gccrs: cleanup our enum type layout to be closer to rustc

This changes our enum type layout so for example:

  enum Foo {
      A,
      B,
      C(char),
      D { x: i32, y: i32 },
  }

Used to get layed out like this in gccrs:

  union {
    struct A { int RUST$ENUM$DISR; };
    struct B { int RUST$ENUM$DISR; };
    struct C { int RUST$ENUM$DISR; char __0; };
    struct D { int RUST$ENUM$DISR; i64 x; i64 y; };
  }

This has some issues notably with the constexpr because this is just a
giant union it means its not simple to constify what enum variant we are
looking at because the discriminant is a mess.

This now gets layed out as:

  struct {
     int RUST$ENUM$DISR;
     union {
         struct A { };
         struct B { };
         struct C { char __0; };
         struct D { i64 x; i64 y; };
     } payload;
  }

This layout is much cleaner and allows for our constexpr to work properly.

gcc/rust/ChangeLog:

	* backend/rust-compile-expr.cc (CompileExpr::visit): new layout
	* backend/rust-compile-pattern.cc (CompilePatternCheckExpr::visit): likewise
	(CompilePatternBindings::visit): likewise
	* backend/rust-compile-resolve-path.cc: likewise
	* backend/rust-compile-type.cc (TyTyResolveCompile::visit): implement new layout
	* rust-gcc.cc (constructor_expression): get rid of useless assert

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

1 files changed, 3 insertions, 2 deletions

diff --git a/gcc/rust/backend/rust-compile-resolve-path.cc b/gcc/rust/backend/rust-compile-resolve-path.cc
index 5f6ba7c..049b0d8 100644
--- a/gcc/rust/backend/rust-compile-resolve-path.cc
+++ b/gcc/rust/backend/rust-compile-resolve-path.cc
@@ -86,8 +86,9 @@ ResolvePathRef::attempt_constructor_expression_lookup (
   tree folded_discrim_expr = fold_expr (discrim_expr_node);
   tree qualifier = folded_discrim_expr;
 
-  return Backend::constructor_expression (compiled_adt_type, true, {qualifier},
-					  union_disriminator, expr_locus);
+  // false for is enum but this is an enum but we have a new layout
+  return Backend::constructor_expression (compiled_adt_type, false, {qualifier},
+					  -1, expr_locus);
 }
 
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