Rollup merge of #133934 - jswrenn:unsafe-fields-auto-traits, r=compiler-errors

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. Consequently, auto implementations of unsafe auto traits should not be generated for types with unsafe fields.

Tracking: #132922

r? `@compiler-errors`

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;