diff --git a/cmd/v/v.v b/cmd/v/v.v
index 206180de5d..e8dfaf74c8 100644
--- a/cmd/v/v.v
+++ b/cmd/v/v.v
@@ -229,6 +229,10 @@ fn setup_vbuild_env_vars(prefs &pref.Preferences) {
 	sdefines := prefs.compile_defines_all.join(',')
 	os.setenv('VBUILD_DEFINES', sdefines, true)
 
+	$if trace_vbuild ? {
+		eprintln('> VBUILD_FACTS: ${sfacts}')
+		eprintln('> VBUILD_DEFINES: ${sdefines}')
+	}
 	unsafe { sdefines.free() }
 	unsafe { sfacts.free() }
 	unsafe { github_job.free() }
diff --git a/vlib/v/build_constraint/lexing.v b/vlib/v/build_constraint/lexing.v
index 4894788e84..765a303993 100644
--- a/vlib/v/build_constraint/lexing.v
+++ b/vlib/v/build_constraint/lexing.v
@@ -38,10 +38,11 @@ fn new_span(kind BTokenKind, mut span []u8) Token {
 	return t
 }
 
-fn lex(s string) ![]Token {
+fn lex(original string) ![]Token {
 	mut res := []Token{}
-	mut span := []u8{cap: s.len}
+	mut span := []u8{cap: original.len}
 	mut op := []u8{}
+	s := original.all_before('//')
 	for c in s {
 		match c {
 			` `, `\t`, `\n` {}