gccrs: Fix ICE with non-trailing const defaults

gcc/rust/parse/rust-parse-impl.h

@@ -3262,21 +3262,42 @@ Parser<ManagedTokenSource>::parse_generic_params (EndTokenPred is_end_token)
 
   // Did we parse a generic type param yet
   auto type_seen = false;
+  // Did we parse a const param with a default value yet
+  auto const_with_default_seen = false;
   // Did the user write a lifetime parameter after a type one
   auto order_error = false;
+  // Did the user write a const param with a default value after a type one
+  auto const_with_default_order_error = false;
 
   // parse lifetime params
   while (!is_end_token (lexer.peek_token ()->get_id ()))
     {
       auto param = parse_generic_param (is_end_token);
       if (param)
 	{
-	  // TODO: Handle `Const` here as well if necessary
 	  if (param->get_kind () == AST::GenericParam::Kind::Type)
-	    type_seen = true;
+	    {
+	      type_seen = true;
+	      if (const_with_default_seen)
+		const_with_default_order_error = true;
+	    }
 	  else if (param->get_kind () == AST::GenericParam::Kind::Lifetime
 		   && type_seen)
-	    order_error = true;
+	    {
+	      order_error = true;
+	      if (const_with_default_seen)
+		const_with_default_order_error = true;
+	    }
+	  else if (param->get_kind () == AST::GenericParam::Kind::Const)
+	    {
+	      type_seen = true;
+	      AST::ConstGenericParam *const_param
+		= static_cast<AST::ConstGenericParam *> (param.get ());
+	      if (const_param->has_default_value ())
+		const_with_default_seen = true;
+	      else if (const_with_default_seen)
+		const_with_default_order_error = true;
+	    }
 
 	  generic_params.emplace_back (std::move (param));
 	  maybe_skip_token (COMMA);
@@ -3293,6 +3314,13 @@ Parser<ManagedTokenSource>::parse_generic_params (EndTokenPred is_end_token)
 		   "must be declared prior to type and const parameters");
       add_error (std::move (error));
     }
+  if (const_with_default_order_error)
+    {
+      Error error (generic_params.front ()->get_locus (),
+		   "invalid order for generic parameters: generic parameters "
+		   "with a default must be trailing");
+      add_error (std::move (error));
+    }
 
   generic_params.shrink_to_fit ();
   return generic_params;

gcc/testsuite/rust/compile/const_generics_17.rs

@@ -0,0 +1,3 @@
+struct Foo<const N: u32 = 1, const O: bool>; // { dg-error "invalid order for generic parameters: generic parameters with a default must be trailing" }
+
+impl<const N: u32> Foo<N> {}

gcc/testsuite/rust/compile/generics14.rs

@@ -0,0 +1 @@
+struct Foo<const N: i32, 'a>; // { dg-error "invalid order for generic parameters: lifetime parameters must be declared prior to type and const parameters" }