mark ty::Const::Error when meet unsupport ty for const generic params

compiler/rustc_const_eval/src/const_eval/valtrees.rs

@@ -82,6 +82,10 @@ pub(crate) fn const_to_valtree_inner<'tcx>(
         return Err(ValTreeCreationError::NodesOverflow);
     }
 
+    if ty::maybe_not_supported_generic_const_param(ty).is_some_and(|v| v) {
+        return Err(ValTreeCreationError::NonSupportedType);
+    }
+
     match ty.kind() {
         ty::FnDef(..) => {
             *num_nodes += 1;
@@ -97,62 +101,38 @@ pub(crate) fn const_to_valtree_inner<'tcx>(
             Ok(ty::ValTree::Leaf(val.assert_int()))
         }
 
-        // Raw pointers are not allowed in type level constants, as we cannot properly test them for
-        // equality at compile-time (see `ptr_guaranteed_cmp`).
-        // Technically we could allow function pointers (represented as `ty::Instance`), but this is not guaranteed to
-        // agree with runtime equality tests.
-        ty::FnPtr(_) | ty::RawPtr(_) => Err(ValTreeCreationError::NonSupportedType),
-
-        ty::Ref(_, _, _)  => {
-            let Ok(derefd_place)= ecx.deref_pointer(place) else {
+        ty::Ref(_, _, _) => {
+            let Ok(derefd_place) = ecx.deref_pointer(place) else {
                 return Err(ValTreeCreationError::Other);
             };
             debug!(?derefd_place);
 
             const_to_valtree_inner(ecx, &derefd_place, num_nodes)
         }
 
-        ty::Str | ty::Slice(_) | ty::Array(_, _) => {
-            slice_branches(ecx, place, num_nodes)
-        }
-        // 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,
-        // but it is unclear if this is useful.
-        ty::Dynamic(..) => Err(ValTreeCreationError::NonSupportedType),
-
-        ty::Tuple(elem_tys) => {
-            branches(ecx, place, elem_tys.len(), None, num_nodes)
-        }
+        ty::Str | ty::Slice(_) | ty::Array(_, _) => slice_branches(ecx, place, num_nodes),
+
+        ty::Tuple(elem_tys) => branches(ecx, place, elem_tys.len(), None, num_nodes),
 
         ty::Adt(def, _) => {
             if def.is_union() {
-                return Err(ValTreeCreationError::NonSupportedType);
+                unreachable!();
             } else if def.variants().is_empty() {
                 bug!("uninhabited types should have errored and never gotten converted to valtree")
             }
 
             let Ok(variant) = ecx.read_discriminant(place) else {
                 return Err(ValTreeCreationError::Other);
             };
-            branches(ecx, place, def.variant(variant).fields.len(), def.is_enum().then_some(variant), num_nodes)
+            branches(
+                ecx,
+                place,
+                def.variant(variant).fields.len(),
+                def.is_enum().then_some(variant),
+                num_nodes,
+            )
         }
-
-        ty::Never
-        | ty::Error(_)
-        | ty::Foreign(..)
-        | ty::Infer(ty::FreshIntTy(_))
-        | ty::Infer(ty::FreshFloatTy(_))
-        // FIXME(oli-obk): we could look behind opaque types
-        | ty::Alias(..)
-        | ty::Param(_)
-        | ty::Bound(..)
-        | ty::Placeholder(..)
-        | ty::Infer(_)
-        // FIXME(oli-obk): we can probably encode closures just like structs
-        | ty::Closure(..)
-        | ty::Coroutine(..)
-        | ty::CoroutineWitness(..) => Err(ValTreeCreationError::NonSupportedType),
+        _ => unreachable!(),
     }
 }
 

compiler/rustc_hir_analysis/src/collect/type_of.rs

@@ -54,10 +54,20 @@ fn anon_const_type_of<'tcx>(tcx: TyCtxt<'tcx>, def_id: LocalDefId) -> Ty<'tcx> {
             kind: GenericParamKind::Const { default: Some(ct), .. },
             ..
         }) if ct.hir_id == hir_id => {
-            return tcx
+            let ty = tcx
                 .type_of(param_def_id)
                 .no_bound_vars()
                 .expect("const parameter types cannot be generic");
+
+            if ty::maybe_not_supported_generic_const_param(ty).is_some_and(|v| v)  {
+                return Ty::new_error_with_message(
+                    tcx,
+                    tcx.def_span(def_id),
+                    "meet unsupported const generics type",
+                )
+            } else {
+                return ty;
+            }
         }
 
         Node::TypeBinding(binding @ &TypeBinding { hir_id: binding_id, .. })

compiler/rustc_middle/src/ty/mod.rs

@@ -462,6 +462,42 @@ pub struct CReaderCacheKey {
 #[rustc_pass_by_value]
 pub struct Ty<'tcx>(Interned<'tcx, WithCachedTypeInfo<TyKind<'tcx>>>);
 
+pub fn maybe_not_supported_generic_const_param(ty: Ty<'_>) -> Option<bool> {
+    match ty.kind() {
+        // Raw pointers are not allowed in type level constants, as we cannot properly test them for
+        // equality at compile-time (see `ptr_guaranteed_cmp`).
+        // Technically we could allow function pointers (represented as `ty::Instance`), but this is not guaranteed to
+        // agree with runtime equality tests.
+        ty::FnPtr(_) | ty::RawPtr(_) => {
+            Some(true)
+        },
+        // 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,
+        // but it is unclear if this is useful.
+        ty::Dynamic(..) => Some(true),
+        ty::Adt(def, _) => {
+            def.is_union().then_some(true)
+        },
+        ty::Never
+        | ty::Error(_)
+        | ty::Foreign(..)
+        | ty::Infer(ty::FreshIntTy(_))
+        | ty::Infer(ty::FreshFloatTy(_))
+        // FIXME(oli-obk): we could look behind opaque types
+        | ty::Alias(..)
+        | ty::Param(_)
+        | ty::Bound(..)
+        | ty::Placeholder(..)
+        | ty::Infer(_)
+        // FIXME(oli-obk): we can probably encode closures just like structs
+        | ty::Closure(..)
+        | ty::Coroutine(..)
+        | ty::CoroutineWitness(..) => Some(true),
+        _ => None,
+    }
+}
+
 impl ty::EarlyBoundRegion {
     /// Does this early bound region have a name? Early bound regions normally
     /// always have names except when using anonymous lifetimes (`'_`).

tests/ui/consts/issue-116796.rs

@@ -0,0 +1,3 @@
+struct X<const FN: fn() = { || {} }>;
+//~^ ERROR using function pointers as const generic parameters is forbidden
+fn main() {}

tests/ui/consts/issue-116796.stderr

@@ -0,0 +1,10 @@
+error: using function pointers as const generic parameters is forbidden
+  --> $DIR/issue-116796.rs:1:20
+   |
+LL | struct X<const FN: fn() = { || {} }>;
+   |                    ^^^^
+   |
+   = note: the only supported types are integers, `bool` and `char`
+
+error: aborting due to previous error
+