Make Copy unsafe to implement for ADTs with unsafe fields

As a rule, the application of `unsafe` to a declaration requires that use-sites
of that declaration also require `unsafe`. For example, a field declared
`unsafe` may only be read in the lexical context of an `unsafe` block.

For nearly all safe traits, the safety obligations of fields are explicitly
discharged when they are mentioned in method definitions. For example,
idiomatically implementing `Clone` (a safe trait) for a type with unsafe fields
will require `unsafe` to clone those fields.

Prior to this commit, `Copy` violated this rule. The trait is marked safe, and
although it has no explicit methods, its implementation permits reads of `Self`.

This commit resolves this by making `Copy` conditionally safe to implement. It
remains safe to implement for ADTs without unsafe fields, but unsafe to
implement for ADTs with unsafe fields.

Tracking: #132922

compiler/rustc_trait_selection/src/traits/misc.rs

@@ -18,6 +18,7 @@ pub enum CopyImplementationError<'tcx> {
     InfringingFields(Vec<(&'tcx ty::FieldDef, Ty<'tcx>, InfringingFieldsReason<'tcx>)>),
     NotAnAdt,
     HasDestructor,
+    HasUnsafeFields,
 }
 
 pub enum ConstParamTyImplementationError<'tcx> {
@@ -39,11 +40,16 @@ pub enum InfringingFieldsReason<'tcx> {
 ///
 /// If it's not an ADT, int ty, `bool`, float ty, `char`, raw pointer, `!`,
 /// a reference or an array returns `Err(NotAnAdt)`.
+///
+/// If the impl is `Safe`, `self_type` must not have unsafe fields. When used to
+/// generate suggestions in lints, `Safe` should be supplied so as to not
+/// suggest implementing `Copy` for types with unsafe fields.
 pub fn type_allowed_to_implement_copy<'tcx>(
     tcx: TyCtxt<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     self_type: Ty<'tcx>,
     parent_cause: ObligationCause<'tcx>,
+    impl_safety: hir::Safety,
 ) -> Result<(), CopyImplementationError<'tcx>> {
     let (adt, args) = match self_type.kind() {
         // These types used to have a builtin impl.
@@ -78,6 +84,10 @@ pub fn type_allowed_to_implement_copy<'tcx>(
         return Err(CopyImplementationError::HasDestructor);
     }
 
+    if impl_safety == hir::Safety::Safe && self_type.has_unsafe_fields() {
+        return Err(CopyImplementationError::HasUnsafeFields);
+    }
+
     Ok(())
 }
 

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

@@ -795,8 +795,6 @@ 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() {