Split part of adt_const_params into unsized_const_params

core/src/lib.rs

@@ -248,6 +248,7 @@
 #![feature(transparent_unions)]
 #![feature(try_blocks)]
 #![feature(unboxed_closures)]
+#![feature(unsized_const_params)]
 #![feature(unsized_fn_params)]
 #![feature(with_negative_coherence)]
 // tidy-alphabetical-end

core/src/marker.rs

@@ -975,41 +975,74 @@ pub trait PointerLike {}
 /// that all fields are also `ConstParamTy`, which implies that recursively, all fields
 /// are `StructuralPartialEq`.
 #[lang = "const_param_ty"]
-#[unstable(feature = "adt_const_params", issue = "95174")]
+#[unstable(feature = "unsized_const_params", issue = "95174")]
 #[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
 #[allow(multiple_supertrait_upcastable)]
-pub trait ConstParamTy: StructuralPartialEq + Eq {}
+// We name this differently than the derive macro so that the `adt_const_params` can
+// be used independently of `unsized_const_params` without requiring a full path
+// to the derive macro every time it is used. This should be renamed on stabilization.
+pub trait ConstParamTy_: UnsizedConstParamTy + StructuralPartialEq + Eq {}
 
 /// Derive macro generating an impl of the trait `ConstParamTy`.
 #[rustc_builtin_macro]
+#[allow_internal_unstable(unsized_const_params)]
 #[unstable(feature = "adt_const_params", issue = "95174")]
 pub macro ConstParamTy($item:item) {
     /* compiler built-in */
 }
 
+#[cfg_attr(not(bootstrap), lang = "unsized_const_param_ty")]
+#[unstable(feature = "unsized_const_params", issue = "95174")]
+#[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
+/// A marker for types which can be used as types of `const` generic parameters.
+///
+/// Equivalent to [`ConstParamTy_`] except that this is used by
+/// the `unsized_const_params` to allow for fake unstable impls.
+pub trait UnsizedConstParamTy: StructuralPartialEq + Eq {}
+
+/// Derive macro generating an impl of the trait `ConstParamTy`.
+#[cfg(not(bootstrap))]
+#[cfg_attr(not(bootstrap), rustc_builtin_macro)]
+#[cfg_attr(not(bootstrap), allow_internal_unstable(unsized_const_params))]
+#[cfg_attr(not(bootstrap), unstable(feature = "unsized_const_params", issue = "95174"))]
+pub macro UnsizedConstParamTy($item:item) {
+    /* compiler built-in */
+}
+
 // FIXME(adt_const_params): handle `ty::FnDef`/`ty::Closure`
 marker_impls! {
     #[unstable(feature = "adt_const_params", issue = "95174")]
-    ConstParamTy for
+    ConstParamTy_ for
         usize, u8, u16, u32, u64, u128,
         isize, i8, i16, i32, i64, i128,
         bool,
         char,
         (),
-        {T: ConstParamTy, const N: usize} [T; N],
+        {T: ConstParamTy_, const N: usize} [T; N],
+}
+#[cfg(bootstrap)]
+marker_impls! {
+    #[unstable(feature = "adt_const_params", issue = "95174")]
+    ConstParamTy_ for
+        str,
+        {T: ConstParamTy_} [T],
+        {T: ConstParamTy_ + ?Sized} &T,
 }
 
-#[unstable(feature = "adt_const_params", issue = "95174")]
-#[rustc_reservation_impl = "types that are not `Sized` are not supported as the type of a const generic parameter"]
-impl<T> ConstParamTy for [T] {}
-
-#[unstable(feature = "adt_const_params", issue = "95174")]
-#[rustc_reservation_impl = "types that are not `Sized` are not supported as the type of a const generic parameter"]
-impl ConstParamTy for str {}
+marker_impls! {
+    #[unstable(feature = "unsized_const_params", issue = "95174")]
+    UnsizedConstParamTy for
+        usize, u8, u16, u32, u64, u128,
+        isize, i8, i16, i32, i64, i128,
+        bool,
+        char,
+        (),
+        {T: UnsizedConstParamTy, const N: usize} [T; N],
 
-#[unstable(feature = "adt_const_params", issue = "95174")]
-#[rustc_reservation_impl = "references are not supported as the type of a const generic parameter"]
-impl<T: ?Sized> ConstParamTy for &T {}
+        str,
+        {T: UnsizedConstParamTy} [T],
+        {T: UnsizedConstParamTy + ?Sized} &T,
+}
 
 /// A common trait implemented by all function pointers.
 #[unstable(

core/src/mem/transmutability.rs

@@ -1,4 +1,4 @@
-use crate::marker::ConstParamTy;
+use crate::marker::{ConstParamTy_, UnsizedConstParamTy};
 
 /// Are values of a type transmutable into values of another type?
 ///
@@ -39,7 +39,9 @@ pub struct Assume {
 }
 
 #[unstable(feature = "transmutability", issue = "99571")]
-impl ConstParamTy for Assume {}
+impl ConstParamTy_ for Assume {}
+#[unstable(feature = "transmutability", issue = "99571")]
+impl UnsizedConstParamTy for Assume {}
 
 impl Assume {
     /// Do not assume that *you* have ensured any safety properties are met.

core/src/tuple.rs

@@ -1,8 +1,9 @@
 // See core/src/primitive_docs.rs for documentation.
 
 use crate::cmp::Ordering::{self, *};
-use crate::marker::ConstParamTy;
+use crate::marker::ConstParamTy_;
 use crate::marker::StructuralPartialEq;
+use crate::marker::UnsizedConstParamTy;
 
 // Recursive macro for implementing n-ary tuple functions and operations
 //
@@ -49,8 +50,15 @@ macro_rules! tuple_impls {
 
         maybe_tuple_doc! {
             $($T)+ @
-            #[unstable(feature = "structural_match", issue = "31434")]
-            impl<$($T: ConstParamTy),+> ConstParamTy for ($($T,)+)
+            #[unstable(feature = "adt_const_params", issue = "95174")]
+            impl<$($T: ConstParamTy_),+> ConstParamTy_ for ($($T,)+)
+            {}
+        }
+
+        maybe_tuple_doc! {
+            $($T)+ @
+            #[unstable(feature = "unsized_const_params", issue = "95174")]
+            impl<$($T: UnsizedConstParamTy),+> UnsizedConstParamTy for ($($T,)+)
             {}
         }