pkg/compiler: support if[expr] attributes

The expression may either include integers/consts or reference other
fields in the structure via value[field1:field2:field3].

The fields on this path must all belong to structures and must not have
any if conditions themselves.

For unions, mandate that the last field has no conditions (it will be
the default one).

For structs, convert conditional fields into fields of a union type of
the following form:

anonymous_union [
 value T (if[expression])
 void void
]

1 files changed, 37 insertions, 0 deletions

diff --git a/pkg/compiler/testdata/errors.txt b/pkg/compiler/testdata/errors.txt
index 1be1bd070..4f3186698 100644
--- a/pkg/compiler/testdata/errors.txt
+++ b/pkg/compiler/testdata/errors.txt
@@ -470,3 +470,40 @@ union$directions [
 
 
 compressed$test(a int32) compressed_image	### compressed_image can't be syscall return
+
+define FLAG1 = 1
+define FLAG2 = 2
+
+some_nested_flags {
+	f1	int32
+}
+
+conditional_fields {
+	f1	int32
+	f2      some_nested_flags (if[value[f1] & FLAG1])
+	f3      some_nested_flags (if[value[f1] & FLAG1], if[value[f1] & FLAG1]) ### duplicate arg/field f3 attribute if
+	f4      some_nested_flags (if[value & FLAG1]) ### value reference must have only one argument
+	f5      some_nested_flags (if[value[f1] & FLAG1, FLAG2]) ### if attribute is expected to have only one argument
+	f6      some_nested_flags (if[value[f1, FLAG2] & FLAG1]) ### value reference must have only one argument
+	f7      some_nested_flags (if[5])
+	f8      some_nested_flags
+	f9      some_nested_flags (if[value[f8:f1] & FLAG1])
+	f10     some_nested_flags (if[value[f8:f1, A] & FLAG1]) ### value reference must have only one argument
+	f11     some_nested_flags (if[value[f8:f1[A]] & FLAG1]) ### value aguments must not have any further arguments
+	f12     some_nested_flags (if[f1 == "A"]) ### the token must be either an integer or an identifier
+	f13     some_nested_flags (if["ABCD"]) ### if argument must be an expression
+	f14     some_nested_flags (if[X[Y]]) ### consts in expressions must not have any arguments
+	f15	conditional_fields_union1
+	f16	conditional_fields_union2
+}
+
+conditional_fields_union1 [
+	u1	int32 (if [value[parent:f1] & FLAG1])
+	u2	int32 (if [value[parent:f1] & FLAG2]) ### unions must not have if conditions on the last field
+]
+
+conditional_fields_union2 [
+	u1	int32 (if [value[parent:f1] & FLAG1])
+	u2	int32 ### either no fields have conditions or all except the last
+	u3	int32
+]