Auto merge of #117537 - GKFX:offset-of-enum-feature, r=cjgillot

Feature gate enums in offset_of

As requested at https://github.com/rust-lang/rust/issues/106655#issuecomment-1790815262, put enums in offset_of behind their own feature gate.

`@rustbot` label F-offset_of

4 files changed, 14 insertions, 2 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes/E0795.md b/compiler/rustc_error_codes/src/error_codes/E0795.md
index 8b4b2dc87fd..20f51441c29 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0795.md
+++ b/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)]
+#![feature(offset_of, offset_of_enum)]
 
 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)]
+#![feature(offset_of, offset_of_enum)]
 
 let x: usize = std::mem::offset_of!(Option<u8>, Some.0);
 ```
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 11782e33d84..64b5a7d2921 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -526,6 +526,8 @@ declare_features! (
     /// In that case, `dyn Trait: Trait` does not hold. Moreover, coercions and
     /// casts in safe Rust to `dyn Trait` for such a `Trait` is also forbidden.
     (unstable, object_safe_for_dispatch, "1.40.0", Some(43561), None),
+    /// Allows using enums in offset_of!
+    (unstable, offset_of_enum, "CURRENT_RUSTC_VERSION", Some(106655), None),
     /// Allows using `#[optimize(X)]`.
     (unstable, optimize_attribute, "1.34.0", Some(54882), None),
     /// Allows exhaustive integer pattern matching on `usize` and `isize`.
diff --git a/compiler/rustc_hir_typeck/src/expr.rs b/compiler/rustc_hir_typeck/src/expr.rs
index a11fe10c01c..8bcec3273d8 100644
--- a/compiler/rustc_hir_typeck/src/expr.rs
+++ b/compiler/rustc_hir_typeck/src/expr.rs
@@ -3119,6 +3119,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     let (ident, _def_scope) =
                         self.tcx.adjust_ident_and_get_scope(field, container_def.did(), block);
 
+                    if !self.tcx.features().offset_of_enum {
+                        rustc_session::parse::feature_err(
+                            &self.tcx.sess.parse_sess,
+                            sym::offset_of_enum,
+                            ident.span,
+                            "using enums in offset_of is experimental",
+                        ).emit();
+                    }
+
                     let Some((index, variant)) = container_def.variants()
                         .iter_enumerated()
                         .find(|(_, v)| v.ident(self.tcx).normalize_to_macros_2_0() == ident) else {
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index ff470ce1fa0..1a1a1668a3c 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -1114,6 +1114,7 @@ symbols! {
         off,
         offset,
         offset_of,
+        offset_of_enum,
         omit_gdb_pretty_printer_section,
         on,
         on_unimplemented,