From f6ce646d3f840181dcc3f5d9edc4e4cff0051b78 Mon Sep 17 00:00:00 2001 From: Trevor Gross Date: Thu, 28 Sep 2023 01:57:01 -0400 Subject: [PATCH 1/2] Stabilize `const_maybe_uninit_zeroed` Make `MaybeUninit::zeroed` const stable. Newly stable API: // core::mem impl MaybeUninit { pub const fn zeroed() -> MaybeUninit; } Use of `const_mut_refs` should be acceptable since we do not leak the mutability. Tracking issue: #91850 --- library/alloc/src/lib.rs | 1 - library/core/src/mem/maybe_uninit.rs | 17 +++++++++++------ library/core/tests/mem.rs | 21 +++++++++++++++++++++ 3 files changed, 32 insertions(+), 7 deletions(-) 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]); +} From 5e5f3341e3749f18a6d6fbb2228a9d3b42c0b280 Mon Sep 17 00:00:00 2001 From: Trevor Gross Date: Fri, 29 Sep 2023 02:40:30 -0400 Subject: [PATCH 2/2] Stabilize `const_mem_zeroed` Make `core::mem::zeroed` const stable. Newly stable API: // core::mem pub const unsafe fn zeroed() -> T; This is stabilized with `const_maybe_uninit_zeroed` since it is a simple wrapper. In order to make this possible, intrinsics `assert_zero_valid` was made const stable under `const_assert_type2`. `assert_mem_uninitialized_valid` was also made const stable since it is under the same gate. --- library/core/src/intrinsics.rs | 4 ++-- library/core/src/mem/mod.rs | 3 ++- tests/ui/consts/assert-type-intrinsics.rs | 1 - tests/ui/consts/assert-type-intrinsics.stderr | 12 ++++++------ 4 files changed, 10 insertions(+), 10 deletions(-) diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs index 4c76662ac0975..40dc323032eda 100644 --- a/library/core/src/intrinsics.rs +++ b/library/core/src/intrinsics.rs @@ -1072,7 +1072,7 @@ extern "rust-intrinsic" { /// zero-initialization: This will statically either panic, or do nothing. /// /// This intrinsic does not have a stable counterpart. - #[rustc_const_unstable(feature = "const_assert_type2", issue = "none")] + #[rustc_const_stable(feature = "const_assert_type2", since = "CURRENT_RUSTC_VERSION")] #[rustc_safe_intrinsic] #[rustc_nounwind] pub fn assert_zero_valid(); @@ -1080,7 +1080,7 @@ extern "rust-intrinsic" { /// A guard for `std::mem::uninitialized`. This will statically either panic, or do nothing. /// /// This intrinsic does not have a stable counterpart. - #[rustc_const_unstable(feature = "const_assert_type2", issue = "none")] + #[rustc_const_stable(feature = "const_assert_type2", since = "CURRENT_RUSTC_VERSION")] #[rustc_safe_intrinsic] #[rustc_nounwind] pub fn assert_mem_uninitialized_valid(); diff --git a/library/core/src/mem/mod.rs b/library/core/src/mem/mod.rs index d7abc9a0e23aa..82ce901140ada 100644 --- a/library/core/src/mem/mod.rs +++ b/library/core/src/mem/mod.rs @@ -647,7 +647,8 @@ pub const fn needs_drop() -> bool { #[allow(deprecated)] #[rustc_diagnostic_item = "mem_zeroed"] #[track_caller] -pub unsafe fn zeroed() -> T { +#[rustc_const_stable(feature = "const_mem_zeroed", since = "CURRENT_RUSTC_VERSION")] +pub const unsafe fn zeroed() -> T { // SAFETY: the caller must guarantee that an all-zero value is valid for `T`. unsafe { intrinsics::assert_zero_valid::(); diff --git a/tests/ui/consts/assert-type-intrinsics.rs b/tests/ui/consts/assert-type-intrinsics.rs index b4fd423becd9d..32b5f5c92c52d 100644 --- a/tests/ui/consts/assert-type-intrinsics.rs +++ b/tests/ui/consts/assert-type-intrinsics.rs @@ -1,5 +1,4 @@ #![feature(never_type)] -#![feature(const_assert_type2)] #![feature(core_intrinsics)] use std::intrinsics; diff --git a/tests/ui/consts/assert-type-intrinsics.stderr b/tests/ui/consts/assert-type-intrinsics.stderr index 3c03b03deee35..66c4f0f9cd65f 100644 --- a/tests/ui/consts/assert-type-intrinsics.stderr +++ b/tests/ui/consts/assert-type-intrinsics.stderr @@ -1,20 +1,20 @@ error[E0080]: evaluation of constant value failed - --> $DIR/assert-type-intrinsics.rs:12:9 + --> $DIR/assert-type-intrinsics.rs:11:9 | LL | MaybeUninit::::uninit().assume_init(); - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'aborted execution: attempted to instantiate uninhabited type `!`', $DIR/assert-type-intrinsics.rs:12:36 + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'aborted execution: attempted to instantiate uninhabited type `!`', $DIR/assert-type-intrinsics.rs:11:36 error[E0080]: evaluation of constant value failed - --> $DIR/assert-type-intrinsics.rs:16:9 + --> $DIR/assert-type-intrinsics.rs:15:9 | LL | intrinsics::assert_mem_uninitialized_valid::<&'static i32>(); - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'aborted execution: attempted to leave type `&i32` uninitialized, which is invalid', $DIR/assert-type-intrinsics.rs:16:9 + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'aborted execution: attempted to leave type `&i32` uninitialized, which is invalid', $DIR/assert-type-intrinsics.rs:15:9 error[E0080]: evaluation of constant value failed - --> $DIR/assert-type-intrinsics.rs:20:9 + --> $DIR/assert-type-intrinsics.rs:19:9 | LL | intrinsics::assert_zero_valid::<&'static i32>(); - | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'aborted execution: attempted to zero-initialize type `&i32`, which is invalid', $DIR/assert-type-intrinsics.rs:20:9 + | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the evaluated program panicked at 'aborted execution: attempted to zero-initialize type `&i32`, which is invalid', $DIR/assert-type-intrinsics.rs:19:9 error: aborting due to 3 previous errors