constraint: example of forcing an error for impossible builds

constraint/cgo_is_required.go

@@ -0,0 +1,7 @@
+//go:build !cgo
+
+package constraint
+
+func _() {
+	"This package requires cgo to build. An error here means cgo is disabled for your build."
+}

constraint/constraint.go

@@ -0,0 +1,3 @@
+//go:build cgo
+
+package constraint

constraint/go.mod

@@ -0,0 +1,3 @@
+module git.sunturtle.xyz/zephyr/errors-my-beloved/syntax
+
+go 1.25.0

go.work

@@ -7,4 +7,5 @@ use (
 	./iface
 	./sizeofstring
 	./stringer
+	./constraint
 )