From d8940f5fd712873a79cce27aea6f0e05e58ae09a Mon Sep 17 00:00:00 2001
From: gingerBill <bill@gingerbill.org>
Date: Sat, 26 Jun 2021 12:08:14 +0100
Subject: [PATCH] Support compound literals for `struct #raw_union` types

---
 src/check_expr.cpp   | 52 ++++++++++++++++++++++++++++++++++++++++----
 src/llvm_backend.cpp | 14 ++++++++++--
 2 files changed, 60 insertions(+), 6 deletions(-)

diff --git a/src/check_expr.cpp b/src/check_expr.cpp
index 16a574b3d..537fb73cc 100644
--- a/src/check_expr.cpp
+++ b/src/check_expr.cpp
@@ -6557,10 +6557,54 @@ ExprKind check_expr_base_internal(CheckerContext *c, Operand *o, Ast *node, Type
 				break; // NOTE(bill): No need to init
 			}
 			if (t->Struct.is_raw_union) {
-				if (cl->elems.count != 0) {
-					gbString type_str = type_to_string(type);
-					error(node, "Illegal compound literal type '%s'", type_str);
-					gb_string_free(type_str);
+				if (cl->elems.count > 0) {
+					// NOTE: unions cannot be constant
+					is_constant = false;
+
+					if (cl->elems[0]->kind != Ast_FieldValue) {
+						gbString type_str = type_to_string(type);
+						error(node, "%s ('struct #raw_union') compound literals are only allowed to contain 'field = value' elements", type_str);
+						gb_string_free(type_str);
+					} else {
+						if (cl->elems.count != 1) {
+							gbString type_str = type_to_string(type);
+							error(node, "%s ('struct #raw_union') compound literals are only allowed to contain up to 1 'field = value' element, got %td", type_str, cl->elems.count);
+							gb_string_free(type_str);
+						} else {
+							Ast *elem = cl->elems[0];
+							ast_node(fv, FieldValue, elem);
+							if (fv->field->kind != Ast_Ident) {
+								gbString expr_str = expr_to_string(fv->field);
+								error(elem, "Invalid field name '%s' in structure literal", expr_str);
+								gb_string_free(expr_str);
+								break;
+							}
+
+							String name = fv->field->Ident.token.string;
+
+							Selection sel = lookup_field(type, name, o->mode == Addressing_Type);
+							bool is_unknown = sel.entity == nullptr;
+							if (is_unknown) {
+								error(elem, "Unknown field '%.*s' in structure literal", LIT(name));
+								break;
+							}
+
+							if (sel.index.count > 1) {
+								error(elem, "Cannot assign to an anonymous field '%.*s' in a structure literal (at the moment)", LIT(name));
+								break;
+							}
+
+							Entity *field = t->Struct.fields[sel.index[0]];
+							add_entity_use(c, fv->field, field);
+
+							Operand o = {};
+							check_expr_or_type(c, &o, fv->value, field->type);
+
+
+							check_assignment(c, &o, field->type, str_lit("structure literal"));
+						}
+
+					}
 				}
 				break;
 			}
diff --git a/src/llvm_backend.cpp b/src/llvm_backend.cpp
index 91d074554..a9a9ad0ac 100644
--- a/src/llvm_backend.cpp
+++ b/src/llvm_backend.cpp
@@ -6839,6 +6839,10 @@ lbValue lb_const_value(lbModule *m, Type *type, ExactValue value, bool allow_loc
 				return lb_const_nil(m, original_type);
 			}
 
+			if (is_type_raw_union(type)) {
+				return lb_const_nil(m, original_type);
+			}
+
 			isize offset = 0;
 			if (type->Struct.custom_align > 0) {
 				offset = 1;
@@ -13464,6 +13468,8 @@ lbAddr lb_build_addr(lbProcedure *p, Ast *expr) {
 			TypeStruct *st = &bt->Struct;
 			if (cl->elems.count > 0) {
 				lb_addr_store(p, v, lb_const_value(p->module, type, exact_value_compound(expr)));
+				lbValue comp_lit_ptr = lb_addr_get_ptr(p, v);
+
 				for_array(field_index, cl->elems) {
 					Ast *elem = cl->elems[field_index];
 
@@ -13492,6 +13498,12 @@ lbAddr lb_build_addr(lbProcedure *p, Ast *expr) {
 
 					field_expr = lb_build_expr(p, elem);
 
+					lbValue gep = {};
+					if (is_raw_union) {
+						gep = lb_emit_conv(p, comp_lit_ptr, alloc_type_pointer(ft));
+					} else {
+						gep = lb_emit_struct_ep(p, comp_lit_ptr, cast(i32)index);
+					}
 
 					Type *fet = field_expr.type;
 					GB_ASSERT(fet->kind != Type_Tuple);
@@ -13500,11 +13512,9 @@ lbAddr lb_build_addr(lbProcedure *p, Ast *expr) {
 					if (is_type_union(ft) && !are_types_identical(fet, ft) && !is_type_untyped(fet)) {
 						GB_ASSERT_MSG(union_variant_index(ft, fet) > 0, "%s", type_to_string(fet));
 
-						lbValue gep = lb_emit_struct_ep(p, lb_addr_get_ptr(p, v), cast(i32)index);
 						lb_emit_store_union_variant(p, gep, field_expr, fet);
 					} else {
 						lbValue fv = lb_emit_conv(p, field_expr, ft);
-						lbValue gep = lb_emit_struct_ep(p, lb_addr_get_ptr(p, v), cast(i32)index);
 						lb_emit_store(p, gep, fv);
 					}
 				}