Implement custom diagnostic for 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_hir_analysis/src/check/wfcheck.rs

@@ -836,7 +836,7 @@ fn check_param_wf(tcx: TyCtxt<'_>, param: &hir::GenericParam<'_>) {
                         ObligationCause::new(
                             hir_ty.span,
                             param.def_id,
-                            ObligationCauseCode::WellFormed(Some(hir_ty.span)),
+                            ObligationCauseCode::ConstParam(ty),
                         ),
                         wfcx.param_env,
                         ty,

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");
             }

src/tools/clippy/tests/ui/same_functions_in_if_condition.rs

@@ -10,6 +10,8 @@
     clippy::uninlined_format_args
 )]
 
+use std::marker::ConstParamTy;
+
 fn function() -> bool {
     true
 }
@@ -96,7 +98,7 @@ fn main() {
     };
     println!("{}", os);
 
-    #[derive(PartialEq, Eq)]
+    #[derive(PartialEq, Eq, ConstParamTy)]
     enum E {
         A,
         B,

src/tools/clippy/tests/ui/same_functions_in_if_condition.stderr

@@ -1,11 +1,11 @@
 error: this `if` has the same function call as a previous `if`
-  --> $DIR/same_functions_in_if_condition.rs:37:15
+  --> $DIR/same_functions_in_if_condition.rs:39:15
    |
 LL |     } else if function() {
    |               ^^^^^^^^^^
    |
 note: same as this
-  --> $DIR/same_functions_in_if_condition.rs:36:8
+  --> $DIR/same_functions_in_if_condition.rs:38:8
    |
 LL |     if function() {
    |        ^^^^^^^^^^
@@ -16,61 +16,61 @@ LL | #![deny(clippy::same_functions_in_if_condition)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: this `if` has the same function call as a previous `if`
-  --> $DIR/same_functions_in_if_condition.rs:42:15
+  --> $DIR/same_functions_in_if_condition.rs:44:15
    |
 LL |     } else if fn_arg(a) {
    |               ^^^^^^^^^
    |
 note: same as this
-  --> $DIR/same_functions_in_if_condition.rs:41:8
+  --> $DIR/same_functions_in_if_condition.rs:43:8
    |
 LL |     if fn_arg(a) {
    |        ^^^^^^^^^
 
 error: this `if` has the same function call as a previous `if`
-  --> $DIR/same_functions_in_if_condition.rs:47:15
+  --> $DIR/same_functions_in_if_condition.rs:49:15
    |
 LL |     } else if obj.method() {
    |               ^^^^^^^^^^^^
    |
 note: same as this
-  --> $DIR/same_functions_in_if_condition.rs:46:8
+  --> $DIR/same_functions_in_if_condition.rs:48:8
    |
 LL |     if obj.method() {
    |        ^^^^^^^^^^^^
 
 error: this `if` has the same function call as a previous `if`
-  --> $DIR/same_functions_in_if_condition.rs:52:15
+  --> $DIR/same_functions_in_if_condition.rs:54:15
    |
 LL |     } else if obj.method_arg(a) {
    |               ^^^^^^^^^^^^^^^^^
    |
 note: same as this
-  --> $DIR/same_functions_in_if_condition.rs:51:8
+  --> $DIR/same_functions_in_if_condition.rs:53:8
    |
 LL |     if obj.method_arg(a) {
    |        ^^^^^^^^^^^^^^^^^
 
 error: this `if` has the same function call as a previous `if`
-  --> $DIR/same_functions_in_if_condition.rs:59:15
+  --> $DIR/same_functions_in_if_condition.rs:61:15
    |
 LL |     } else if v.pop().is_none() {
    |               ^^^^^^^^^^^^^^^^^
    |
 note: same as this
-  --> $DIR/same_functions_in_if_condition.rs:57:8
+  --> $DIR/same_functions_in_if_condition.rs:59:8
    |
 LL |     if v.pop().is_none() {
    |        ^^^^^^^^^^^^^^^^^
 
 error: this `if` has the same function call as a previous `if`
-  --> $DIR/same_functions_in_if_condition.rs:64:15
+  --> $DIR/same_functions_in_if_condition.rs:66:15
    |
 LL |     } else if v.len() == 42 {
    |               ^^^^^^^^^^^^^
    |
 note: same as this
-  --> $DIR/same_functions_in_if_condition.rs:62:8
+  --> $DIR/same_functions_in_if_condition.rs:64:8
    |
 LL |     if v.len() == 42 {
    |        ^^^^^^^^^^^^^

tests/incremental/const-generics/hash-tyvid-regression-1.rs

@@ -1,8 +1,20 @@
 // revisions: cfail
 #![feature(generic_const_exprs, adt_const_params)]
 #![allow(incomplete_features)]
+
+use std::marker::ConstParamTy;
+
+#[derive(PartialEq, Eq, ConstParamTy)]
+struct NonZeroUsize(usize);
+
+impl NonZeroUsize {
+    const fn get(self) -> usize {
+        self.0
+    }
+}
+
 // regression test for #77650
-fn c<T, const N: std::num::NonZeroUsize>()
+fn c<T, const N: NonZeroUsize>()
 where
     [T; N.get()]: Sized,
 {

tests/incremental/const-generics/hash-tyvid-regression-2.rs

@@ -1,11 +1,23 @@
 // revisions: cfail
 #![feature(generic_const_exprs, adt_const_params)]
 #![allow(incomplete_features)]
+
+use std::marker::ConstParamTy;
+
+#[derive(PartialEq, Eq, ConstParamTy)]
+struct NonZeroUsize(usize);
+
+impl NonZeroUsize {
+    const fn get(self) -> usize {
+        self.0
+    }
+}
+
 // regression test for #77650
-struct C<T, const N: core::num::NonZeroUsize>([T; N.get()])
+struct C<T, const N: NonZeroUsize>([T; N.get()])
 where
     [T; N.get()]: Sized;
-impl<'a, const N: core::num::NonZeroUsize, A, B: PartialEq<A>> PartialEq<&'a [A]> for C<B, N>
+impl<'a, const N: NonZeroUsize, A, B: PartialEq<A>> PartialEq<&'a [A]> for C<B, N>
 where
     [B; N.get()]: Sized,
 {

tests/incremental/const-generics/try_unify_abstract_const_regression_tests/issue-77708-3.rs

@@ -2,7 +2,18 @@
 #![feature(generic_const_exprs, adt_const_params)]
 #![allow(incomplete_features)]
 
-use std::{convert::TryFrom, num::NonZeroUsize};
+use std::{convert::TryFrom};
+
+use std::marker::ConstParamTy;
+
+#[derive(PartialEq, Eq, ConstParamTy)]
+struct NonZeroUsize(usize);
+
+impl NonZeroUsize {
+    const fn get(self) -> usize {
+        self.0
+    }
+}
 
 struct A<const N: NonZeroUsize>([u8; N.get()])
 where