add mem::conjure_zst

Author: Qelxiros

library/core/src/mem/mod.rs

@@ -1407,3 +1407,55 @@ pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
     // The `{}` is for better error messages
     {builtin # offset_of($Container, $($fields)+)}
 }
+
+/// Create a fresh instance of the inhabited ZST type `T`.
+///
+/// Prefer this to [`zeroed`] or [`uninitialized`] or [`transmute_copy`]
+/// in places where you know that `T` is zero-sized, but don't have a bound
+/// (such as [`Default`]) that would allow you to instantiate it using safe code.
+///
+/// If you're not sure whether `T` is an inhabited ZST, then you should be
+/// using [`MaybeUninit`], not this function.
+///
+/// # Safety
+///
+/// - `size_of::<T>()` must be zero.
+///
+/// - `T` must be *inhabited*.  (It must not be a zero-variant `enum`, for example.)
+///
+/// - You must use the value only in ways which do not violate any *safety*
+///   invariants of the type.
+///
+/// While it's easy to create a *valid* instance of an inhabited ZST, since having
+/// no bits in its representation means there's only one possible value, that
+/// doesn't mean that it's always *sound* to do so.
+///
+/// For example, a library with a global semaphore could give out ZST tokens
+/// on `acquire`, and by them being `!Default`+`!Clone` could consume them
+/// in `release` to ensure that it's called at most once per `acquire`.
+/// Or a library could use a `!Default`+`!Send` token to ensure it's used only
+/// from the thread on which it was initialized.
+///
+/// # Examples
+///
+/// ```
+/// #![feature(mem_conjure_zst)]
+/// use std::mem::conjure_zst;
+///
+/// assert_eq!(unsafe { conjure_zst::<()>() }, ());
+/// assert_eq!(unsafe { conjure_zst::<[i32; 0]>() }, []);
+/// ```
+#[unstable(feature = "mem_conjure_zst", issue = "95383")]
+pub const unsafe fn conjure_zst<T>() -> T {
+    // This is not a guarantee exposed to clients, but it'll easily optimize out
+    // in the sound cases, so we might as well check because we can.
+    assert!(size_of::<T>() == 0); // FIXME: use assert_eq! once it's const
+
+    // SAFETY: because the caller must guarantee that it's inhabited and zero-sized,
+    // there's nothing in the representation that needs to be set.
+    // `assume_init` calls `assert_inhabited`, so we don't need to here.
+    unsafe {
+        #[allow(clippy::uninit_assumed_init)]
+        MaybeUninit::uninit().assume_init()
+    }
+}

tests/ui/consts/std/conjure_zst.rs

@@ -0,0 +1,10 @@
+#![feature(mem_conjure_zst)]
+
+use std::{convert::Infallible, mem};
+
+const INVALID: Infallible = unsafe { mem::conjure_zst() };
+//~^ ERROR attempted to instantiate uninhabited type
+
+const VALID: () = unsafe { mem::conjure_zst() };
+
+fn main() {}

tests/ui/consts/std/conjure_zst.stderr

@@ -0,0 +1,12 @@
+error[E0080]: evaluation panicked: aborted execution: attempted to instantiate uninhabited type `Infallible`
+  --> $DIR/conjure_zst.rs:5:38
+   |
+LL | const INVALID: Infallible = unsafe { mem::conjure_zst() };
+   |                                      ^^^^^^^^^^^^^^^^^^ evaluation of `INVALID` failed inside this call
+   |
+note: inside `conjure_zst::<Infallible>`
+  --> $SRC_DIR/core/src/mem/mod.rs:LL:COL
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0080`.