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

compiler/rustc_hir_analysis/src/check/check.rs

@@ -1511,31 +1511,46 @@ 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)` or uninhabited. 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 +1565,36 @@ 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 +1606,36 @@ 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,
                     tcx.local_def_id_to_hir_id(adt.did().expect_local()),
-                    span,
+                    field.span,
                     |lint| {
                         lint.primary_message(
                             "zero-sized fields in `repr(transparent)` cannot \
                              contain external non-exhaustive types",
                         );
-                        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 marked with `#[repr(C)]`, which means it cannot be ignored for all ABIs.",
                         };
-                        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_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

@@ -1,6 +1,6 @@
 #![feature(rustc_private)]
 
-#![allow(private_interfaces)]
+#![allow(private_interfaces, repr_transparent_external_private_fields)]
 #![deny(improper_ctypes)]
 
 use std::cell::UnsafeCell;

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

@@ -1,4 +1,4 @@
-#![allow(private_interfaces)]
+#![allow(private_interfaces, repr_transparent_external_private_fields)]
 #![deny(improper_ctypes_definitions)]
 
 use std::default::Default;