Preserve input_filename I/O variable

[marcel-steinbach-mpf]

jq can sets the variable input_filename, which may be useful when processing multiple files, e.g.

$ echo "{}" > a.json
$ echo "{}" > b.json
$ jq '{ file:input_filename }' {a,b}.json
{
  "file": "a.json"
}
{
  "file": "b.json"
}

(see https://stedolan.github.io/jq/manual/#IO)

yq seems to pipe the contents in which removes the information about the currently processed file:

$ echo '---' > a.yaml
$ echo '---' > b.yaml
$ yq '{ file:input_filename }' {a,b}.yaml
{
  "file": "<stdin>"
}
{
  "file": "<stdin>"
}

[marcel-steinbach-mpf]

I know this is a fringe case. Not sure how to properly solve this, a workaround would be to pass it as an argument to jq, e.g. --arg input_filename a.json, it would be a different accessor though: { file: $input_filename } (with the $ prefix)

Also, I haven't checked if jq provides some other implicit variables.