work towards rejecting consts in patterns that do not implement PartialEq

compiler/rustc_lint_defs/src/builtin.rs

@@ -2310,6 +2310,57 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    /// The `match_without_partial_eq` lint detects constants that are used in patterns,
+    /// whose type does not implement `PartialEq`.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(match_without_partial_eq)]
+    ///
+    /// trait EnumSetType {
+    ///    type Repr;
+    /// }
+    ///
+    /// enum Enum8 { }
+    /// impl EnumSetType for Enum8 {
+    ///     type Repr = u8;
+    /// }
+    ///
+    /// #[derive(PartialEq, Eq)]
+    /// struct EnumSet<T: EnumSetType> {
+    ///     __enumset_underlying: T::Repr,
+    /// }
+    ///
+    /// const CONST_SET: EnumSet<Enum8> = EnumSet { __enumset_underlying: 3 };
+    ///
+    /// fn main() {
+    ///     match CONST_SET {
+    ///         CONST_SET => { /* ok */ }
+    ///         _ => panic!("match fell through?"),
+    ///     }
+    /// }
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Previous versions of Rust accepted constants in patterns, even if those constants' types
+    /// did not have `PartialEq` implemented. The compiler falls back to comparing the value
+    /// field-by-field. In the future we'd like to ensure that pattern matching always
+    /// follows `PartialEq` semantics, so that trait bound will become a requirement for
+    /// matching on constants.
+    pub MATCH_WITHOUT_PARTIAL_EQ,
+    Warn,
+    "constant in pattern does not implement `PartialEq`",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::FutureReleaseErrorReportInDeps,
+        reference: "issue #X <https://github.com/rust-lang/rust/issues/X>",
+    };
+}
+
 declare_lint! {
     /// The `ambiguous_associated_items` lint detects ambiguity between
     /// [associated items] and [enum variants].
@@ -3389,6 +3440,7 @@ declare_lint_pass! {
         LOSSY_PROVENANCE_CASTS,
         MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
         MACRO_USE_EXTERN_CRATE,
+        MATCH_WITHOUT_PARTIAL_EQ,
         META_VARIABLE_MISUSE,
         MISSING_ABI,
         MISSING_FRAGMENT_SPECIFIER,

compiler/rustc_mir_build/messages.ftl

@@ -229,6 +229,9 @@ mir_build_non_exhaustive_patterns_type_not_empty = non-exhaustive patterns: type
     .suggestion = ensure that all possible cases are being handled by adding a match arm with a wildcard pattern as shown
     .help = ensure that all possible cases are being handled by adding a match arm with a wildcard pattern
 
+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)]`
 

compiler/rustc_mir_build/src/errors.rs

@@ -748,6 +748,12 @@ pub struct NontrivialStructuralMatch<'tcx> {
     pub non_sm_ty: Ty<'tcx>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(mir_build_non_partial_eq_match)]
+pub struct NonPartialEqMatch<'tcx> {
+    pub non_peq_ty: Ty<'tcx>,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(mir_build_overlapping_range_endpoints)]
 #[note]

compiler/rustc_mir_build/src/thir/pattern/const_to_pat.rs

@@ -16,8 +16,8 @@ use std::cell::Cell;
 
 use super::PatCtxt;
 use crate::errors::{
-    FloatPattern, IndirectStructuralMatch, InvalidPattern, NontrivialStructuralMatch,
-    PointerPattern, TypeNotStructural, UnionPattern, UnsizedPattern,
+    FloatPattern, IndirectStructuralMatch, InvalidPattern, NonPartialEqMatch,
+    NontrivialStructuralMatch, PointerPattern, TypeNotStructural, UnionPattern, UnsizedPattern,
 };
 
 impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
@@ -235,6 +235,16 @@ impl<'tcx> ConstToPat<'tcx> {
                             PointerPattern,
                         );
                     }
+                    _ if !self.type_may_have_partial_eq_impl(cv.ty()) => {
+                        // Value is structural-match but the type doesn't even implement `PartialEq`...
+                        self.saw_const_match_lint.set(true);
+                        self.tcx().emit_spanned_lint(
+                            lint::builtin::MATCH_WITHOUT_PARTIAL_EQ,
+                            self.id,
+                            self.span,
+                            NonPartialEqMatch { non_peq_ty: cv.ty() },
+                        );
+                    }
                     _ => {}
                 }
             }

tests/ui/consts/const_in_pattern/issue-65466.rs

@@ -15,7 +15,8 @@ const C: &[O<B>] = &[O::None];
 fn main() {
     let x = O::None;
     match &[x][..] {
-        C => (),
+        C => (), //~WARN: the type must implement `PartialEq`
+        //~| previously accepted
         _ => (),
     }
 }

tests/ui/consts/const_in_pattern/issue-65466.stderr

@@ -0,0 +1,23 @@
+warning: to use a constant of type `&[O<B>]` in a pattern, the type must implement `PartialEq`
+  --> $DIR/issue-65466.rs:18:9
+   |
+LL |         C => (),
+   |         ^
+   |
+   = 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 #X <https://github.com/rust-lang/rust/issues/X>
+   = note: `#[warn(match_without_partial_eq)]` on by default
+
+warning: 1 warning emitted
+
+Future incompatibility report: Future breakage diagnostic:
+warning: to use a constant of type `&[O<B>]` in a pattern, the type must implement `PartialEq`
+  --> $DIR/issue-65466.rs:18:9
+   |
+LL |         C => (),
+   |         ^
+   |
+   = 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 #X <https://github.com/rust-lang/rust/issues/X>
+   = note: `#[warn(match_without_partial_eq)]` on by default
+

tests/ui/match/issue-72896-non-partial-eq-const.rs

@@ -17,7 +17,8 @@ const CONST_SET: EnumSet<Enum8> = EnumSet { __enumset_underlying: 3 };
 
 fn main() {
     match CONST_SET {
-        CONST_SET => { /* ok */ }
+        CONST_SET => { /* ok */ } //~WARN: must implement `PartialEq`
+        //~| previously accepted
         _ => panic!("match fell through?"),
     }
 }

tests/ui/match/issue-72896-non-partial-eq-const.stderr

@@ -0,0 +1,23 @@
+warning: to use a constant of type `EnumSet<Enum8>` in a pattern, the type must implement `PartialEq`
+  --> $DIR/issue-72896-non-partial-eq-const.rs:20:9
+   |
+LL |         CONST_SET => { /* ok */ }
+   |         ^^^^^^^^^
+   |
+   = 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 #X <https://github.com/rust-lang/rust/issues/X>
+   = note: `#[warn(match_without_partial_eq)]` on by default
+
+warning: 1 warning emitted
+
+Future incompatibility report: Future breakage diagnostic:
+warning: to use a constant of type `EnumSet<Enum8>` in a pattern, the type must implement `PartialEq`
+  --> $DIR/issue-72896-non-partial-eq-const.rs:20:9
+   |
+LL |         CONST_SET => { /* ok */ }
+   |         ^^^^^^^^^
+   |
+   = 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 #X <https://github.com/rust-lang/rust/issues/X>
+   = note: `#[warn(match_without_partial_eq)]` on by default
+