deprecate Eq::assert_receiver_is_total_eq and emit FCW on manual impls

compiler/rustc_lint/messages.ftl

@@ -318,6 +318,9 @@ lint_enum_intrinsics_mem_variant =
     the return value of `mem::variant_count` is unspecified when called with a non-enum type
     .note = the type parameter of `variant_count` should be an enum, but it was instantiated with the type `{$ty_param}`, which is not an enum
 
+lint_eq_internal_method = `Eq::assert_receiver_is_total_eq` should never be implemented by hand
+    .note = this method is only used to add checks to the `Eq` derive macro
+
 lint_expectation = this lint expectation is unfulfilled
     .note = the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
     .rationale = {$rationale}

compiler/rustc_lint/src/builtin.rs

@@ -24,12 +24,11 @@ use rustc_ast_pretty::pprust::expr_to_string;
 use rustc_attr_parsing::AttributeParser;
 use rustc_errors::{Applicability, LintDiagnostic};
 use rustc_feature::GateIssue;
-use rustc_hir as hir;
 use rustc_hir::attrs::{AttributeKind, DocAttribute};
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
 use rustc_hir::intravisit::FnKind as HirFnKind;
-use rustc_hir::{Body, FnDecl, ImplItemImplKind, PatKind, PredicateOrigin, find_attr};
+use rustc_hir::{self as hir, Body, FnDecl, ImplItemImplKind, PatKind, PredicateOrigin, find_attr};
 use rustc_middle::bug;
 use rustc_middle::lint::LevelAndSource;
 use rustc_middle::ty::layout::LayoutOf;
@@ -59,7 +58,7 @@ use crate::lints::{
     BuiltinSpecialModuleNameUsed, BuiltinTrivialBounds, BuiltinTypeAliasBounds,
     BuiltinUngatedAsyncFnTrackCaller, BuiltinUnpermittedTypeInit, BuiltinUnpermittedTypeInitSub,
     BuiltinUnreachablePub, BuiltinUnsafe, BuiltinUnstableFeatures, BuiltinUnusedDocComment,
-    BuiltinUnusedDocCommentSub, BuiltinWhileTrue, InvalidAsmLabel,
+    BuiltinUnusedDocCommentSub, BuiltinWhileTrue, EqInternalMethodImplemented, InvalidAsmLabel,
 };
 use crate::{
     EarlyContext, EarlyLintPass, LateContext, LateLintPass, Level, LintContext,
@@ -3191,3 +3190,64 @@ impl EarlyLintPass for SpecialModuleName {
         }
     }
 }
+
+declare_lint! {
+    /// The `eq_assert_receiver_is_total_eq_impl` lint detects manual
+    /// implementations of `Eq::assert_receiver_is_total_eq`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust
+    /// #[derive(PartialEq)]
+    /// pub struct Foo;
+    ///
+    /// impl Eq for Foo {
+    ///     fn assert_receiver_is_total_eq(&self) {}
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// This method exists so that `#[derive(Eq)]` can check that all
+    /// fields of a type implement `Eq`. Other users were never supposed
+    /// to implement it and it was hidden from documentation.
+    ///
+    /// Unfortunately, it was not explicitly marked as unstable and some
+    /// people have now mistakenly assumed they had to implement this method.
+    ///
+    /// As the method is never called by the standard library, you can safely
+    /// remove any implementations of the method and just write `impl Eq for Foo {}`.
+    ///
+    /// This is a [future-incompatible] lint to transition this to a hard
+    /// error in the future. See [issue #150000] for more details.
+    ///
+    /// [issue #150000]: https://github.com/rust-lang/rust/issues/150000
+    pub EQ_ASSERT_RECEIVER_IS_TOTAL_EQ_IMPL,
+    Warn,
+    "manual implementation of the internal `Eq::assert_receiver_is_total_eq` method",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: fcw!(FutureReleaseError #150000),
+        report_in_deps: false,
+    };
+}
+
+declare_lint_pass!(EqAssertReceiverIsTotalEq => [EQ_ASSERT_RECEIVER_IS_TOTAL_EQ_IMPL]);
+
+impl<'tcx> LateLintPass<'tcx> for EqAssertReceiverIsTotalEq {
+    fn check_impl_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx rustc_hir::ImplItem<'tcx>) {
+        if let ImplItemImplKind::Trait { defaultness: _, trait_item_def_id: Ok(trait_item_def_id) } =
+            item.impl_kind
+            && item.ident.name == sym::assert_receiver_is_total_eq
+            && cx.tcx.is_diagnostic_item(sym::assert_receiver_is_total_eq, trait_item_def_id)
+            && !item.span.from_expansion()
+        {
+            cx.emit_span_lint(
+                EQ_ASSERT_RECEIVER_IS_TOTAL_EQ_IMPL,
+                item.span,
+                EqInternalMethodImplemented,
+            );
+        }
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -249,6 +249,7 @@ late_lint_methods!(
             FunctionCastsAsInteger: FunctionCastsAsInteger,
             CheckTransmutes: CheckTransmutes,
             LifetimeSyntax: LifetimeSyntax,
+            EqAssertReceiverIsTotalEq: EqAssertReceiverIsTotalEq,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -3298,3 +3298,8 @@ pub(crate) struct DocTestUnknown {
 #[derive(LintDiagnostic)]
 #[diag(lint_doc_test_literal)]
 pub(crate) struct DocTestLiteral;
+
+#[derive(LintDiagnostic)]
+#[diag(lint_eq_internal_method)]
+#[note]
+pub(crate) struct EqInternalMethodImplemented;

library/core/src/cmp.rs

@@ -345,6 +345,8 @@ pub const trait Eq: [const] PartialEq<Self> + PointeeSized {
     #[coverage(off)]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_diagnostic_item = "assert_receiver_is_total_eq"]
+    #[deprecated(since = "1.94.0", note = "implementation detail of `#[derive(Eq)]`")]
     fn assert_receiver_is_total_eq(&self) {}
 }
 

tests/ui/deriving/eq_assert_receiver_is_total_eq_impl.rs

@@ -0,0 +1,42 @@
+#![deny(deprecated, eq_assert_receiver_is_total_eq_impl)]
+pub struct Bad;
+
+impl PartialEq for Bad {
+    fn eq(&self, _: &Self) -> bool {
+        true
+    }
+}
+
+impl Eq for Bad {
+    fn assert_receiver_is_total_eq(&self) {}
+    //~^ ERROR: `Eq::assert_receiver_is_total_eq` should never be implemented by hand [eq_assert_receiver_is_total_eq_impl]
+    //~| WARN: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+}
+
+#[derive(PartialEq, Eq)]
+pub struct Good;
+
+#[derive(PartialEq)]
+pub struct Good2;
+
+impl Eq for Good2 {}
+
+pub struct Foo;
+
+pub trait SameName {
+    fn assert_receiver_is_total_eq(&self) {}
+}
+
+impl SameName for Foo {
+    fn assert_receiver_is_total_eq(&self) {}
+}
+
+pub fn main() {
+    Foo.assert_receiver_is_total_eq();
+    Good2.assert_receiver_is_total_eq();
+    //~^ ERROR: use of deprecated method `std::cmp::Eq::assert_receiver_is_total_eq`: implementation detail of `#[derive(Eq)]` [deprecated]
+    Good.assert_receiver_is_total_eq();
+    //~^ ERROR: use of deprecated method `std::cmp::Eq::assert_receiver_is_total_eq`: implementation detail of `#[derive(Eq)]` [deprecated]
+    Bad.assert_receiver_is_total_eq();
+    //~^ ERROR: use of deprecated method `std::cmp::Eq::assert_receiver_is_total_eq`: implementation detail of `#[derive(Eq)]` [deprecated]
+}

tests/ui/deriving/eq_assert_receiver_is_total_eq_impl.stderr

@@ -0,0 +1,41 @@
+error: use of deprecated method `std::cmp::Eq::assert_receiver_is_total_eq`: implementation detail of `#[derive(Eq)]`
+  --> $DIR/eq_assert_receiver_is_total_eq_impl.rs:36:11
+   |
+LL |     Good2.assert_receiver_is_total_eq();
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: the lint level is defined here
+  --> $DIR/eq_assert_receiver_is_total_eq_impl.rs:1:9
+   |
+LL | #![deny(deprecated, eq_assert_receiver_is_total_eq_impl)]
+   |         ^^^^^^^^^^
+
+error: use of deprecated method `std::cmp::Eq::assert_receiver_is_total_eq`: implementation detail of `#[derive(Eq)]`
+  --> $DIR/eq_assert_receiver_is_total_eq_impl.rs:38:10
+   |
+LL |     Good.assert_receiver_is_total_eq();
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: use of deprecated method `std::cmp::Eq::assert_receiver_is_total_eq`: implementation detail of `#[derive(Eq)]`
+  --> $DIR/eq_assert_receiver_is_total_eq_impl.rs:40:9
+   |
+LL |     Bad.assert_receiver_is_total_eq();
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: `Eq::assert_receiver_is_total_eq` should never be implemented by hand
+  --> $DIR/eq_assert_receiver_is_total_eq_impl.rs:11:5
+   |
+LL |     fn assert_receiver_is_total_eq(&self) {}
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
+   = note: for more information, see issue #150000 <https://github.com/rust-lang/rust/issues/150000>
+   = note: this method is only used to add checks to the `Eq` derive macro
+note: the lint level is defined here
+  --> $DIR/eq_assert_receiver_is_total_eq_impl.rs:1:21
+   |
+LL | #![deny(deprecated, eq_assert_receiver_is_total_eq_impl)]
+   |                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 4 previous errors
+