repr(transparent): do not consider repr(C) types to be 1-ZST

Author: RalfJung

compiler/rustc_hir_analysis/src/check/check.rs

@@ -12,9 +12,7 @@ use rustc_hir::def::{CtorKind, DefKind};
 use rustc_hir::{LangItem, Node, attrs, find_attr, intravisit};
 use rustc_infer::infer::{RegionVariableOrigin, TyCtxtInferExt};
 use rustc_infer::traits::{Obligation, ObligationCauseCode, WellFormedLoc};
-use rustc_lint_defs::builtin::{
-    REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS, UNSUPPORTED_CALLING_CONVENTIONS,
-};
+use rustc_lint_defs::builtin::{REPR_TRANSPARENT_NON_ZST_FIELDS, UNSUPPORTED_CALLING_CONVENTIONS};
 use rustc_middle::hir::nested_filter;
 use rustc_middle::middle::resolve_bound_vars::ResolvedArg;
 use rustc_middle::middle::stability::EvalResult;
@@ -1511,31 +1509,46 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
     }
 
     let typing_env = ty::TypingEnv::non_body_analysis(tcx, adt.did());
-    // For each field, figure out if it's known to have "trivial" layout (i.e., is a 1-ZST), with
-    // "known" respecting #[non_exhaustive] attributes.
+    // For each field, figure out if it has "trivial" layout (i.e., is a 1-ZST).
+    // Even some 1-ZST fields are not allowed though, if they have `non_exhaustive` or private
+    // fields or `repr(C)`. We call those fields "unsuited".
+    struct FieldInfo<'tcx> {
+        span: Span,
+        trivial: bool,
+        unsuited: Option<UnsuitedInfo<'tcx>>,
+    }
+    struct UnsuitedInfo<'tcx> {
+        /// The source of the problem, a type that is found somewhere within the field type.
+        ty: Ty<'tcx>,
+        reason: UnsuitedReason,
+    }
+    enum UnsuitedReason {
+        NonExhaustive,
+        PrivateField,
+        ReprC,
+    }
+
     let field_infos = adt.all_fields().map(|field| {
         let ty = field.ty(tcx, GenericArgs::identity_for_item(tcx, field.did));
         let layout = tcx.layout_of(typing_env.as_query_input(ty));
         // We are currently checking the type this field came from, so it must be local
         let span = tcx.hir_span_if_local(field.did).unwrap();
         let trivial = layout.is_ok_and(|layout| layout.is_1zst());
         if !trivial {
-            return (span, trivial, None);
+            // No need to even compute `unsuited`.
+            return FieldInfo { span, trivial, unsuited: None };
         }
-        // Even some 1-ZST fields are not allowed though, if they have `non_exhaustive`.
 
-        fn check_non_exhaustive<'tcx>(
+        fn check_unsuited<'tcx>(
             tcx: TyCtxt<'tcx>,
             typing_env: ty::TypingEnv<'tcx>,
-            t: Ty<'tcx>,
-        ) -> ControlFlow<(&'static str, DefId, GenericArgsRef<'tcx>, bool)> {
+            ty: Ty<'tcx>,
+        ) -> ControlFlow<UnsuitedInfo<'tcx>> {
             // We can encounter projections during traversal, so ensure the type is normalized.
-            let t = tcx.try_normalize_erasing_regions(typing_env, t).unwrap_or(t);
-            match t.kind() {
-                ty::Tuple(list) => {
-                    list.iter().try_for_each(|t| check_non_exhaustive(tcx, typing_env, t))
-                }
-                ty::Array(ty, _) => check_non_exhaustive(tcx, typing_env, *ty),
+            let ty = tcx.try_normalize_erasing_regions(typing_env, ty).unwrap_or(ty);
+            match ty.kind() {
+                ty::Tuple(list) => list.iter().try_for_each(|t| check_unsuited(tcx, typing_env, t)),
+                ty::Array(ty, _) => check_unsuited(tcx, typing_env, *ty),
                 ty::Adt(def, args) => {
                     if !def.did().is_local()
                         && !find_attr!(
@@ -1550,28 +1563,36 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
                                 .any(ty::VariantDef::is_field_list_non_exhaustive);
                         let has_priv = def.all_fields().any(|f| !f.vis.is_public());
                         if non_exhaustive || has_priv {
-                            return ControlFlow::Break((
-                                def.descr(),
-                                def.did(),
-                                args,
-                                non_exhaustive,
-                            ));
+                            return ControlFlow::Break(UnsuitedInfo {
+                                ty,
+                                reason: if non_exhaustive {
+                                    UnsuitedReason::NonExhaustive
+                                } else {
+                                    UnsuitedReason::PrivateField
+                                },
+                            });
                         }
                     }
+                    if def.repr().c() {
+                        return ControlFlow::Break(UnsuitedInfo {
+                            ty,
+                            reason: UnsuitedReason::ReprC,
+                        });
+                    }
                     def.all_fields()
                         .map(|field| field.ty(tcx, args))
-                        .try_for_each(|t| check_non_exhaustive(tcx, typing_env, t))
+                        .try_for_each(|t| check_unsuited(tcx, typing_env, t))
                 }
                 _ => ControlFlow::Continue(()),
             }
         }
 
-        (span, trivial, check_non_exhaustive(tcx, typing_env, ty).break_value())
+        FieldInfo { span, trivial, unsuited: check_unsuited(tcx, typing_env, ty).break_value() }
     });
 
     let non_trivial_fields = field_infos
         .clone()
-        .filter_map(|(span, trivial, _non_exhaustive)| if !trivial { Some(span) } else { None });
+        .filter_map(|field| if !field.trivial { Some(field.span) } else { None });
     let non_trivial_count = non_trivial_fields.clone().count();
     if non_trivial_count >= 2 {
         bad_non_zero_sized_fields(
@@ -1583,36 +1604,40 @@ pub(super) fn check_transparent<'tcx>(tcx: TyCtxt<'tcx>, adt: ty::AdtDef<'tcx>)
         );
         return;
     }
-    let mut prev_non_exhaustive_1zst = false;
-    for (span, _trivial, non_exhaustive_1zst) in field_infos {
-        if let Some((descr, def_id, args, non_exhaustive)) = non_exhaustive_1zst {
+
+    let mut prev_unsuited_1zst = false;
+    for field in field_infos {
+        if let Some(unsuited) = field.unsuited {
+            assert!(field.trivial);
             // If there are any non-trivial fields, then there can be no non-exhaustive 1-zsts.
             // Otherwise, it's only an issue if there's >1 non-exhaustive 1-zst.
-            if non_trivial_count > 0 || prev_non_exhaustive_1zst {
+            if non_trivial_count > 0 || prev_unsuited_1zst {
                 tcx.node_span_lint(
-                    REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS,
+                    REPR_TRANSPARENT_NON_ZST_FIELDS,
                     tcx.local_def_id_to_hir_id(adt.did().expect_local()),
-                    span,
+                    field.span,
                     |lint| {
+                        let title = match unsuited.reason {
+                            UnsuitedReason::NonExhaustive => "external non-exhaustive types",
+                            UnsuitedReason::PrivateField => "external types with private fields",
+                            UnsuitedReason::ReprC => "`repr(C)` types",
+                        };
                         lint.primary_message(
-                            "zero-sized fields in `repr(transparent)` cannot \
-                             contain external non-exhaustive types",
+                            format!("zero-sized fields in `repr(transparent)` cannot contain {title}"),
                         );
-                        let note = if non_exhaustive {
-                            "is marked with `#[non_exhaustive]`"
-                        } else {
-                            "contains private fields"
+                        let note = match unsuited.reason {
+                            UnsuitedReason::NonExhaustive => "is marked with `#[non_exhaustive]`, so it could become non-zero-sized in the future.",
+                            UnsuitedReason::PrivateField => "contains private fields, so it could become non-zero-sized in the future.",
+                            UnsuitedReason::ReprC => "is a `#[repr(C)]` type, so it is not guaranteed to be zero-sized on all targets.",
                         };
-                        let field_ty = tcx.def_path_str_with_args(def_id, args);
                         lint.note(format!(
-                            "this {descr} contains `{field_ty}`, which {note}, \
-                                and makes it not a breaking change to become \
-                                non-zero-sized in the future."
+                            "this field contains `{field_ty}`, which {note}",
+                            field_ty = unsuited.ty,
                         ));
                     },
-                )
+                );
             } else {
-                prev_non_exhaustive_1zst = true;
+                prev_unsuited_1zst = true;
             }
         }
     }

compiler/rustc_lint/src/lib.rs

@@ -362,6 +362,10 @@ fn register_builtins(store: &mut LintStore) {
     store.register_renamed("static_mut_ref", "static_mut_refs");
     store.register_renamed("temporary_cstring_as_ptr", "dangling_pointers_from_temporaries");
     store.register_renamed("elided_named_lifetimes", "mismatched_lifetime_syntaxes");
+    store.register_renamed(
+        "repr_transparent_external_private_fields",
+        "repr_transparent_non_zst_fields",
+    );
 
     // These were moved to tool lints, but rustc still sees them when compiling normally, before
     // tool lints are registered, so `check_tool_name_for_backwards_compat` doesn't work. Use

compiler/rustc_lint_defs/src/builtin.rs

@@ -86,7 +86,7 @@ declare_lint_pass! {
         REFINING_IMPL_TRAIT_INTERNAL,
         REFINING_IMPL_TRAIT_REACHABLE,
         RENAMED_AND_REMOVED_LINTS,
-        REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS,
+        REPR_TRANSPARENT_NON_ZST_FIELDS,
         RUST_2021_INCOMPATIBLE_CLOSURE_CAPTURES,
         RUST_2021_INCOMPATIBLE_OR_PATTERNS,
         RUST_2021_PREFIXES_INCOMPATIBLE_SYNTAX,
@@ -3011,19 +3011,23 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `repr_transparent_external_private_fields` lint
+    /// The `repr_transparent_non_zst_fields` lint
     /// detects types marked `#[repr(transparent)]` that (transitively)
-    /// contain an external ZST type marked `#[non_exhaustive]` or containing
-    /// private fields
+    /// contain a type that is not guaranteed to remain a ZST type under all configurations.
     ///
     /// ### Example
     ///
     /// ```rust,ignore (needs external crate)
     /// #![deny(repr_transparent_external_private_fields)]
     /// use foo::NonExhaustiveZst;
     ///
+    /// #[repr(C)]
+    /// struct CZst([u8; 0]);
+    ///
     /// #[repr(transparent)]
     /// struct Bar(u32, ([u32; 0], NonExhaustiveZst));
+    /// #[repr(transparent)]
+    /// struct Baz(u32, CZst);
     /// ```
     ///
     /// This will produce:
@@ -3042,26 +3046,39 @@ declare_lint! {
     ///   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     ///   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
     ///   = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
-    ///   = note: this struct contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, and makes it not a breaking change to become non-zero-sized in the future.
+    ///   = note: this field contains `NonExhaustiveZst`, which is marked with `#[non_exhaustive]`, so it could become non-zero-sized in the future.
+    ///
+    /// error: zero-sized fields in repr(transparent) cannot contain `#[repr(C)]` types
+    ///  --> src/main.rs:5:28
+    ///   |
+    /// 5 | struct Baz(u32, CZst);
+    ///   |                 ^^^^
+    ///   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+    ///   = note: for more information, see issue #78586 <https://github.com/rust-lang/rust/issues/78586>
+    ///   = note: this field contains `CZst`, which is a `#[repr(C)]` type, so it is not guaranteed to be zero-sized on all targets.
     /// ```
     ///
     /// ### Explanation
     ///
-    /// Previous, Rust accepted fields that contain external private zero-sized types,
-    /// even though it should not be a breaking change to add a non-zero-sized field to
-    /// that private type.
+    /// Previous, Rust accepted fields that contain external private zero-sized types, even though
+    /// those types could gain a non-zero-sized field in a future, semver-compatible update.
+    ///
+    /// Rust also accepted fields that contain `repr(C)` zero-sized types, even though those types
+    /// are not guaranteed to be zero-sized on all targets, and even though those types can
+    /// make a difference for the ABI (and therefore cannot be ignored by `repr(transparent)`).
     ///
     /// This is a [future-incompatible] lint to transition this
     /// to a hard error in the future. See [issue #78586] for more details.
     ///
     /// [issue #78586]: https://github.com/rust-lang/rust/issues/78586
     /// [future-incompatible]: ../index.md#future-incompatible-lints
-    pub REPR_TRANSPARENT_EXTERNAL_PRIVATE_FIELDS,
-    Warn,
+    pub REPR_TRANSPARENT_NON_ZST_FIELDS,
+    Deny,
     "transparent type contains an external ZST that is marked #[non_exhaustive] or contains private fields",
     @future_incompatible = FutureIncompatibleInfo {
         reason: FutureIncompatibilityReason::FutureReleaseError,
         reference: "issue #78586 <https://github.com/rust-lang/rust/issues/78586>",
+        report_in_deps: true,
     };
 }
 

compiler/rustc_lint_defs/src/lib.rs

@@ -403,7 +403,7 @@ pub enum FutureIncompatibilityReason {
     ///
     /// After a lint has been in this state for a while and you feel like it is ready to graduate
     /// to warning everyone, consider setting [`FutureIncompatibleInfo::report_in_deps`] to true.
-    /// (see it's documentation for more guidance)
+    /// (see its documentation for more guidance)
     ///
     /// After some period of time, lints with this variant can be turned into
     /// hard errors (and the lint removed). Preferably when there is some

tests/ui/lint/improper-ctypes/lint-ctypes.rs

@@ -38,6 +38,7 @@ pub struct TransparentLifetime<'a>(*const u8, PhantomData<&'a ()>);
 #[repr(transparent)]
 pub struct TransparentUnit<U>(f32, PhantomData<U>);
 #[repr(transparent)]
+#[allow(repr_transparent_non_zst_fields)]
 pub struct TransparentCustomZst(i32, ZeroSize);
 
 #[repr(C)]