Rollup merge of rust-lang#118799 - GKFX:stabilize-simple-offsetof, r=wesleywiser

Stabilize single-field offset_of

This PR stabilizes offset_of for a single field. There has been some further discussion at rust-lang#106655 about whether this is advisable; I'm opening the PR anyway so that the code is available.

Author: matthiaskrgr

core/src/lib.rs

@@ -111,6 +111,7 @@
 //
 // Library features:
 // tidy-alphabetical-start
+#![cfg_attr(not(bootstrap), feature(offset_of_nested))]
 #![feature(char_indices_offset)]
 #![feature(const_align_of_val)]
 #![feature(const_align_of_val_raw)]
@@ -176,7 +177,6 @@
 #![feature(isqrt)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(non_null_convenience)]
-#![feature(offset_of)]
 #![feature(offset_of_enum)]
 #![feature(panic_internals)]
 #![feature(ptr_alignment_type)]

core/src/mem/mod.rs

@@ -1303,11 +1303,12 @@ impl<T> SizedTypeProperties for T {}
 /// Enum variants may be traversed as if they were fields. Variants themselves do
 /// not have an offset.
 ///
+/// However, on stable only a single field name is supported, which blocks the use of
+/// enum support.
+///
 /// Visibility is respected - all types and fields must be visible to the call site:
 ///
 /// ```
-/// #![feature(offset_of)]
-///
 /// mod nested {
 ///     #[repr(C)]
 ///     pub struct Struct {
@@ -1330,8 +1331,6 @@ impl<T> SizedTypeProperties for T {}
 /// not *identical*, e.g.:
 ///
 /// ```
-/// #![feature(offset_of)]
-///
 /// struct Wrapper<T, U>(T, U);
 ///
 /// type A = Wrapper<u8, u8>;
@@ -1359,8 +1358,7 @@ impl<T> SizedTypeProperties for T {}
 /// # Examples
 ///
 /// ```
-/// #![feature(offset_of)]
-/// # #![feature(offset_of_enum)]
+/// #![feature(offset_of_enum, offset_of_nested)]
 ///
 /// use std::mem;
 /// #[repr(C)]
@@ -1396,15 +1394,15 @@ impl<T> SizedTypeProperties for T {}
 /// assert_eq!(mem::offset_of!(Option<&u8>, Some.0), 0);
 /// ```
 #[cfg(not(bootstrap))]
-#[unstable(feature = "offset_of", issue = "106655")]
+#[stable(feature = "offset_of", since = "CURRENT_RUSTC_VERSION")]
 #[allow_internal_unstable(builtin_syntax, hint_must_use)]
 pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
     // The `{}` is for better error messages
     crate::hint::must_use({builtin # offset_of($Container, $($fields)+)})
 }
 
 #[cfg(bootstrap)]
-#[unstable(feature = "offset_of", issue = "106655")]
+#[stable(feature = "offset_of", since = "CURRENT_RUSTC_VERSION")]
 #[allow_internal_unstable(builtin_syntax, hint_must_use)]
 #[allow(missing_docs)]
 pub macro offset_of($Container:ty, $($fields:tt).+ $(,)?) {

core/tests/lib.rs

@@ -59,6 +59,7 @@
 #![feature(noop_waker)]
 #![feature(numfmt)]
 #![feature(num_midpoint)]
+#![cfg_attr(not(bootstrap), feature(offset_of_nested))]
 #![feature(isqrt)]
 #![feature(step_trait)]
 #![feature(str_internals)]
@@ -115,7 +116,6 @@
 #![feature(utf8_chunks)]
 #![feature(is_ascii_octdigit)]
 #![feature(get_many_mut)]
-#![feature(offset_of)]
 #![feature(iter_map_windows)]
 #![allow(internal_features)]
 #![deny(unsafe_op_in_unsafe_fn)]

std/src/lib.rs

@@ -329,7 +329,6 @@
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_write_slice)]
-#![feature(offset_of)]
 #![feature(panic_can_unwind)]
 #![feature(panic_info_message)]
 #![feature(panic_internals)]