{"ast_node_type":"other","tokens":[{"end":78,"kind":"keyword","start":0,"text":"//"}]}
    // E.g. "{0} {0:x} {0} {1}" -> [(0, Display), (0, LowerHex), (1, Display)]
    // Create a list of all _unique_ (argument, format trait) combinations.
    let mut argmap = FxIndexMap::default();