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Stabilize offset_of_slice #139673

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2 changes: 2 additions & 0 deletions compiler/rustc_feature/src/accepted.rs
View file Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -318,6 +318,8 @@ declare_features! (
(accepted, non_modrs_mods, "1.30.0", Some(44660)),
/// Allows using multiple nested field accesses in offset_of!
(accepted, offset_of_nested, "1.82.0", Some(120140)),
/// Allows using fields with slice type in offset_of!
(accepted, offset_of_slice, "CURRENT_RUSTC_VERSION", Some(126151)),
/// Allows the use of or-patterns (e.g., `0 | 1`).
(accepted, or_patterns, "1.53.0", Some(54883)),
/// Allows using `+bundle,+whole-archive` link modifiers with native libs.
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2 changes: 0 additions & 2 deletions compiler/rustc_feature/src/unstable.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -589,8 +589,6 @@ declare_features! (
(incomplete, non_lifetime_binders, "1.69.0", Some(108185)),
/// Allows using enums in offset_of!
(unstable, offset_of_enum, "1.75.0", Some(120141)),
/// Allows using fields with slice type in offset_of!
(unstable, offset_of_slice, "1.81.0", Some(126151)),
/// Allows using `#[optimize(X)]`.
(unstable, optimize_attribute, "1.34.0", Some(54882)),
/// Allows specifying nop padding on functions for dynamic patching.
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20 changes: 2 additions & 18 deletions compiler/rustc_hir_typeck/src/expr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3946,15 +3946,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
{
let field_ty = self.field_ty(expr.span, field, args);

if self.tcx.features().offset_of_slice() {
self.require_type_has_static_alignment(field_ty, expr.span);
} else {
self.require_type_is_sized(
field_ty,
expr.span,
ObligationCauseCode::Misc,
);
}
self.require_type_has_static_alignment(field_ty, expr.span);

if field.vis.is_accessible_from(def_scope, self.tcx) {
self.tcx.check_stability(field.did, Some(expr.hir_id), expr.span, None);
Expand All @@ -3975,15 +3967,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
&& field.name == sym::integer(index)
{
if let Some(&field_ty) = tys.get(index) {
if self.tcx.features().offset_of_slice() {
self.require_type_has_static_alignment(field_ty, expr.span);
} else {
self.require_type_is_sized(
field_ty,
expr.span,
ObligationCauseCode::Misc,
);
}
self.require_type_has_static_alignment(field_ty, expr.span);
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@compiler-errors compiler-errors Apr 11, 2025

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Could you please add a comment onto require_type_has_static_alignment that notes that changing the behavior will extend the functionality of offset_of and is insta-stable?

WaffleLapkin reacted with thumbs up emoji

field_indices.push((FIRST_VARIANT, index.into()));
current_container = field_ty;
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15 changes: 7 additions & 8 deletions library/core/src/mem/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1263,10 +1263,10 @@ impl<T> SizedTypeProperties for T {}
///
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@compiler-errors compiler-errors Apr 11, 2025

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Duplicate this item, mark one as #[cfg(bootstrap)] and one as #[cfg(not(bootstrap))]. Give the #[cfg(bootstrap)] one dummy documentation like /// This item will not be documented so it doesn't get doc-tested in stage 0.

/// # Offsets of, and in, dynamically sized types
///
/// The field’s type must be [`Sized`], but it may be located in a [dynamically sized] container.
/// If the field type is dynamically sized, then you cannot use `offset_of!` (since the field's
/// alignment, and therefore its offset, may also be dynamic) and must take the offset from an
/// actual pointer to the container instead.
/// The field’s type must have a statically known alignment. In other words, it is [`Sized`], a slice,
/// or some wrapper around a slice. Notably, this is not the case if the field is a trait object.
/// The alignment of trait objects can be different based on what the underlying type is, which can
/// affect the offset of the field. Therefore you cannot use `offset_of!`.
///
/// ```
/// # use core::mem;
Expand All @@ -1281,8 +1281,9 @@ impl<T> SizedTypeProperties for T {}
/// #[repr(C, align(4))]
/// struct Align4(u32);
///
/// assert_eq!(mem::offset_of!(Struct<dyn Debug>, a), 0); // OK — Sized field
/// assert_eq!(mem::offset_of!(Struct<Align4>, b), 4); // OK — not DST
/// assert_eq!(mem::offset_of!(Struct<Align4>, b), 4); // OK — the last field is Sized
/// assert_eq!(mem::offset_of!(Struct<[u8]>, b), 1); // OK — the last field is a slice
/// assert_eq!(mem::offset_of!(Struct<dyn Debug>, a), 0); // OK — the struct is unsized, but the field `a` is Sized
///
/// // assert_eq!(mem::offset_of!(Struct<dyn Debug>, b), 1);
/// // ^^^ error[E0277]: ... cannot be known at compilation time
Expand Down Expand Up @@ -1344,7 +1345,6 @@ impl<T> SizedTypeProperties for T {}
/// The following unstable features expand the functionality of `offset_of!`:
///
/// * [`offset_of_enum`] — allows `enum` variants to be traversed as if they were fields.
/// * [`offset_of_slice`] — allows getting the offset of a field of type `[T]`.
///
/// # Examples
///
Expand Down Expand Up @@ -1374,7 +1374,6 @@ impl<T> SizedTypeProperties for T {}
///
/// [dynamically sized]: https://doc.rust-lang.org/reference/dynamically-sized-types.html
/// [`offset_of_enum`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-enum.html
/// [`offset_of_slice`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-slice.html
#[stable(feature = "offset_of", since = "1.77.0")]
#[allow_internal_unstable(builtin_syntax)]
pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
Expand Down
30 changes: 0 additions & 30 deletions src/doc/unstable-book/src/language-features/offset-of-slice.md
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This file was deleted.

2 changes: 0 additions & 2 deletions tests/ui/offset-of/offset-of-slice-normalized.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,8 +3,6 @@
//@[next] compile-flags: -Znext-solver
//@ run-pass

#![feature(offset_of_slice)]

use std::mem::offset_of;

trait Mirror {
Expand Down
1 change: 0 additions & 1 deletion tests/ui/offset-of/offset-of-slice.rs
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Original file line number Diff line number Diff line change
@@ -1,5 +1,4 @@
//@run-pass
#![feature(offset_of_slice)]

use std::mem::offset_of;

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