Auto merge of #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The documentation given for the trait is outdated: *all* function pointers implement `PartialEq` and `Eq` these days.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 #115893 to check for `Eq`, and rule out float matching for good.

Fixes #115881

Author: bors

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

@@ -19,19 +19,6 @@ pub fn expand_deriving_eq(
 ) {
     let span = cx.with_def_site_ctxt(span);
 
-    let structural_trait_def = TraitDef {
-        span,
-        path: path_std!(marker::StructuralEq),
-        skip_path_as_bound: true, // crucial!
-        needs_copy_as_bound_if_packed: false,
-        additional_bounds: Vec::new(),
-        supports_unions: true,
-        methods: Vec::new(),
-        associated_types: Vec::new(),
-        is_const: false,
-    };
-    structural_trait_def.expand(cx, mitem, item, push);
-
     let trait_def = TraitDef {
         span,
         path: path_std!(cmp::Eq),

compiler/rustc_codegen_cranelift/example/mini_core.rs

@@ -104,9 +104,6 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 #[lang = "structural_peq"]
 pub trait StructuralPartialEq {}
 
-#[lang = "structural_teq"]
-pub trait StructuralEq {}
-
 #[lang = "not"]
 pub trait Not {
     type Output;

compiler/rustc_codegen_gcc/example/mini_core.rs

@@ -100,9 +100,6 @@ unsafe impl<T: ?Sized> Freeze for &mut T {}
 #[lang = "structural_peq"]
 pub trait StructuralPartialEq {}
 
-#[lang = "structural_teq"]
-pub trait StructuralEq {}
-
 #[lang = "not"]
 pub trait Not {
     type Output;

compiler/rustc_codegen_gcc/tests/run/static.rs

@@ -61,9 +61,6 @@ mod libc {
 #[lang = "structural_peq"]
 pub trait StructuralPartialEq {}
 
-#[lang = "structural_teq"]
-pub trait StructuralEq {}
-
 #[lang = "drop_in_place"]
 #[allow(unconditional_recursion)]
 pub unsafe fn drop_in_place<T: ?Sized>(to_drop: *mut T) {

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -138,7 +138,7 @@ pub(crate) fn const_to_valtree_inner<'tcx>(
         }
         // Trait objects are not allowed in type level constants, as we have no concept for
         // resolving their backing type, even if we can do that at const eval time. We may
-        // hypothetically be able to allow `dyn StructuralEq` trait objects in the future,
+        // hypothetically be able to allow `dyn StructuralPartialEq` trait objects in the future,
         // but it is unclear if this is useful.
         ty::Dynamic(..) => Err(ValTreeCreationError::NonSupportedType),
 

compiler/rustc_hir/src/lang_items.rs

@@ -143,8 +143,6 @@ language_item_table! {
     Unsize,                  sym::unsize,              unsize_trait,               Target::Trait,          GenericRequirement::Minimum(1);
     /// Trait injected by `#[derive(PartialEq)]`, (i.e. "Partial EQ").
     StructuralPeq,           sym::structural_peq,      structural_peq_trait,       Target::Trait,          GenericRequirement::None;
-    /// Trait injected by `#[derive(Eq)]`, (i.e. "Total EQ"; no, I will not apologize).
-    StructuralTeq,           sym::structural_teq,      structural_teq_trait,       Target::Trait,          GenericRequirement::None;
     Copy,                    sym::copy,                copy_trait,                 Target::Trait,          GenericRequirement::Exact(0);
     Clone,                   sym::clone,               clone_trait,                Target::Trait,          GenericRequirement::None;
     Sync,                    sym::sync,                sync_trait,                 Target::Trait,          GenericRequirement::Exact(0);

compiler/rustc_middle/src/query/mod.rs

@@ -1360,9 +1360,9 @@ rustc_queries! {
     ///
     /// This is only correct for ADTs. Call `is_structural_eq_shallow` to handle all types
     /// correctly.
-    query has_structural_eq_impls(ty: Ty<'tcx>) -> bool {
+    query has_structural_eq_impl(ty: Ty<'tcx>) -> bool {
         desc {
-            "computing whether `{}` implements `PartialStructuralEq` and `StructuralEq`",
+            "computing whether `{}` implements `StructuralPartialEq`",
             ty
         }
     }

compiler/rustc_middle/src/ty/util.rs

@@ -1249,19 +1249,18 @@ impl<'tcx> Ty<'tcx> {
     /// Primitive types (`u32`, `str`) have structural equality by definition. For composite data
     /// types, equality for the type as a whole is structural when it is the same as equality
     /// between all components (fields, array elements, etc.) of that type. For ADTs, structural
-    /// equality is indicated by an implementation of `PartialStructuralEq` and `StructuralEq` for
-    /// that type.
+    /// equality is indicated by an implementation of `StructuralPartialEq` for that type.
     ///
     /// This function is "shallow" because it may return `true` for a composite type whose fields
-    /// are not `StructuralEq`. For example, `[T; 4]` has structural equality regardless of `T`
+    /// are not `StructuralPartialEq`. For example, `[T; 4]` has structural equality regardless of `T`
     /// because equality for arrays is determined by the equality of each array element. If you
     /// want to know whether a given call to `PartialEq::eq` will proceed structurally all the way
     /// down, you will need to use a type visitor.
     #[inline]
     pub fn is_structural_eq_shallow(self, tcx: TyCtxt<'tcx>) -> bool {
         match self.kind() {
-            // Look for an impl of both `PartialStructuralEq` and `StructuralEq`.
-            ty::Adt(..) => tcx.has_structural_eq_impls(self),
+            // Look for an impl of `StructuralPartialEq`.
+            ty::Adt(..) => tcx.has_structural_eq_impl(self),
 
             // Primitive types that satisfy `Eq`.
             ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Str | ty::Never => true,

compiler/rustc_mir_build/messages.ftl

@@ -110,7 +110,7 @@ mir_build_extern_static_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
 mir_build_float_pattern = floating-point types cannot be used in patterns
 
 mir_build_indirect_structural_match =
-    to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq, Eq)]`
+    to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq)]`
 
 mir_build_inform_irrefutable = `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
 
@@ -254,7 +254,7 @@ mir_build_non_partial_eq_match =
     to use a constant of type `{$non_peq_ty}` in a pattern, the type must implement `PartialEq`
 
 mir_build_nontrivial_structural_match =
-    to use a constant of type `{$non_sm_ty}` in a pattern, the constant's initializer must be trivial or `{$non_sm_ty}` must be annotated with `#[derive(PartialEq, Eq)]`
+    to use a constant of type `{$non_sm_ty}` in a pattern, the constant's initializer must be trivial or `{$non_sm_ty}` must be annotated with `#[derive(PartialEq)]`
 
 mir_build_pattern_not_covered = refutable pattern in {$origin}
     .pattern_ty = the matched value is of type `{$pattern_ty}`
@@ -297,9 +297,9 @@ mir_build_trailing_irrefutable_let_patterns = trailing irrefutable {$count ->
     } into the body
 
 mir_build_type_not_structural =
-     to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq, Eq)]`
+     to use a constant of type `{$non_sm_ty}` in a pattern, `{$non_sm_ty}` must be annotated with `#[derive(PartialEq)]`
 
-mir_build_type_not_structural_more_info = see https://doc.rust-lang.org/stable/std/marker/trait.StructuralEq.html for details
+mir_build_type_not_structural_more_info = see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details
 
 mir_build_type_not_structural_tip = the traits must be derived, manual `impl`s are not sufficient
 

compiler/rustc_span/src/symbol.rs

@@ -310,7 +310,6 @@ symbols! {
         Some,
         SpanCtxt,
         String,
-        StructuralEq,
         StructuralPartialEq,
         SubdiagnosticMessage,
         Sync,
@@ -1625,7 +1624,6 @@ symbols! {
         struct_variant,
         structural_match,
         structural_peq,
-        structural_teq,
         sub,
         sub_assign,
         sub_with_overflow,

compiler/rustc_trait_selection/src/traits/structural_match.rs

@@ -39,7 +39,7 @@ pub fn search_for_structural_match_violation<'tcx>(
 
 /// This implements the traversal over the structure of a given type to try to
 /// find instances of ADTs (specifically structs or enums) that do not implement
-/// the structural-match traits (`StructuralPartialEq` and `StructuralEq`).
+/// `StructuralPartialEq`.
 struct Search<'tcx> {
     span: Span,
 

compiler/rustc_ty_utils/src/structural_match.rs

@@ -6,13 +6,12 @@ use rustc_infer::infer::TyCtxtInferExt;
 use rustc_trait_selection::traits::{ObligationCause, ObligationCtxt};
 
 /// This method returns true if and only if `adt_ty` itself has been marked as
-/// eligible for structural-match: namely, if it implements both
-/// `StructuralPartialEq` and `StructuralEq` (which are respectively injected by
-/// `#[derive(PartialEq)]` and `#[derive(Eq)]`).
+/// eligible for structural-match: namely, if it implements
+/// `StructuralPartialEq` (which is injected by `#[derive(PartialEq)]`).
 ///
 /// Note that this does *not* recursively check if the substructure of `adt_ty`
-/// implements the traits.
-fn has_structural_eq_impls<'tcx>(tcx: TyCtxt<'tcx>, adt_ty: Ty<'tcx>) -> bool {
+/// implements the trait.
+fn has_structural_eq_impl<'tcx>(tcx: TyCtxt<'tcx>, adt_ty: Ty<'tcx>) -> bool {
     let infcx = &tcx.infer_ctxt().build();
     let cause = ObligationCause::dummy();
 
@@ -21,11 +20,6 @@ fn has_structural_eq_impls<'tcx>(tcx: TyCtxt<'tcx>, adt_ty: Ty<'tcx>) -> bool {
     let structural_peq_def_id =
         infcx.tcx.require_lang_item(LangItem::StructuralPeq, Some(cause.span));
     ocx.register_bound(cause.clone(), ty::ParamEnv::empty(), adt_ty, structural_peq_def_id);
-    // for now, require `#[derive(Eq)]`. (Doing so is a hack to work around
-    // the type `for<'a> fn(&'a ())` failing to implement `Eq` itself.)
-    let structural_teq_def_id =
-        infcx.tcx.require_lang_item(LangItem::StructuralTeq, Some(cause.span));
-    ocx.register_bound(cause, ty::ParamEnv::empty(), adt_ty, structural_teq_def_id);
 
     // We deliberately skip *reporting* fulfillment errors (via
     // `report_fulfillment_errors`), for two reasons:
@@ -40,5 +34,5 @@ fn has_structural_eq_impls<'tcx>(tcx: TyCtxt<'tcx>, adt_ty: Ty<'tcx>) -> bool {
 }
 
 pub(crate) fn provide(providers: &mut Providers) {
-    providers.has_structural_eq_impls = has_structural_eq_impls;
+    providers.has_structural_eq_impl = has_structural_eq_impl;
 }

library/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`.

library/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,)+)
             {}
         }
 

src/tools/clippy/tests/ui/crashes/ice-6254.rs

@@ -11,8 +11,6 @@ fn main() {
     // This used to cause an ICE (https://github.com/rust-lang/rust/issues/78071)
     match FOO_REF_REF {
         FOO_REF_REF => {},
-        //~^ ERROR: to use a constant of type `Foo` in a pattern, `Foo` must be annotated
-        //~| NOTE: for more information, see issue #62411 <https://github.com/rust-lang/ru
         Foo(_) => {},
     }
 }

src/tools/clippy/tests/ui/crashes/ice-6254.stderr

@@ -8,13 +8,11 @@ impl std::marker::ConstParamTy for ImplementsConstParamTy {}
 struct CantParam(ImplementsConstParamTy);
 
 impl std::marker::ConstParamTy for CantParam {}
-//~^ error: the type `CantParam` does not `#[derive(Eq)]`
-//~| error: the type `CantParam` does not `#[derive(PartialEq)]`
+//~^ error: the type `CantParam` does not `#[derive(PartialEq)]`
 //~| the trait bound `CantParam: Eq` is not satisfied
 
 #[derive(std::marker::ConstParamTy)]
-//~^ error: the type `CantParamDerive` does not `#[derive(Eq)]`
-//~| error: the type `CantParamDerive` does not `#[derive(PartialEq)]`
+//~^ error: the type `CantParamDerive` does not `#[derive(PartialEq)]`
 //~| the trait bound `CantParamDerive: Eq` is not satisfied
 struct CantParamDerive(ImplementsConstParamTy);
 

tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.rs

@@ -21,17 +21,8 @@ LL | impl std::marker::ConstParamTy for CantParam {}
 note: required by a bound in `ConstParamTy`
   --> $SRC_DIR/core/src/marker.rs:LL:COL
 
-error[E0277]: the type `CantParam` does not `#[derive(Eq)]`
-  --> $DIR/const_param_ty_impl_no_structural_eq.rs:10:36
-   |
-LL | impl std::marker::ConstParamTy for CantParam {}
-   |                                    ^^^^^^^^^ the trait `StructuralEq` is not implemented for `CantParam`
-   |
-note: required by a bound in `ConstParamTy`
-  --> $SRC_DIR/core/src/marker.rs:LL:COL
-
 error[E0277]: the trait bound `CantParamDerive: Eq` is not satisfied
-  --> $DIR/const_param_ty_impl_no_structural_eq.rs:15:10
+  --> $DIR/const_param_ty_impl_no_structural_eq.rs:14:10
    |
 LL | #[derive(std::marker::ConstParamTy)]
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Eq` is not implemented for `CantParamDerive`
@@ -46,7 +37,7 @@ LL | struct CantParamDerive(ImplementsConstParamTy);
    |
 
 error[E0277]: the type `CantParamDerive` does not `#[derive(PartialEq)]`
-  --> $DIR/const_param_ty_impl_no_structural_eq.rs:15:10
+  --> $DIR/const_param_ty_impl_no_structural_eq.rs:14:10
    |
 LL | #[derive(std::marker::ConstParamTy)]
    |          ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `StructuralPartialEq` is not implemented for `CantParamDerive`
@@ -55,16 +46,6 @@ 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[E0277]: the type `CantParamDerive` does not `#[derive(Eq)]`
-  --> $DIR/const_param_ty_impl_no_structural_eq.rs:15:10
-   |
-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 6 previous errors
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0277`.

tests/ui/const-generics/adt_const_params/const_param_ty_impl_no_structural_eq.stderr

@@ -1,5 +1,5 @@
 #![allow(incomplete_features)]
-#![feature(adt_const_params, structural_match)]
+#![feature(adt_const_params)]
 
 union Union {
     a: u8,
@@ -11,7 +11,6 @@ impl PartialEq for Union {
     }
 }
 impl Eq for Union {}
-impl std::marker::StructuralEq for Union {}
 
 impl std::marker::ConstParamTy for Union {}
 //~^ ERROR the type `Union` does not `#[derive(PartialEq)]`
@@ -28,7 +27,6 @@ impl PartialEq for UnionDerive {
     }
 }
 impl Eq for UnionDerive {}
-impl std::marker::StructuralEq for UnionDerive {}
 
 
 fn main() {}