! cue vet -c file.cue

cmp stderr expect-stderr

-- expect-stderr --
incomplete value 'X.y' in interpolation:
    ./file.cue:16:3
    ./file.cue:3:5
-- file.cue --
X :: {
	x: string
	y: string
	z: string
}

X :: {
	x: "x"
	z: "z"
}

"""
hello
world
\(X.x)
\(X.y)
\(X.z)
"""