Fix Slice Type Layout

Slices in Rust are represented by TypePaths such as '[i32]'. Though if you
actually try to use this explicit type-path you will hit errors such as
this type has an unknown size at compile time. This is because this is
actually what Rust calls a dynamically sized type. This means when you use
types such as: '&[i32]' it is not actually a reference type to a slice. Its
a slice in its entirety this means for lack of a better word when you use
'*const [i32]' or '&mut [i32]' we end up actually passing around a struct
by value _not_ at pointer/reference to it.

This patch changes the type-layout so that we handle this layout change
properly. This patch will also need to be applied to str types which I
believe have a similar layout for safety.

The patch also sets up TYPE_MAIN_VARIANT so that we can avoid unnessecary
view_convert_expressions between *const [i32] and &mut [i32] which will
have the same layout.

Reference:

https://doc.rust-lang.org/reference/dynamically-sized-types.html

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=672adac002939a2dab43b8d231adc1dc

Fixes #1232

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/rust/backend/rust-compile.cc b/gcc/rust/backend/rust-compile.cc
index d9349d5..18a2df6 100644
--- a/gcc/rust/backend/rust-compile.cc
+++ b/gcc/rust/backend/rust-compile.cc
@@ -218,6 +218,11 @@ HIRCompileBase::coercion_site (tree rvalue, const TyTy::BaseType *rval,
 	= static_cast<const TyTy::ReferenceType *> (expected);
       const TyTy::ReferenceType *act
 	= static_cast<const TyTy::ReferenceType *> (actual);
+      if (act->is_dyn_slice_type ())
+	{
+	  // nothing to do
+	  return rvalue;
+	}
 
       tree expected_type = TyTyResolveCompile::compile (ctx, act->get_base ());
       tree deref_rvalue
@@ -227,6 +232,8 @@ HIRCompileBase::coercion_site (tree rvalue, const TyTy::BaseType *rval,
       tree coerced
 	= coercion_site (deref_rvalue, act->get_base (), exp->get_base (),
 			 lvalue_locus, rvalue_locus);
+      if (exp->is_dyn_slice_type () && SLICE_TYPE_P (TREE_TYPE (coerced)))
+	return coerced;
 
       return address_expression (coerced,
 				 build_reference_type (TREE_TYPE (coerced)),
@@ -249,6 +256,12 @@ HIRCompileBase::coercion_site (tree rvalue, const TyTy::BaseType *rval,
 	{
 	  const TyTy::ReferenceType *act
 	    = static_cast<const TyTy::ReferenceType *> (actual);
+	  if (act->is_dyn_slice_type ())
+	    {
+	      // nothing to do
+	      return rvalue;
+	    }
+
 	  actual_base = act->get_base ();
 	  expected_type = TyTyResolveCompile::compile (ctx, act->get_base ());
 	}
@@ -256,6 +269,12 @@ HIRCompileBase::coercion_site (tree rvalue, const TyTy::BaseType *rval,
 	{
 	  const TyTy::PointerType *act
 	    = static_cast<const TyTy::PointerType *> (actual);
+	  if (act->is_dyn_slice_type ())
+	    {
+	      // nothing to do
+	      return rvalue;
+	    }
+
 	  actual_base = act->get_base ();
 	  expected_type = TyTyResolveCompile::compile (ctx, act->get_base ());
 	}