rust-index: remove one collect when converting back
Turns out this is slightly faster. Sending the results back to Python is still
the most costly (like 75% of the time) of the whole method, but it's about
as fast as it can be now.
hg perf::phases on mozilla-try-2023-03-22
before: 0.267114
after: 0.247101
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
end_of_line = lf
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true
end_of_line = lf
[*.t]
indent_size = 2
indent_style = space
trim_trailing_whitespace = false
end_of_line = lf