gofumpt -w foo.go
[gofumpt] cmp foo.go foo.go.gofumpt
[!gofumpt] cmp foo.go foo.go.gofumports

-- foo.go --
package p

var _ bytes.Buffer
-- foo.go.gofumpt --
package p

var _ bytes.Buffer
-- foo.go.gofumports --
package p

import "bytes"

var _ bytes.Buffer