add const_param_ty_unchecked gate

Author: zedddie

compiler/rustc_feature/src/unstable.rs

@@ -229,6 +229,8 @@ declare_features! (
     (internal, cfg_target_has_reliable_f16_f128, "1.88.0", None),
     /// Allows identifying the `compiler_builtins` crate.
     (internal, compiler_builtins, "1.13.0", None),
+    /// Allows skipping `ConstParamTy_` trait implementation checks
+    (internal, const_param_ty_unchecked, "CURRENT_RUSTC_VERSION", None),
     /// Allows writing custom MIR
     (internal, custom_mir, "1.65.0", None),
     /// Implementation details of externally implementable items

compiler/rustc_hir_analysis/src/coherence/builtin.rs

@@ -184,6 +184,10 @@ fn visit_implementation_of_const_param_ty(checker: &Checker<'_>) -> Result<(), E
         return Ok(());
     }
 
+    if tcx.features().const_param_ty_unchecked() {
+        return Ok(());
+    }
+
     let cause = traits::ObligationCause::misc(DUMMY_SP, impl_did);
     match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, cause) {
         Ok(()) => Ok(()),

compiler/rustc_span/src/symbol.rs

@@ -676,6 +676,7 @@ symbols! {
         const_panic,
         const_panic_fmt,
         const_param_ty,
+        const_param_ty_unchecked,
         const_precise_live_drops,
         const_ptr_cast,
         const_raw_ptr_deref,

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

@@ -0,0 +1,20 @@
+//@check-pass
+//! Ensure that const_param_ty_unchecked gate allow
+//! bypassing `ConstParamTy_` implementation check
+
+#![allow(incomplete_features)]
+//FIXME: change to const_param_ty_trait gate when it will be merged
+#![feature(unsized_const_params, adt_const_params)]
+#![feature(const_param_ty_unchecked)]
+
+use std::marker::ConstParamTy_;
+
+#[derive(PartialEq, Eq)]
+struct Miow; // Miow does not implement ConstParamTy_.
+
+#[derive(PartialEq, Eq)]
+struct Meoww(Miow);
+
+impl ConstParamTy_ for Meoww {}
+
+fn main() {}

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

@@ -0,0 +1,19 @@
+// gate-test-const_param_ty_unchecked
+//! Ensure this fails when const_param_ty_unchecked isn't used
+
+#![allow(incomplete_features)]
+//FIXME: change to const_param_ty_trait gate when it will be merged
+#![feature(unsized_const_params, adt_const_params)]
+
+use std::marker::ConstParamTy_;
+
+#[derive(PartialEq, Eq)]
+struct Miow; // Miow does not implement ConstParamTy_.
+
+#[derive(PartialEq, Eq)]
+struct Meoww(Miow);
+
+impl ConstParamTy_ for Meoww {}
+                    //~^ ERROR: the trait `ConstParamTy_` cannot be implemented for this type [E0204]
+
+fn main() {}

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

@@ -0,0 +1,12 @@
+error[E0204]: the trait `ConstParamTy_` cannot be implemented for this type
+  --> $DIR/const_param_ty_unchecked_fail.rs:16:24
+   |
+LL | struct Meoww(Miow);
+   |              ---- this field does not implement `ConstParamTy_`
+LL |
+LL | impl ConstParamTy_ for Meoww {}
+   |                        ^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0204`.