Auto merge of rust-lang#116167 - RalfJung:structural-eq, r=lcnr

remove StructuralEq trait

The documentation given for the trait is outdated: *all* function pointers implement `PartialEq` and `Eq` these days. So the `StructuralEq` trait doesn't really seem to have any reason to exist any more.

One side-effect of this PR is that we allow matching on some consts that do not implement `Eq`. However, we already allowed matching on floats and consts containing floats, so this is not new, it is just allowed in more cases now. IMO it makes no sense at all to allow float matching but also sometimes require an `Eq` instance. If we want to require `Eq` we should adjust rust-lang#115893 to check for `Eq`, and rule out float matching for good.

Fixes rust-lang#115881

Author: bors

core/src/marker.rs

@@ -187,7 +187,7 @@ pub trait Unsize<T: ?Sized> {
 /// Required trait for constants used in pattern matches.
 ///
 /// Any type that derives `PartialEq` automatically implements this trait,
-/// *regardless* of whether its type-parameters implement `Eq`.
+/// *regardless* of whether its type-parameters implement `PartialEq`.
 ///
 /// If a `const` item contains some type that does not implement this trait,
 /// then that type either (1.) does not implement `PartialEq` (which means the
@@ -200,7 +200,7 @@ pub trait Unsize<T: ?Sized> {
 /// a pattern match.
 ///
 /// See also the [structural match RFC][RFC1445], and [issue 63438] which
-/// motivated migrating from attribute-based design to this trait.
+/// motivated migrating from an attribute-based design to this trait.
 ///
 /// [RFC1445]: https://github.com/rust-lang/rfcs/blob/master/text/1445-restrict-constants-in-patterns.md
 /// [issue 63438]: https://github.com/rust-lang/rust/issues/63438
@@ -218,7 +218,7 @@ marker_impls! {
         isize, i8, i16, i32, i64, i128,
         bool,
         char,
-        str /* Technically requires `[u8]: StructuralEq` */,
+        str /* Technically requires `[u8]: StructuralPartialEq` */,
         (),
         {T, const N: usize} [T; N],
         {T} [T],
@@ -275,13 +275,15 @@ marker_impls! {
 #[unstable(feature = "structural_match", issue = "31434")]
 #[diagnostic::on_unimplemented(message = "the type `{Self}` does not `#[derive(Eq)]`")]
 #[lang = "structural_teq"]
+#[cfg(bootstrap)]
 pub trait StructuralEq {
     // Empty.
 }
 
 // FIXME: Remove special cases of these types from the compiler pattern checking code and always check `T: StructuralEq` instead
 marker_impls! {
     #[unstable(feature = "structural_match", issue = "31434")]
+    #[cfg(bootstrap)]
     StructuralEq for
         usize, u8, u16, u32, u64, u128,
         isize, i8, i16, i32, i64, i128,
@@ -859,6 +861,7 @@ impl<T: ?Sized> Default for PhantomData<T> {
 impl<T: ?Sized> StructuralPartialEq for PhantomData<T> {}
 
 #[unstable(feature = "structural_match", issue = "31434")]
+#[cfg(bootstrap)]
 impl<T: ?Sized> StructuralEq for PhantomData<T> {}
 
 /// Compiler-internal trait used to indicate the type of enum discriminants.
@@ -1038,6 +1041,20 @@ pub trait PointerLike {}
 #[unstable(feature = "adt_const_params", issue = "95174")]
 #[diagnostic::on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
 #[allow(multiple_supertrait_upcastable)]
+#[cfg(not(bootstrap))]
+pub trait ConstParamTy: StructuralPartialEq + Eq {}
+
+/// A marker for types which can be used as types of `const` generic parameters.
+///
+/// These types must have a proper equivalence relation (`Eq`) and it must be automatically
+/// derived (`StructuralPartialEq`). There's a hard-coded check in the compiler ensuring
+/// 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")]
+#[rustc_on_unimplemented(message = "`{Self}` can't be used as a const parameter type")]
+#[allow(multiple_supertrait_upcastable)]
+#[cfg(bootstrap)]
 pub trait ConstParamTy: StructuralEq + StructuralPartialEq + Eq {}
 
 /// Derive macro generating an impl of the trait `ConstParamTy`.

core/src/tuple.rs

@@ -2,7 +2,7 @@
 
 use crate::cmp::Ordering::{self, *};
 use crate::marker::ConstParamTy;
-use crate::marker::{StructuralEq, StructuralPartialEq};
+use crate::marker::StructuralPartialEq;
 
 // Recursive macro for implementing n-ary tuple functions and operations
 //
@@ -64,7 +64,8 @@ macro_rules! tuple_impls {
         maybe_tuple_doc! {
             $($T)+ @
             #[unstable(feature = "structural_match", issue = "31434")]
-            impl<$($T),+> StructuralEq for ($($T,)+)
+            #[cfg(bootstrap)]
+            impl<$($T),+> crate::marker::StructuralEq for ($($T,)+)
             {}
         }