do not implement unsafe auto traits for types with unsafe fields

If a type has unsafe fields, its safety invariants are not simply
the conjunction of its field types' safety invariants. Consequently,
it's invalid to reason about the safety properties of these types
in a purely structural manner — i.e., the manner in which `auto`
traits are implemented.

Makes progress towards #132922.

compiler/rustc_middle/src/ty/context.rs

@@ -585,6 +585,10 @@ impl<'tcx> Interner for TyCtxt<'tcx> {
         self.trait_def(trait_def_id).implement_via_object
     }
 
+    fn trait_is_unsafe(self, trait_def_id: Self::DefId) -> bool {
+        self.trait_def(trait_def_id).safety == hir::Safety::Unsafe
+    }
+
     fn is_impl_trait_in_trait(self, def_id: DefId) -> bool {
         self.is_impl_trait_in_trait(def_id)
     }

compiler/rustc_middle/src/ty/sty.rs

@@ -978,6 +978,14 @@ impl<'tcx> rustc_type_ir::inherent::Ty<TyCtxt<'tcx>> for Ty<'tcx> {
     fn async_destructor_ty(self, interner: TyCtxt<'tcx>) -> Ty<'tcx> {
         self.async_destructor_ty(interner)
     }
+
+    fn has_unsafe_fields(self) -> bool {
+        if let ty::Adt(adt_def, ..) = self.kind() {
+            adt_def.all_fields().any(|x| x.safety == hir::Safety::Unsafe)
+        } else {
+            false
+        }
+    }
 }
 
 /// Type utilities

compiler/rustc_next_trait_solver/src/solve/trait_goals.rs

@@ -169,6 +169,14 @@ where
             return result;
         }
 
+        // Only consider auto impls of unsafe traits when there are no unsafe
+        // fields.
+        if ecx.cx().trait_is_unsafe(goal.predicate.def_id())
+            && goal.predicate.self_ty().has_unsafe_fields()
+        {
+            return Err(NoSolution);
+        }
+
         // We only look into opaque types during analysis for opaque types
         // outside of their defining scope. Doing so for opaques in the
         // defining scope may require calling `typeck` on the same item we're

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -18,6 +18,7 @@ use rustc_infer::traits::{
 use rustc_middle::ty::fast_reject::DeepRejectCtxt;
 use rustc_middle::ty::{self, ToPolyTraitRef, Ty, TypeVisitableExt, TypingMode};
 use rustc_middle::{bug, span_bug};
+use rustc_type_ir::Interner;
 use tracing::{debug, instrument, trace};
 
 use super::SelectionCandidate::*;
@@ -794,6 +795,14 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | ty::Never
                 | ty::Tuple(_)
                 | ty::CoroutineWitness(..) => {
+                    use rustc_type_ir::inherent::*;
+
+                    // Only consider auto impls of unsafe traits when there are
+                    // no unsafe fields.
+                    if self.tcx().trait_is_unsafe(def_id) && self_ty.has_unsafe_fields() {
+                        return;
+                    }
+
                     // Only consider auto impls if there are no manual impls for the root of `self_ty`.
                     //
                     // For example, we only consider auto candidates for `&i32: Auto` if no explicit impl

compiler/rustc_type_ir/src/inherent.rs

@@ -136,6 +136,9 @@ pub trait Ty<I: Interner<Ty = Self>>:
         matches!(self.kind(), ty::FnPtr(..))
     }
 
+    /// Checks whether this type is an ADT that has unsafe fields.
+    fn has_unsafe_fields(self) -> bool;
+
     fn fn_sig(self, interner: I) -> ty::Binder<I, ty::FnSig<I>> {
         match self.kind() {
             ty::FnPtr(sig_tys, hdr) => sig_tys.with(hdr),

compiler/rustc_type_ir/src/interner.rs

@@ -270,6 +270,9 @@ pub trait Interner:
 
     fn trait_may_be_implemented_via_object(self, trait_def_id: Self::DefId) -> bool;
 
+    /// Returns `true` if this is an `unsafe trait`.
+    fn trait_is_unsafe(self, trait_def_id: Self::DefId) -> bool;
+
     fn is_impl_trait_in_trait(self, def_id: Self::DefId) -> bool;
 
     fn delay_bug(self, msg: impl ToString) -> Self::ErrorGuaranteed;

tests/ui/unsafe-fields/auto-traits.current.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `UnsafeEnum: UnsafeAuto` is not satisfied
+  --> $DIR/auto-traits.rs:24:22
+   |
+LL |     impl_unsafe_auto(UnsafeEnum::Safe(42));
+   |     ---------------- ^^^^^^^^^^^^^^^^^^^^ the trait `UnsafeAuto` is not implemented for `UnsafeEnum`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `impl_unsafe_auto`
+  --> $DIR/auto-traits.rs:20:29
+   |
+LL | fn impl_unsafe_auto(_: impl UnsafeAuto) {}
+   |                             ^^^^^^^^^^ required by this bound in `impl_unsafe_auto`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/unsafe-fields/auto-traits.next.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `UnsafeEnum: UnsafeAuto` is not satisfied
+  --> $DIR/auto-traits.rs:24:22
+   |
+LL |     impl_unsafe_auto(UnsafeEnum::Safe(42));
+   |     ---------------- ^^^^^^^^^^^^^^^^^^^^ the trait `UnsafeAuto` is not implemented for `UnsafeEnum`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required by a bound in `impl_unsafe_auto`
+  --> $DIR/auto-traits.rs:20:29
+   |
+LL | fn impl_unsafe_auto(_: impl UnsafeAuto) {}
+   |                             ^^^^^^^^^^ required by this bound in `impl_unsafe_auto`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/unsafe-fields/auto-traits.rs

@@ -0,0 +1,26 @@
+//@ compile-flags: --crate-type=lib
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+
+#![feature(auto_traits)]
+#![feature(unsafe_fields)]
+#![allow(incomplete_features)]
+
+enum UnsafeEnum {
+    Safe(u8),
+    Unsafe { unsafe field: u8 },
+}
+
+auto trait SafeAuto {}
+
+fn impl_safe_auto(_: impl SafeAuto) {}
+
+unsafe auto trait UnsafeAuto {}
+
+fn impl_unsafe_auto(_: impl UnsafeAuto) {}
+
+fn tests() {
+    impl_safe_auto(UnsafeEnum::Safe(42));
+    impl_unsafe_auto(UnsafeEnum::Safe(42));
+    //~^ ERROR the trait bound `UnsafeEnum: UnsafeAuto` is not satisfied
+}