Introduce some tests

tests/ui/const-generics/adt_const_params/const_param_ty_object_safety.rs

@@ -0,0 +1,9 @@
+#![feature(adt_const_params, unsized_const_parameters)]
+#![allow(incomplete_features)]
+
+use std::marker::ConstParamTy_;
+
+fn foo(a: &dyn ConstParamTy_) {}
+//~^ ERROR: the trait `ConstParamTy_`
+
+fn main() {}

tests/ui/const-generics/adt_const_params/const_param_ty_object_safety.stderr

@@ -0,0 +1,18 @@
+error[E0038]: the trait `ConstParamTy_` cannot be made into an object
+  --> $DIR/const_param_ty_object_safety.rs:6:12
+   |
+LL | fn foo(a: &dyn ConstParamTy_) {}
+   |            ^^^^^^^^^^^^^^^^^ `ConstParamTy_` cannot be made into an object
+   |
+note: for a trait to be "object safe" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
+  --> $SRC_DIR/core/src/cmp.rs:LL:COL
+   |
+   = note: the trait cannot be made into an object because it uses `Self` as a type parameter
+help: consider using an opaque type instead
+   |
+LL | fn foo(a: &impl ConstParamTy_) {}
+   |            ~~~~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0038`.

tests/ui/const-generics/adt_const_params/reference_pointee_is_const_param-1.rs

@@ -0,0 +1,11 @@
+#![feature(adt_const_params, unsized_const_parameters)]
+#![allow(incomplete_features)]
+
+use std::marker::ConstParamTy_;
+
+struct Foo;
+
+impl ConstParamTy_ for &'static Foo {}
+//~^ ERROR: the trait `ConstParamTy_` cannot be implemented for this type
+
+fn main() {}

tests/ui/const-generics/adt_const_params/reference_pointee_is_const_param-1.stderr

@@ -0,0 +1,9 @@
+error[E0204]: the trait `ConstParamTy_` cannot be implemented for this type
+  --> $DIR/reference_pointee_is_const_param-1.rs:8:24
+   |
+LL | impl ConstParamTy_ for &'static Foo {}
+   |                        ^^^^^^^^^^^^ this field does not implement `ConstParamTy_`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0204`.

tests/crashes/119299.rs → tests/ui/const-generics/adt_const_params/reference_pointee_is_const_param-2.rs

@@ -1,13 +1,15 @@
-//@ known-bug: #119299
-#![feature(adt_const_params)]
+#![feature(adt_const_params, unsized_const_parameters)]
 #![allow(incomplete_features)]
 
+// Regression test for #119299
+
 use std::marker::ConstParamTy_;
 
 #[derive(Eq, PartialEq)]
 struct ConstStrU(*const u8, usize);
 
 impl ConstParamTy_ for &'static ConstStrU {}
+//~^ ERROR: the trait `ConstParamTy_` cannot be implemented for this type
 
 impl ConstStrU {
     const fn from_bytes(bytes: &'static [u8]) -> Self {

tests/ui/const-generics/adt_const_params/reference_pointee_is_const_param-2.stderr

@@ -0,0 +1,9 @@
+error[E0204]: the trait `ConstParamTy_` cannot be implemented for this type
+  --> $DIR/reference_pointee_is_const_param-2.rs:11:24
+   |
+LL | impl ConstParamTy_ for &'static ConstStrU {}
+   |                        ^^^^^^^^^^^^^^^^^^ this field does not implement `ConstParamTy_`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0204`.