565,3 +565,24 @@ fn offset_of_addr() { assert_eq!(ptr::addr_of!(base).addr() + offset_of!(Foo, z.0), ptr::addr_of!(base.z.0).addr()); assert_eq!(ptr::addr_of!(base).addr() + offset_of!(Foo, z.1), ptr::addr_of!(base.z.1).addr()); } + +#[test] +fn const_maybe_uninit_zeroed() { + // Sanity check for `MaybeUninit::zeroed` in a realistic const situation (plugin array term) + #[repr(C)] + struct Foo { + a: Option<&'static str>, + b: Bar, + c: f32, + d: *const u8, + } + #[repr(C)] + struct Bar(usize); + struct FooPtr(*const Foo); + unsafe impl Sync for FooPtr {} + + static UNINIT: FooPtr = FooPtr([unsafe { MaybeUninit::zeroed().assume_init() }].as_ptr()); + const SIZE: usize = size_of::(); + + assert_eq!(unsafe { (*UNINIT.0.cast::<[[u8; SIZE]; 1]>())[0] }, [0u8; SIZE]); +}