diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 108394545d..e12cca7ddd 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -45,6 +45,12 @@ pub struct string {
 pub:
 	str &byte = 0 // points to a C style 0 terminated string of bytes.
 	len int   // the length of the .str field, excluding the ending 0 byte. It is always equal to strlen(.str).
+	// NB string.is_lit is an enumeration of the following:
+	// .is_lit == 0 => a fresh string, should be freed by autofree
+	// .is_lit == 1 => a literal string from .rodata, should NOT be freed
+	// .is_lit == -98761234 => already freed string, protects against double frees.
+	// ---------> ^^^^^^^^^ calling free on these is a bug.
+	// Any other value means that the string has been corrupted.
 mut:
 	is_lit int
 }