Rollup merge of #134008 - jswrenn:unsafe-fields-copy, r=compiler-errors

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 entail `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

r? ```@compiler-errors```

2 files changed, 10 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)
     }
 }
 
diff --git a/compiler/rustc_middle/src/ty/util.rs b/compiler/rustc_middle/src/ty/util.rs
index 57054bd1a0b..b9a45ea3c2c 100644
--- a/compiler/rustc_middle/src/ty/util.rs
+++ b/compiler/rustc_middle/src/ty/util.rs
@@ -1288,6 +1288,15 @@ impl<'tcx> Ty<'tcx> {
         }
     }
 
+    /// Checks whether this type is an ADT that has unsafe fields.
+    pub 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
+        }
+    }
+
     /// Get morphology of the async drop glue, needed for types which do not
     /// use async drop. To get async drop glue morphology for a definition see
     /// [`TyCtxt::async_drop_glue_morphology`]. Used for `AsyncDestruct::Destructor`