cgen: fix array type as interface (fix #14677) (#14681)

vlib/v/gen/c/cgen.v

@@ -5654,12 +5654,17 @@ fn (mut g Gen) interface_table() string {
 					field_styp := g.typ(field.typ)
 					if _ := st_sym.find_field(field.name) {
 						cast_struct.writeln('\t\t.$cname = ($field_styp*)((char*)x + __offsetof_ptr(x, $cctype, $cname)),')
+					} else if st_sym.kind == .array
+						&& field.name in ['element_size', 'data', 'offset', 'len', 'cap', 'flags'] {
+						// Manaully checking, we already knows array contains above fields
+						cast_struct.writeln('\t\t.$cname = ($field_styp*)((char*)x + __offsetof_ptr(x, $cctype, $cname)),')
 					} else {
 						// the field is embedded in another struct
 						cast_struct.write_string('\t\t.$cname = ($field_styp*)((char*)x')
 						if st == ast.voidptr_type {
 							cast_struct.write_string('/*.... ast.voidptr_type */')
 						} else {
+							if st_sym.kind == .struct_ {
 								for embed_type in st_sym.struct_info().embeds {
 									embed_sym := g.table.sym(embed_type)
 									if _ := embed_sym.find_field(field.name) {
@@ -5668,6 +5673,7 @@ fn (mut g Gen) interface_table() string {
 									}
 								}
 							}
+						}
 						cast_struct.writeln('),')
 					}
 				}

vlib/v/gen/c/testdata/array_as_interface.out

@@ -0,0 +1,2 @@
+4
+1

vlib/v/gen/c/testdata/array_as_interface.vv

@@ -0,0 +1,8 @@
+interface Source {
+    len int
+}
+
+fn main() {
+    println(Source('test').len)
+    println(Source([`a`]).len) 
+}

vlib/v/tests/array_as_interface_test.v

@@ -0,0 +1,19 @@
+interface Source {
+	element_size int
+	data voidptr
+	offset int
+	len int
+	cap int
+	flags ArrayFlags
+}
+
+fn test_array_as_interface() {
+	arr := []rune{len: 1}
+	src := Source(arr)
+	assert arr.element_size == src.element_size
+	assert arr.data == src.data
+	assert arr.offset == src.offset
+	assert arr.len == src.len
+	assert arr.cap == src.cap
+	assert arr.flags == src.flags
+}