Ensure the coercion sites are applied correctly on unions

When we construct a union, we were wrongly compiling the constructor
without taking into account the associated union index. This means we ended
up trying to coerce the wrong field which was always zero here which
resulted in a silent failure by returning error mark node.

The offending test case was: gcc/testsuite/execute/torture/slice1.rs

1 files changed, 41 insertions, 7 deletions

diff --git a/gcc/rust/backend/rust-compile-expr.h b/gcc/rust/backend/rust-compile-expr.h
index bb81d4e..79d9e57 100644
--- a/gcc/rust/backend/rust-compile-expr.h
+++ b/gcc/rust/backend/rust-compile-expr.h
@@ -419,18 +419,19 @@ public:
     // compile it
     tree compiled_adt_type = TyTyResolveCompile::compile (ctx, tyty);
 
-    // this assumes all fields are in order from type resolution and if a base
-    // struct was specified those fields are filed via accesors
     std::vector<tree> arguments;
-    for (size_t i = 0; i < struct_expr.get_fields ().size (); i++)
+    if (adt->is_union ())
       {
+	rust_assert (struct_expr.get_fields ().size () == 1);
+
 	// assignments are coercion sites so lets convert the rvalue if
 	// necessary
-	auto respective_field = variant->get_field_at_index (i);
+	auto respective_field
+	  = variant->get_field_at_index (union_disriminator);
 	auto expected = respective_field->get_field_type ();
 
 	// process arguments
-	auto &argument = struct_expr.get_fields ().at (i);
+	auto &argument = struct_expr.get_fields ().at (0);
 	auto lvalue_locus
 	  = ctx->get_mappings ()->lookup_location (expected->get_ty_ref ());
 	auto rvalue_locus = argument->get_locus ();
@@ -440,8 +441,6 @@ public:
 	bool ok = ctx->get_tyctx ()->lookup_type (
 	  argument->get_mappings ().get_hirid (), &actual);
 
-	// coerce it if required/possible see
-	// compile/torture/struct_base_init_1.rs
 	if (ok)
 	  {
 	    rvalue = coercion_site (rvalue, actual, expected, lvalue_locus,
@@ -451,6 +450,41 @@ public:
 	// add it to the list
 	arguments.push_back (rvalue);
       }
+    else
+      {
+	// this assumes all fields are in order from type resolution and if a
+	// base struct was specified those fields are filed via accesors
+	for (size_t i = 0; i < struct_expr.get_fields ().size (); i++)
+	  {
+	    // assignments are coercion sites so lets convert the rvalue if
+	    // necessary
+	    auto respective_field = variant->get_field_at_index (i);
+	    auto expected = respective_field->get_field_type ();
+
+	    // process arguments
+	    auto &argument = struct_expr.get_fields ().at (i);
+	    auto lvalue_locus
+	      = ctx->get_mappings ()->lookup_location (expected->get_ty_ref ());
+	    auto rvalue_locus = argument->get_locus ();
+	    auto rvalue
+	      = CompileStructExprField::Compile (argument.get (), ctx);
+
+	    TyTy::BaseType *actual = nullptr;
+	    bool ok = ctx->get_tyctx ()->lookup_type (
+	      argument->get_mappings ().get_hirid (), &actual);
+
+	    // coerce it if required/possible see
+	    // compile/torture/struct_base_init_1.rs
+	    if (ok)
+	      {
+		rvalue = coercion_site (rvalue, actual, expected, lvalue_locus,
+					rvalue_locus);
+	      }
+
+	    // add it to the list
+	    arguments.push_back (rvalue);
+	  }
+      }
 
     // the constructor depends on whether this is actually an enum or not if
     // its an enum we need to setup the discriminator