gccrs: add discriminant value intrinsic

This is pretty nasty piece of rust in my opinion the return type of this
intrinsic results into a lang item associated type:

  <T as DiscriminantKind>::Discriminant

This is a special case which needs to support mapping onto the repr type
of the associated ADT that is passed in, but defaults to iszie otherwise.

This patch assumes we only come accross this case in a HIR::CALL_EXPR, so
and makes assumutions that its always of this function signiture. I will
do some checking in libcore to verify that assumption. More work is needed
to parse the repr type on enums but the code is there to support this when
its in to change the types etc.

Addresses Rust-GCC#3348

gcc/rust/ChangeLog:

	* backend/rust-compile-intrinsic.cc (discriminant_value_handler): new handler
	* typecheck/rust-hir-trait-resolve.cc (TraitItemReference::resolve_item): track the defid
	* typecheck/rust-hir-type-check-base.cc (TypeCheckBase::parse_repr_options): default isize
	* typecheck/rust-hir-type-check-expr.cc (TypeCheckExpr::visit): special case CallExpr
	* typecheck/rust-hir-type-check-item.cc (TypeCheckItem::visit): parse repr options enum
	* typecheck/rust-hir-type-check-type.cc (TypeCheckType::visit): remove bad diagnostic
	* typecheck/rust-tyty.cc (PlaceholderType::PlaceholderType): track defid
	(PlaceholderType::clone): likewise
	(PlaceholderType::get_def_id): likeiwse
	* typecheck/rust-tyty.h: placeholder track defid
	* util/rust-lang-item.cc: add new lang items
	* util/rust-lang-item.h: likewise

gcc/testsuite/ChangeLog:

	* rust/execute/torture/enum_intrinsics1.rs: New test.

Signed-off-by: Philip Herron <herron.philip@googlemail.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/gcc/rust/util/rust-lang-item.h b/gcc/rust/util/rust-lang-item.h
index 61d6454e..1aaa887 100644
--- a/gcc/rust/util/rust-lang-item.h
+++ b/gcc/rust/util/rust-lang-item.h
@@ -147,6 +147,9 @@ public:
 
     STRUCTURAL_PEQ,
     STRUCTURAL_TEQ,
+
+    DISCRIMINANT_TYPE,
+    DISCRIMINANT_KIND,
   };
 
   static const BiMap<std::string, Kind> lang_items;