Specialize derive(Clone) to use Copy when applicable

compiler/rustc_builtin_macros/src/deriving/bounds.rs

@@ -17,6 +17,7 @@ pub fn expand_deriving_copy(
         span,
         attributes: Vec::new(),
         path: path_std!(marker::Copy),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -15,27 +15,17 @@ pub fn expand_deriving_clone(
     item: &Annotatable,
     push: &mut dyn FnMut(Annotatable),
 ) {
-    // check if we can use a short form
-    //
-    // the short form is `fn clone(&self) -> Self { *self }`
-    //
-    // we can use the short form if:
-    // - the item is Copy (unfortunately, all we can check is whether it's also deriving Copy)
-    // - there are no generic parameters (after specialization this limitation can be removed)
-    //      if we used the short form with generics, we'd have to bound the generics with
-    //      Clone + Copy, and then there'd be no Clone impl at all if the user fills in something
-    //      that is Clone but not Copy. and until specialization we can't write both impls.
-    // - the item is a union with Copy fields
-    //      Unions with generic parameters still can derive Clone because they require Copy
-    //      for deriving, Clone alone is not enough.
-    //      Wherever Clone is implemented for fields is irrelevant so we don't assert it.
     let bounds;
     let substructure;
     let is_shallow;
     match *item {
         Annotatable::Item(ref annitem) => match annitem.kind {
             ItemKind::Struct(_, Generics { ref params, .. })
             | ItemKind::Enum(_, Generics { ref params, .. }) => {
+                // FIXME: although the use of specialization already removes the need for checking whether
+                // the type already derives `Copy`, `rustc_scalar_valid_range_*` types derive
+                // `Clone` AND `Copy` and cannot be constructed unless unsafe blocks surround the expression,
+                // thus this part is preserved.
                 let container_id = cx.current_expansion.id.expn_data().parent.expect_local();
                 let has_derive_copy = cx.resolver.has_derive_copy(container_id);
                 if has_derive_copy
@@ -46,27 +36,24 @@ pub fn expand_deriving_clone(
                     bounds = vec![];
                     is_shallow = true;
                     substructure = combine_substructure(Box::new(|c, s, sub| {
-                        cs_clone_shallow("Clone", c, s, sub, false)
+                        cs_clone_shallow(c, s, sub, false)
                     }));
                 } else {
-                    bounds = vec![];
+                    bounds = vec![Literal(path_std!(clone::Clone))];
                     is_shallow = false;
-                    substructure =
-                        combine_substructure(Box::new(|c, s, sub| cs_clone("Clone", c, s, sub)));
+                    substructure = combine_substructure(Box::new(|c, s, sub| cs_clone(c, s, sub)));
                 }
             }
             ItemKind::Union(..) => {
                 bounds = vec![Literal(path_std!(marker::Copy))];
                 is_shallow = true;
-                substructure = combine_substructure(Box::new(|c, s, sub| {
-                    cs_clone_shallow("Clone", c, s, sub, true)
-                }));
+                substructure =
+                    combine_substructure(Box::new(|c, s, sub| cs_clone_shallow(c, s, sub, true)));
             }
             _ => {
-                bounds = vec![];
+                bounds = vec![Literal(path_std!(clone::Clone))];
                 is_shallow = false;
-                substructure =
-                    combine_substructure(Box::new(|c, s, sub| cs_clone("Clone", c, s, sub)));
+                substructure = combine_substructure(Box::new(|c, s, sub| cs_clone(c, s, sub)));
             }
         },
 
@@ -78,7 +65,8 @@ pub fn expand_deriving_clone(
     let trait_def = TraitDef {
         span,
         attributes: Vec::new(),
-        path: path_std!(clone::Clone),
+        path: if is_shallow { path_std!(clone::Clone) } else { path_std!(clone::DerivedClone) },
+        bound_current_trait: false,
         additional_bounds: bounds,
         generics: Bounds::empty(),
         is_unsafe: false,
@@ -89,19 +77,55 @@ pub fn expand_deriving_clone(
             explicit_self: borrowed_explicit_self(),
             args: Vec::new(),
             ret_ty: Self_,
-            attributes: attrs,
+            attributes: attrs.clone(),
             is_unsafe: false,
             unify_fieldless_variants: false,
             combine_substructure: substructure,
         }],
         associated_types: Vec::new(),
     };
 
-    trait_def.expand_ext(cx, mitem, item, push, is_shallow)
+    trait_def.expand_ext(cx, mitem, item, push, is_shallow);
+
+    if !is_shallow {
+        TraitDef {
+            span,
+            attributes: Vec::new(),
+            path: path_std!(clone::Clone),
+            bound_current_trait: true,
+            additional_bounds: vec![],
+            generics: Bounds::empty(),
+            is_unsafe: false,
+            supports_unions: false,
+            methods: vec![MethodDef {
+                name: sym::clone,
+                generics: Bounds::empty(),
+                explicit_self: borrowed_explicit_self(),
+                args: Vec::new(),
+                ret_ty: Self_,
+                attributes: attrs,
+                is_unsafe: false,
+                unify_fieldless_variants: false,
+                combine_substructure: combine_substructure(Box::new(|c, s, _| {
+                    c.expr_call(
+                        s,
+                        c.expr_path(c.path(s, c.std_path(&[sym::clone, sym::try_copy]))),
+                        vec![
+                            c.expr_self(s),
+                            c.expr_path(
+                                c.path(s, c.std_path(&[sym::clone, sym::DerivedClone, sym::clone])),
+                            ),
+                        ],
+                    )
+                })),
+            }],
+            associated_types: vec![],
+        }
+        .expand_ext(cx, mitem, item, push, true)
+    }
 }
 
 fn cs_clone_shallow(
-    name: &str,
     cx: &mut ExtCtxt<'_>,
     trait_span: Span,
     substr: &Substructure<'_>,
@@ -149,20 +173,15 @@ fn cs_clone_shallow(
             }
             _ => cx.span_bug(
                 trait_span,
-                &format!("unexpected substructure in shallow `derive({})`", name),
+                &format!("unexpected substructure in shallow `derive(Clone)`"),
             ),
         }
     }
     stmts.push(cx.stmt_expr(cx.expr_deref(trait_span, cx.expr_self(trait_span))));
     cx.expr_block(cx.block(trait_span, stmts))
 }
 
-fn cs_clone(
-    name: &str,
-    cx: &mut ExtCtxt<'_>,
-    trait_span: Span,
-    substr: &Substructure<'_>,
-) -> P<Expr> {
+fn cs_clone(cx: &mut ExtCtxt<'_>, trait_span: Span, substr: &Substructure<'_>) -> P<Expr> {
     let ctor_path;
     let all_fields;
     let fn_path = cx.std_path(&[sym::clone, sym::Clone, sym::clone]);
@@ -184,10 +203,10 @@ fn cs_clone(
             vdata = &variant.data;
         }
         EnumNonMatchingCollapsed(..) => {
-            cx.span_bug(trait_span, &format!("non-matching enum variants in `derive({})`", name,))
+            cx.span_bug(trait_span, "non-matching enum variants in `derive(Clone)`")
         }
         StaticEnum(..) | StaticStruct(..) => {
-            cx.span_bug(trait_span, &format!("associated function in `derive({})`", name))
+            cx.span_bug(trait_span, "associated function in `derive(Clone)`")
         }
     }
 
@@ -199,7 +218,7 @@ fn cs_clone(
                     let Some(ident) = field.name else {
                         cx.span_bug(
                             trait_span,
-                            &format!("unnamed field in normal struct in `derive({})`", name,),
+                            "unnamed field in normal struct in `derive(Clone)`",
                         );
                     };
                     let call = subcall(cx, field);

compiler/rustc_builtin_macros/src/deriving/cmp/eq.rs

@@ -25,6 +25,7 @@ pub fn expand_deriving_eq(
         span,
         attributes: Vec::new(),
         path: path_std!(cmp::Eq),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/cmp/ord.rs

@@ -21,6 +21,7 @@ pub fn expand_deriving_ord(
         span,
         attributes: Vec::new(),
         path: path_std!(cmp::Ord),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/cmp/partial_eq.rs

@@ -100,6 +100,7 @@ pub fn expand_deriving_partial_eq(
         span,
         attributes: Vec::new(),
         path: path_std!(cmp::PartialEq),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/cmp/partial_ord.rs

@@ -44,6 +44,7 @@ pub fn expand_deriving_partial_ord(
         span,
         attributes: vec![],
         path: path_std!(cmp::PartialOrd),
+        bound_current_trait: true,
         additional_bounds: vec![],
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -27,6 +27,7 @@ pub fn expand_deriving_debug(
         span,
         attributes: Vec::new(),
         path: path_std!(fmt::Debug),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/decodable.rs

@@ -24,6 +24,7 @@ pub fn expand_deriving_rustc_decodable(
         span,
         attributes: Vec::new(),
         path: Path::new_(vec![krate, sym::Decodable], None, vec![], PathKind::Global),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/default.rs

@@ -28,6 +28,7 @@ pub fn expand_deriving_default(
         span,
         attributes: Vec::new(),
         path: Path::new(vec![kw::Default, sym::Default]),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/encodable.rs

@@ -109,6 +109,7 @@ pub fn expand_deriving_rustc_encodable(
         span,
         attributes: Vec::new(),
         path: Path::new_(vec![krate, sym::Encodable], None, vec![], PathKind::Global),
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_builtin_macros/src/deriving/generic/mod.rs

@@ -205,6 +205,8 @@ pub struct TraitDef<'a> {
     /// Path of the trait, including any type parameters
     pub path: Path,
 
+    pub bound_current_trait: bool,
+
     /// Additional bounds required of any type parameters of the type,
     /// other than the current trait
     pub additional_bounds: Vec<Ty>,
@@ -591,7 +593,7 @@ impl<'a> TraitDef<'a> {
                         cx.trait_bound(p.to_path(cx, self.span, type_ident, generics))
                     }).chain(
                         // require the current trait
-                        iter::once(cx.trait_bound(trait_path.clone()))
+                        self.bound_current_trait.then(|| cx.trait_bound(trait_path.clone()))
                     ).chain(
                         // also add in any bounds from the declaration
                         param.bounds.iter().cloned()
@@ -671,8 +673,10 @@ impl<'a> TraitDef<'a> {
                             .map(|p| cx.trait_bound(p.to_path(cx, self.span, type_ident, generics)))
                             .collect();
 
-                        // require the current trait
-                        bounds.push(cx.trait_bound(trait_path.clone()));
+                        if self.bound_current_trait {
+                            // require the current trait
+                            bounds.push(cx.trait_bound(trait_path.clone()));
+                        }
 
                         let predicate = ast::WhereBoundPredicate {
                             span: self.span,

compiler/rustc_builtin_macros/src/deriving/hash.rs

@@ -24,6 +24,7 @@ pub fn expand_deriving_hash(
         span,
         attributes: Vec::new(),
         path,
+        bound_current_trait: true,
         additional_bounds: Vec::new(),
         generics: Bounds::empty(),
         is_unsafe: false,

compiler/rustc_passes/src/dead.rs

@@ -268,6 +268,11 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                     }
                     return true;
                 }
+
+                // ignore generated `DerivedClone` but do not expose this to users.
+                if Some(trait_of) == self.tcx.get_diagnostic_item(sym::DerivedClone) {
+                    return true;
+                }
             }
         }
 

compiler/rustc_span/src/symbol.rs

@@ -168,6 +168,7 @@ symbols! {
         Decoder,
         Default,
         Deref,
+        DerivedClone,
         DirBuilder,
         Display,
         DoubleEndedIterator,
@@ -1412,6 +1413,7 @@ symbols! {
         truncf32,
         truncf64,
         try_blocks,
+        try_copy,
         try_from,
         try_into,
         try_trait_v2,

library/core/src/clone.rs

@@ -172,6 +172,43 @@ pub struct AssertParamIsCopy<T: Copy + ?Sized> {
     _field: crate::marker::PhantomData<T>,
 }
 
+#[doc(hidden)]
+#[unstable(
+    feature = "derive_clone_copy",
+    reason = "deriving hack, should not be public",
+    issue = "none"
+)]
+#[rustc_diagnostic_item = "DerivedClone"]
+pub trait DerivedClone: Sized {
+    fn clone(&self) -> Self;
+}
+
+#[doc(hidden)]
+#[unstable(
+    feature = "derive_clone_copy",
+    reason = "deriving hack, should not be public",
+    issue = "none"
+)]
+pub fn try_copy<T: Clone>(x: &T, clone: fn(&T) -> T) -> T {
+    trait TryCopy {
+        fn try_copy(&self, clone: fn(&Self) -> Self) -> Self;
+    }
+
+    impl<X: Clone> TryCopy for X {
+        default fn try_copy(&self, clone: fn(&Self) -> Self) -> Self {
+            clone(self)
+        }
+    }
+
+    impl<X: Copy> TryCopy for X {
+        fn try_copy(&self, _clone: fn(&Self) -> Self) -> Self {
+            *self
+        }
+    }
+
+    TryCopy::try_copy(x, clone)
+}
+
 /// Implementations of `Clone` for primitive types.
 ///
 /// Implementations that cannot be described in Rust