Rollup merge of #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 #106655 about whether this is advisable; I'm opening the PR anyway so that the code is available.

Author: matthiaskrgr

compiler/rustc_error_codes/src/error_codes/E0795.md

@@ -3,7 +3,7 @@ Invalid argument for the `offset_of!` macro.
 Erroneous code example:
 
 ```compile_fail,E0795
-#![feature(offset_of, offset_of_enum)]
+#![feature(offset_of_enum, offset_of_nested)]
 
 let x = std::mem::offset_of!(Option<u8>, Some);
 ```
@@ -16,7 +16,7 @@ The offset of the contained `u8` in the `Option<u8>` can be found by specifying
 the field name `0`:
 
 ```
-#![feature(offset_of, offset_of_enum)]
+#![feature(offset_of_enum, offset_of_nested)]
 
 let x: usize = std::mem::offset_of!(Option<u8>, Some.0);
 ```

compiler/rustc_feature/src/unstable.rs

@@ -549,7 +549,9 @@ declare_features! (
     /// casts in safe Rust to `dyn Trait` for such a `Trait` is also forbidden.
     (unstable, object_safe_for_dispatch, "1.40.0", Some(43561)),
     /// Allows using enums in offset_of!
-    (unstable, offset_of_enum, "1.75.0", Some(106655)),
+    (unstable, offset_of_enum, "1.75.0", Some(120141)),
+    /// Allows using multiple nested field accesses in offset_of!
+    (unstable, offset_of_nested, "CURRENT_RUSTC_VERSION", Some(120140)),
     /// Allows using `#[optimize(X)]`.
     (unstable, optimize_attribute, "1.34.0", Some(54882)),
     /// Allows macro attributes on expressions, statements and non-inline modules.

compiler/rustc_hir_typeck/src/expr.rs

@@ -3246,6 +3246,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     ) -> Ty<'tcx> {
         let container = self.to_ty(container).normalized;
 
+        if let Some(ident_2) = fields.get(1)
+            && !self.tcx.features().offset_of_nested
+        {
+            rustc_session::parse::feature_err(
+                &self.tcx.sess,
+                sym::offset_of_nested,
+                ident_2.span,
+                "only a single ident or integer is stable as the field in offset_of",
+            )
+            .emit();
+        }
+
         let mut field_indices = Vec::with_capacity(fields.len());
         let mut current_container = container;
         let mut fields = fields.into_iter();

compiler/rustc_span/src/symbol.rs

@@ -1153,6 +1153,7 @@ symbols! {
         offset,
         offset_of,
         offset_of_enum,
+        offset_of_nested,
         ok_or_else,
         omit_gdb_pretty_printer_section,
         on,

library/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)]

library/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).+ $(,)?) {

library/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)]

library/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)]

tests/mir-opt/const_prop/offset_of.rs

@@ -2,7 +2,7 @@
 // unit-test: GVN
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 
-#![feature(offset_of, offset_of_enum)]
+#![feature(offset_of_enum, offset_of_nested)]
 
 use std::marker::PhantomData;
 use std::mem::offset_of;

tests/mir-opt/dataflow-const-prop/offset_of.rs

@@ -2,7 +2,7 @@
 // unit-test: DataflowConstProp
 // EMIT_MIR_FOR_EACH_PANIC_STRATEGY
 
-#![feature(offset_of)]
+#![feature(offset_of_nested)]
 
 use std::marker::PhantomData;
 use std::mem::offset_of;