Require that const param tys implement ConstParamTy

compiler/rustc_error_codes/src/error_codes/E0741.md

@@ -10,15 +10,19 @@ struct A;
 struct B<const X: A>; // error!
 ```
 
-Only structural-match types (that is, types that derive `PartialEq` and `Eq`)
-may be used as the types of const generic parameters.
+Only structural-match types, which are types that derive `PartialEq` and `Eq`
+and implement `ConstParamTy`, may be used as the types of const generic
+parameters.
 
-To fix the previous code example, we derive `PartialEq` and `Eq`:
+To fix the previous code example, we derive `PartialEq`, `Eq`, and
+`ConstParamTy`:
 
 ```
 #![feature(adt_const_params)]
 
-#[derive(PartialEq, Eq)] // We derive both traits here.
+use std::marker::ConstParamTy;
+
+#[derive(PartialEq, Eq, ConstParamTy)] // We derive both traits here.
 struct A;
 
 struct B<const X: A>; // ok!

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -115,36 +115,22 @@ pub trait EmissionGuarantee: Sized {
     ) -> DiagnosticBuilder<'_, Self>;
 }
 
-/// Private module for sealing the `IsError` helper trait.
-mod sealed_level_is_error {
-    use crate::Level;
-
-    /// Sealed helper trait for statically checking that a `Level` is an error.
-    pub(crate) trait IsError<const L: Level> {}
-
-    impl IsError<{ Level::Bug }> for () {}
-    impl IsError<{ Level::DelayedBug }> for () {}
-    impl IsError<{ Level::Fatal }> for () {}
-    // NOTE(eddyb) `Level::Error { lint: true }` is also an error, but lints
-    // don't need error guarantees, as their levels are always dynamic.
-    impl IsError<{ Level::Error { lint: false } }> for () {}
-}
-
 impl<'a> DiagnosticBuilder<'a, ErrorGuaranteed> {
     /// Convenience function for internal use, clients should use one of the
     /// `struct_*` methods on [`Handler`].
     #[track_caller]
-    pub(crate) fn new_guaranteeing_error<M: Into<DiagnosticMessage>, const L: Level>(
+    pub(crate) fn new_guaranteeing_error<M: Into<DiagnosticMessage>>(
         handler: &'a Handler,
         message: M,
-    ) -> Self
-    where
-        (): sealed_level_is_error::IsError<L>,
-    {
+    ) -> Self {
         Self {
             inner: DiagnosticBuilderInner {
                 state: DiagnosticBuilderState::Emittable(handler),
-                diagnostic: Box::new(Diagnostic::new_with_code(L, None, message)),
+                diagnostic: Box::new(Diagnostic::new_with_code(
+                    Level::Error { lint: false },
+                    None,
+                    message,
+                )),
             },
             _marker: PhantomData,
         }
@@ -203,9 +189,7 @@ impl EmissionGuarantee for ErrorGuaranteed {
         handler: &Handler,
         msg: impl Into<DiagnosticMessage>,
     ) -> DiagnosticBuilder<'_, Self> {
-        DiagnosticBuilder::new_guaranteeing_error::<_, { Level::Error { lint: false } }>(
-            handler, msg,
-        )
+        DiagnosticBuilder::new_guaranteeing_error(handler, msg)
     }
 }
 

compiler/rustc_errors/src/lib.rs

@@ -6,7 +6,6 @@
 #![feature(array_windows)]
 #![feature(drain_filter)]
 #![feature(if_let_guard)]
-#![feature(adt_const_params)]
 #![feature(let_chains)]
 #![feature(never_type)]
 #![feature(result_option_inspect)]
@@ -845,7 +844,7 @@ impl Handler {
         &self,
         msg: impl Into<DiagnosticMessage>,
     ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
-        DiagnosticBuilder::new_guaranteeing_error::<_, { Level::Error { lint: false } }>(self, msg)
+        DiagnosticBuilder::new_guaranteeing_error(self, msg)
     }
 
     /// This should only be used by `rustc_middle::lint::struct_lint_level`. Do not use it for hard errors.

compiler/rustc_hir_analysis/src/check/wfcheck.rs

@@ -829,83 +829,20 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
             let ty = tcx.type_of(param.def_id).subst_identity();
 
             if tcx.features().adt_const_params {
-                if let Some(non_structural_match_ty) =
-                    traits::search_for_adt_const_param_violation(param.span, tcx, ty)
-                {
-                    // We use the same error code in both branches, because this is really the same
-                    // issue: we just special-case the message for type parameters to make it
-                    // clearer.
-                    match non_structural_match_ty.kind() {
-                        ty::Param(_) => {
-                            // Const parameters may not have type parameters as their types,
-                            // because we cannot be sure that the type parameter derives `PartialEq`
-                            // and `Eq` (just implementing them is not enough for `structural_match`).
-                            struct_span_err!(
-                                tcx.sess,
-                                hir_ty.span,
-                                E0741,
-                                "`{ty}` is not guaranteed to `#[derive(PartialEq, Eq)]`, so may not be \
-                                used as the type of a const parameter",
-                            )
-                            .span_label(
-                                hir_ty.span,
-                                format!("`{ty}` may not derive both `PartialEq` and `Eq`"),
-                            )
-                            .note(
-                                "it is not currently possible to use a type parameter as the type of a \
-                                const parameter",
-                            )
-                            .emit();
-                        }
-                        ty::Float(_) => {
-                            struct_span_err!(
-                                tcx.sess,
-                                hir_ty.span,
-                                E0741,
-                                "`{ty}` is forbidden as the type of a const generic parameter",
-                            )
-                            .note("floats do not derive `Eq` or `Ord`, which are required for const parameters")
-                            .emit();
-                        }
-                        ty::FnPtr(_) => {
-                            struct_span_err!(
-                                tcx.sess,
-                                hir_ty.span,
-                                E0741,
-                                "using function pointers as const generic parameters is forbidden",
-                            )
-                            .emit();
-                        }
-                        ty::RawPtr(_) => {
-                            struct_span_err!(
-                                tcx.sess,
-                                hir_ty.span,
-                                E0741,
-                                "using raw pointers as const generic parameters is forbidden",
-                            )
-                            .emit();
-                        }
-                        _ => {
-                            let mut diag = struct_span_err!(
-                                tcx.sess,
-                                hir_ty.span,
-                                E0741,
-                                "`{}` must be annotated with `#[derive(PartialEq, Eq)]` to be used as \
-                                the type of a const parameter",
-                                non_structural_match_ty,
-                            );
-
-                            if ty == non_structural_match_ty {
-                                diag.span_label(
-                                    hir_ty.span,
-                                    format!("`{ty}` doesn't derive both `PartialEq` and `Eq`"),
-                                );
-                            }
-
-                            diag.emit();
-                        }
-                    }
-                }
+                enter_wf_checking_ctxt(tcx, hir_ty.span, param.def_id, |wfcx| {
+                    let trait_def_id =
+                        tcx.require_lang_item(LangItem::ConstParamTy, Some(hir_ty.span));
+                    wfcx.register_bound(
+                        ObligationCause::new(
+                            hir_ty.span,
+                            param.def_id,
+                            ObligationCauseCode::ConstParam(ty),
+                        ),
+                        wfcx.param_env,
+                        ty,
+                        trait_def_id,
+                    );
+                });
             } else {
                 let err_ty_str;
                 let mut is_ptr = true;

compiler/rustc_middle/src/traits/mod.rs

@@ -445,6 +445,9 @@ pub enum ObligationCauseCode<'tcx> {
     /// Obligations to prove that a `std::ops::Drop` impl is not stronger than
     /// the ADT it's being implemented for.
     DropImpl,
+
+    /// Requirement for a `const N: Ty` to implement `Ty: ConstParamTy`
+    ConstParam(Ty<'tcx>),
 }
 
 /// The 'location' at which we try to perform HIR-based wf checking.

compiler/rustc_trait_selection/src/traits/error_reporting/mod.rs

@@ -149,6 +149,12 @@ pub trait TypeErrCtxtExt<'tcx> {
         root_obligation: &PredicateObligation<'tcx>,
         error: &SelectionError<'tcx>,
     );
+
+    fn report_const_param_not_wf(
+        &self,
+        ty: Ty<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed>;
 }
 
 impl<'tcx> InferCtxtExt<'tcx> for InferCtxt<'tcx> {
@@ -641,6 +647,7 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                         span = obligation.cause.span;
                     }
                 }
+
                 if let ObligationCauseCode::CompareImplItemObligation {
                     impl_item_def_id,
                     trait_item_def_id,
@@ -657,6 +664,13 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
                     return;
                 }
 
+                // Report a const-param specific error
+                if let ObligationCauseCode::ConstParam(ty) = *obligation.cause.code().peel_derives()
+                {
+                    self.report_const_param_not_wf(ty, &obligation).emit();
+                    return;
+                }
+
                 let bound_predicate = obligation.predicate.kind();
                 match bound_predicate.skip_binder() {
                     ty::PredicateKind::Clause(ty::Clause::Trait(trait_predicate)) => {
@@ -1163,6 +1177,102 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
         self.point_at_returns_when_relevant(&mut err, &obligation);
         err.emit();
     }
+
+    fn report_const_param_not_wf(
+        &self,
+        ty: Ty<'tcx>,
+        obligation: &PredicateObligation<'tcx>,
+    ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
+        let span = obligation.cause.span;
+
+        let mut diag = match ty.kind() {
+            _ if ty.has_param() => {
+                span_bug!(span, "const param tys cannot mention other generic parameters");
+            }
+            ty::Float(_) => {
+                struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0741,
+                    "`{ty}` is forbidden as the type of a const generic parameter",
+                )
+            }
+            ty::FnPtr(_) => {
+                struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0741,
+                    "using function pointers as const generic parameters is forbidden",
+                )
+            }
+            ty::RawPtr(_) => {
+                struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0741,
+                    "using raw pointers as const generic parameters is forbidden",
+                )
+            }
+            ty::Adt(def, _) => {
+                // We should probably see if we're *allowed* to derive `ConstParamTy` on the type...
+                let mut diag = struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0741,
+                    "`{ty}` must implement `ConstParamTy` to be used as the type of a const generic parameter",
+                );
+                // Only suggest derive if this isn't a derived obligation,
+                // and the struct is local.
+                if let Some(span) = self.tcx.hir().span_if_local(def.did())
+                    && obligation.cause.code().parent().is_none()
+                {
+                    if ty.is_structural_eq_shallow(self.tcx) {
+                        diag.span_suggestion(
+                            span,
+                            "add `#[derive(ConstParamTy)]` to the struct",
+                            "#[derive(ConstParamTy)]\n",
+                            Applicability::MachineApplicable,
+                        );
+                    } else {
+                        // FIXME(adt_const_params): We should check there's not already an
+                        // overlapping `Eq`/`PartialEq` impl.
+                        diag.span_suggestion(
+                            span,
+                            "add `#[derive(ConstParamTy, PartialEq, Eq)]` to the struct",
+                            "#[derive(ConstParamTy, PartialEq, Eq)]\n",
+                            Applicability::MachineApplicable,
+                        );
+                    }
+                }
+                diag
+            }
+            _ => {
+                struct_span_err!(
+                    self.tcx.sess,
+                    span,
+                    E0741,
+                    "`{ty}` can't be used as a const parameter type",
+                )
+            }
+        };
+
+        let mut code = obligation.cause.code();
+        let mut pred = obligation.predicate.to_opt_poly_trait_pred();
+        while let Some((next_code, next_pred)) = code.parent() {
+            if let Some(pred) = pred {
+                let pred = self.instantiate_binder_with_placeholders(pred);
+                diag.note(format!(
+                    "`{}` must implement `{}`, but it does not",
+                    pred.self_ty(),
+                    pred.print_modifiers_and_trait_path()
+                ));
+            }
+            code = next_code;
+            pred = next_pred;
+        }
+
+        diag
+    }
 }
 
 trait InferCtxtPrivExt<'tcx> {

compiler/rustc_trait_selection/src/traits/error_reporting/suggestions.rs

@@ -2655,7 +2655,8 @@ impl<'tcx> TypeErrCtxtExt<'tcx> for TypeErrCtxt<'_, 'tcx> {
             | ObligationCauseCode::BinOp { .. }
             | ObligationCauseCode::AscribeUserTypeProvePredicate(..)
             | ObligationCauseCode::RustCall
-            | ObligationCauseCode::DropImpl => {}
+            | ObligationCauseCode::DropImpl
+            | ObligationCauseCode::ConstParam(_) => {}
             ObligationCauseCode::SliceOrArrayElem => {
                 err.note("slice and array elements must have `Sized` type");
             }

compiler/rustc_trait_selection/src/traits/misc.rs

@@ -100,7 +100,8 @@ pub fn type_allowed_to_implement_const_param_ty<'tcx>(
         | ty::Str
         | ty::Array(..)
         | ty::Slice(_)
-        | ty::Ref(.., hir::Mutability::Not) => return Ok(()),
+        | ty::Ref(.., hir::Mutability::Not)
+        | ty::Tuple(_) => return Ok(()),
 
         &ty::Adt(adt, substs) => (adt, substs),
 

compiler/rustc_trait_selection/src/traits/mod.rs

@@ -62,9 +62,7 @@ pub use self::specialize::specialization_graph::FutureCompatOverlapErrorKind;
 pub use self::specialize::{
     specialization_graph, translate_substs, translate_substs_with_cause, OverlapError,
 };
-pub use self::structural_match::{
-    search_for_adt_const_param_violation, search_for_structural_match_violation,
-};
+pub use self::structural_match::search_for_structural_match_violation;
 pub use self::structural_normalize::StructurallyNormalizeExt;
 pub use self::util::elaborate;
 pub use self::util::{expand_trait_aliases, TraitAliasExpander};