ability to cast explicitly from int to enum

This commit also fixes a bug where pure functions are marked with
the read-only attribute in debug mode. This resulted in incorrect
codegen because calls to read-only functions with unused values
were not generated.

For example, a call to assert() would not be generated if assert
is marked with read-only. Which it *is* marked with in release
mode.

src/all_types.hpp

@@ -383,6 +383,7 @@ enum CastOp {
     CastOpFloatToInt,
     CastOpBoolToInt,
     CastOpResizeSlice,
+    CastOpIntToEnum,
 };
 
 struct AstNodeFnCallExpr {

src/analyze.cpp

@@ -4428,6 +4428,14 @@ static TypeTableEntry *analyze_cast_expr(CodeGen *g, ImportTableEntry *import, B
         }
     }
 
+    // explicit cast from integer to enum type with no payload
+    if (actual_type->id == TypeTableEntryIdInt &&
+        wanted_type->id == TypeTableEntryIdEnum &&
+        wanted_type->data.enumeration.gen_field_count == 0)
+    {
+        return resolve_cast(g, context, node, expr_node, wanted_type, CastOpIntToEnum, false);
+    }
+
     add_node_error(g, node,
         buf_sprintf("invalid cast from type '%s' to '%s'",
             buf_ptr(&actual_type->name),

src/codegen.cpp

@@ -1032,6 +1032,8 @@ static LLVMValueRef gen_cast_expr(CodeGen *g, AstNode *node) {
             set_debug_source_node(g, node);
             return LLVMBuildZExt(g->builder, expr_val, wanted_type->type_ref, "");
 
+        case CastOpIntToEnum:
+            return gen_widen_or_shorten(g, node, actual_type, wanted_type->data.enumeration.tag_type, expr_val);
     }
     zig_unreachable();
 }
@@ -3913,7 +3915,7 @@ static void do_code_gen(CodeGen *g) {
             LLVMZigAddNonNullAttr(fn_table_entry->fn_value, 1);
             is_sret = true;
         }
-        if (fn_table_entry->is_pure && !is_sret) {
+        if (fn_table_entry->is_pure && !is_sret && g->is_release_build) {
             LLVMAddFunctionAttr(fn_table_entry->fn_value, LLVMReadOnlyAttribute);
         }
 

src/eval.cpp

@@ -657,6 +657,16 @@ void eval_const_expr_implicit_cast(CastOp cast_op,
             bignum_init_unsigned(&const_val->data.x_bignum, other_val->data.x_bool ? 1 : 0);
             const_val->ok = true;
             break;
+        case CastOpIntToEnum:
+            {
+                uint64_t value = other_val->data.x_bignum.data.x_uint;
+                assert(new_type->id == TypeTableEntryIdEnum);
+                assert(value < new_type->data.enumeration.field_count);
+                const_val->data.x_enum.tag = value;
+                const_val->data.x_enum.payload = NULL;
+                const_val->ok = true;
+                break;
+            }
     }
 }
 

test/self_hosted.zig

@@ -1740,3 +1740,23 @@ fn int_type_builtin() {
     assert(!usize.is_signed);
 
 }
+
+#attribute("test")
+fn int_to_enum() {
+    test_int_to_enum_eval(3);
+    test_int_to_enum_noeval(3);
+}
+fn test_int_to_enum_eval(x: i32) {
+    assert(IntToEnumNumber(x) == IntToEnumNumber.Three);
+}
+#static_eval_enable(false)
+fn test_int_to_enum_noeval(x: i32) {
+    assert(IntToEnumNumber(x) == IntToEnumNumber.Three);
+}
+enum IntToEnumNumber {
+    Zero,
+    One,
+    Two,
+    Three,
+    Four,
+}