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

1 files changed, 1 insertions, 5 deletions

diff --git a/compiler/rustc_middle/src/ty/sty.rs b/compiler/rustc_middle/src/ty/sty.rs
index 474062218c9..3fbc23924f5 100644
--- a/compiler/rustc_middle/src/ty/sty.rs
+++ b/compiler/rustc_middle/src/ty/sty.rs
@@ -980,11 +980,7 @@ impl<'tcx> rustc_type_ir::inherent::Ty<TyCtxt<'tcx>> for Ty<'tcx> {
     }
 
     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
-        }
+        Ty::has_unsafe_fields(self)
     }
 }