I was trying to do

scala_library(
  dependencies=[
  ],

  scalac_plugins=[
    '3rdparty/jvm/org/spire-math:kind-projector',
  ]
)

But I needed to do

scala_library(
  dependencies=[
    '3rdparty/jvm/org/spire-math:kind-projector',
  ],

  scalac_plugins=[
    'kind-projector',
  ]
)