Add derive for `core::marker::ConstParamTy`

This makes it easier to implement it for a type, just like `Copy`.

11 files changed, 115 insertions, 2 deletions

diff --git a/compiler/rustc_builtin_macros/src/deriving/bounds.rs b/compiler/rustc_builtin_macros/src/deriving/bounds.rs
index 0481a118906..2c8e6f99c67 100644
--- a/compiler/rustc_builtin_macros/src/deriving/bounds.rs
+++ b/compiler/rustc_builtin_macros/src/deriving/bounds.rs
@@ -27,3 +27,26 @@ pub fn expand_deriving_copy(
 
     trait_def.expand(cx, mitem, item, push);
 }
+
+pub fn expand_deriving_const_param_ty(
+    cx: &mut ExtCtxt<'_>,
+    span: Span,
+    mitem: &MetaItem,
+    item: &Annotatable,
+    push: &mut dyn FnMut(Annotatable),
+    is_const: bool,
+) {
+    let trait_def = TraitDef {
+        span,
+        path: path_std!(marker::ConstParamTy),
+        skip_path_as_bound: false,
+        needs_copy_as_bound_if_packed: false,
+        additional_bounds: Vec::new(),
+        supports_unions: false,
+        methods: Vec::new(),
+        associated_types: Vec::new(),
+        is_const,
+    };
+
+    trait_def.expand(cx, mitem, item, push);
+}
diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 4e5edb4d6b1..ebf1448f55c 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -115,6 +115,7 @@ pub fn register_builtin_macros(resolver: &mut dyn ResolverExpand) {
     register_derive! {
         Clone: clone::expand_deriving_clone,
         Copy: bounds::expand_deriving_copy,
+        ConstParamTy: bounds::expand_deriving_const_param_ty,
         Debug: debug::expand_deriving_debug,
         Default: default::expand_deriving_default,
         Eq: eq::expand_deriving_eq,
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 60efcb768cb..b3e54b10900 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -164,6 +164,7 @@ symbols! {
         Capture,
         Center,
         Clone,
+        ConstParamTy,
         Context,
         Continue,
         Copy,
diff --git a/library/core/src/marker.rs b/library/core/src/marker.rs
index 97f9d01e016..ede0e5db50c 100644
--- a/library/core/src/marker.rs
+++ b/library/core/src/marker.rs
@@ -991,6 +991,14 @@ pub trait PointerLike {}
 #[rustc_on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
 pub trait ConstParamTy: StructuralEq {}
 
+/// Derive macro generating an impl of the trait `Copy`.
+#[rustc_builtin_macro]
+#[unstable(feature = "adt_const_params", issue = "95174")]
+#[cfg(not(bootstrap))]
+pub macro ConstParamTy($item:item) {
+    /* compiler built-in */
+}
+
 // FIXME(generic_const_parameter_types): handle `ty::FnDef`/`ty::Closure`
 // FIXME(generic_const_parameter_types): handle `ty::Tuple`
 marker_impls! {
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_good.rs b/tests/ui/const-generics/adt_const_params/const_param_ty_good.rs
index a1b711a3024..87ae83dd966 100644
--- a/tests/ui/const-generics/adt_const_params/const_param_ty_good.rs
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_good.rs
@@ -11,6 +11,13 @@ struct S<T> {
 
 impl<T: ConstParamTy> ConstParamTy for S<T> {}
 
+#[derive(PartialEq, Eq, ConstParamTy)]
+struct D<T> {
+    field: u8,
+    gen: T,
+}
+
+
 fn check<T: ConstParamTy + ?Sized>() {}
 
 fn main() {
@@ -39,5 +46,8 @@ fn main() {
     check::<S<u8>>();
     check::<S<[&[bool]; 8]>>();
 
+    check::<D<u8>>();
+    check::<D<[&[bool]; 8]>>();
+
     // FIXME: test tuples
 }
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.rs b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.rs
index 07fd243737e..74283a37afc 100644
--- a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.rs
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.rs
@@ -10,4 +10,8 @@ struct CantParam(NotParam);
 impl std::marker::ConstParamTy for CantParam {}
 //~^ error: the trait `ConstParamTy` cannot be implemented for this type
 
+#[derive(std::marker::ConstParamTy, Eq, PartialEq)]
+//~^ error: the trait `ConstParamTy` cannot be implemented for this type
+struct CantParamDerive(NotParam);
+
 fn main() {}
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.stderr b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.stderr
index c8e065848b1..52b65d6061a 100644
--- a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.stderr
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_bad_field.stderr
@@ -7,6 +7,17 @@ LL |
 LL | impl std::marker::ConstParamTy for CantParam {}
    |                                    ^^^^^^^^^
 
-error: aborting due to previous error
+error[E0204]: the trait `ConstParamTy` cannot be implemented for this type
+  --> $DIR/const_param_ty_impl_bad_field.rs:13:10
+   |
+LL | #[derive(std::marker::ConstParamTy, Eq, PartialEq)]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL | struct CantParamDerive(NotParam);
+   |                        -------- this field does not implement `ConstParamTy`
+   |
+   = note: this error originates in the derive macro `std::marker::ConstParamTy` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0204`.
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.rs b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.rs
index 17ef396164e..37986de481f 100644
--- a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.rs
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.rs
@@ -10,6 +10,10 @@ struct CantParam(ImplementsConstParamTy);
 impl std::marker::ConstParamTy for CantParam {}
 //~^ error: the type `CantParam` does not `#[derive(Eq)]`
 
+#[derive(std::marker::ConstParamTy)]
+//~^ error: the type `CantParamDerive` does not `#[derive(Eq)]`
+struct CantParamDerive(ImplementsConstParamTy);
+
 fn check<T: std::marker::ConstParamTy>() {}
 
 fn main() {
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.stderr b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.stderr
index ca5abf5e254..52701d55914 100644
--- a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.stderr
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.stderr
@@ -7,6 +7,16 @@ LL | impl std::marker::ConstParamTy for CantParam {}
 note: required by a bound in `ConstParamTy`
   --> $SRC_DIR/core/src/marker.rs:LL:COL
 
-error: aborting due to previous error
+error[E0277]: the type `CantParamDerive` does not `#[derive(Eq)]`
+  --> $DIR/const_param_ty_impl_no_structural_eq.rs:13:10
+   |
+LL | #[derive(std::marker::ConstParamTy)]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `StructuralEq` is not implemented for `CantParamDerive`
+   |
+note: required by a bound in `ConstParamTy`
+  --> $SRC_DIR/core/src/marker.rs:LL:COL
+   = note: this error originates in the derive macro `std::marker::ConstParamTy` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0277`.
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_union.rs b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_union.rs
new file mode 100644
index 00000000000..d70377a20c1
--- /dev/null
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_union.rs
@@ -0,0 +1,33 @@
+#![allow(incomplete_features)]
+#![feature(adt_const_params, structural_match)]
+
+union Union {
+    a: u8,
+}
+
+impl PartialEq for Union {
+    fn eq(&self, other: &Union) -> bool {
+        true
+    }
+}
+impl Eq for Union {}
+impl std::marker::StructuralEq for Union {}
+
+impl std::marker::ConstParamTy for Union {}
+
+#[derive(std::marker::ConstParamTy)]
+//~^ ERROR this trait cannot be derived for unions
+union UnionDerive {
+    a: u8,
+}
+
+impl PartialEq for UnionDerive {
+    fn eq(&self, other: &UnionDerive) -> bool {
+        true
+    }
+}
+impl Eq for UnionDerive {}
+impl std::marker::StructuralEq for UnionDerive {}
+
+
+fn main() {}
diff --git a/tests/ui/const-generics/adt_const_params/const_param_ty_impl_union.stderr b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_union.stderr
new file mode 100644
index 00000000000..29370304605
--- /dev/null
+++ b/tests/ui/const-generics/adt_const_params/const_param_ty_impl_union.stderr
@@ -0,0 +1,8 @@
+error: this trait cannot be derived for unions
+  --> $DIR/const_param_ty_impl_union.rs:18:10
+   |
+LL | #[derive(std::marker::ConstParamTy)]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+