Change unit-type from void_type_node to a zero sized unsigned integer

GCC does not support unit-type when it is represented as void, unsigned
integer of precision zero seems to work. The missing piece here is that
functions with return type of unit are still represented by a void type
node there is an ICE in

```
0x1d551d5 wi::min_value(unsigned int,signop)
    ../../gccrs/gcc/wide-int.cc:346
```

Addresses #155

6 files changed, 33 insertions, 8 deletions

diff --git a/gcc/rust/backend/rust-compile-context.h b/gcc/rust/backend/rust-compile-context.h
index 5bd1e96..e0c9352 100644
--- a/gcc/rust/backend/rust-compile-context.h
+++ b/gcc/rust/backend/rust-compile-context.h
@@ -423,7 +423,7 @@ public:
   {
     if (type.num_fields () == 0)
       {
-	translated = ctx->get_backend ()->void_type ();
+	translated = ctx->get_backend ()->unit_type ();
 	return;
       }
 
@@ -544,7 +544,7 @@ public:
 
   void visit (TyTy::NeverType &) override
   {
-    translated = ctx->get_backend ()->void_type ();
+    translated = ctx->get_backend ()->unit_type ();
   }
 
 private:
diff --git a/gcc/rust/backend/rust-compile-tyty.h b/gcc/rust/backend/rust-compile-tyty.h
index ba98ac0..8576235 100644
--- a/gcc/rust/backend/rust-compile-tyty.h
+++ b/gcc/rust/backend/rust-compile-tyty.h
@@ -42,8 +42,6 @@ public:
     return compiler.translated;
   }
 
-  ~TyTyCompile () {}
-
   void visit (TyTy::ErrorType &) override { gcc_unreachable (); }
 
   void visit (TyTy::InferType &) override { gcc_unreachable (); }
@@ -53,7 +51,7 @@ public:
   void visit (TyTy::TupleType &type) override
   {
     if (type.num_fields () == 0)
-      translated = backend->void_type ();
+      translated = backend->unit_type ();
     else
       gcc_unreachable ();
   }
@@ -224,7 +222,7 @@ public:
 
   void visit (TyTy::NeverType &) override
   {
-    translated = backend->void_type ();
+    translated = backend->unit_type ();
   }
 
 private:
diff --git a/gcc/rust/rust-backend.h b/gcc/rust/rust-backend.h
index e71d81e..35271b6 100644
--- a/gcc/rust/rust-backend.h
+++ b/gcc/rust/rust-backend.h
@@ -109,6 +109,9 @@ public:
   // unsafe.Pointer is represented as *void.
   virtual Btype *void_type () = 0;
 
+  // get unit-type
+  virtual Btype *unit_type () = 0;
+
   // Get the unnamed boolean type.
   virtual Btype *bool_type () = 0;
 
diff --git a/gcc/rust/rust-gcc.cc b/gcc/rust/rust-gcc.cc
index 3158c11..794660e 100644
--- a/gcc/rust/rust-gcc.cc
+++ b/gcc/rust/rust-gcc.cc
@@ -171,6 +171,19 @@ public:
 
   Btype *void_type () { return this->make_type (void_type_node); }
 
+  Btype *unit_type ()
+  {
+    static Btype *unit_type;
+    if (unit_type == nullptr)
+      {
+	auto unit_type_node = integer_type (true, 0);
+	unit_type = named_type ("()", unit_type_node,
+				::Linemap::predeclared_location ());
+      }
+
+    return unit_type;
+  }
+
   Btype *bool_type () { return this->make_type (boolean_type_node); }
 
   Btype *char_type () { return this->make_type (char_type_node); }
@@ -297,7 +310,10 @@ public:
     return this->make_expression (null_pointer_node);
   }
 
-  Bexpression *unit_expression () { return this->make_expression (void_node); }
+  Bexpression *unit_expression ()
+  {
+    return this->make_expression (integer_zero_node);
+  }
 
   Bexpression *var_expression (Bvariable *var, Location);
 
diff --git a/gcc/rust/typecheck/rust-hir-const-fold.h b/gcc/rust/typecheck/rust-hir-const-fold.h
index 4c030c5..c134d51 100644
--- a/gcc/rust/typecheck/rust-hir-const-fold.h
+++ b/gcc/rust/typecheck/rust-hir-const-fold.h
@@ -54,7 +54,7 @@ public:
   void visit (TyTy::TupleType &type) override
   {
     if (type.num_fields () == 0)
-      translated = backend->void_type ();
+      translated = backend->unit_type ();
     else
       gcc_unreachable ();
   }
diff --git a/gcc/testsuite/rust/compile/torture/unit_type2.rs b/gcc/testsuite/rust/compile/torture/unit_type2.rs
new file mode 100644
index 0000000..b5f9259
--- /dev/null
+++ b/gcc/testsuite/rust/compile/torture/unit_type2.rs
@@ -0,0 +1,8 @@
+fn test(a: ()) -> () {
+    a
+}
+
+fn main() {
+    let a;
+    a = test(());
+}