This produces pretty code in output.rs:
$ bindgen input.h --rustfmt-bindings -o output.rs
This does not print pretty code to stdout:
$ bindgen input.h --rustfmt-bindings
See the rustfmt_generated_file function in src/lib.rs for where we currently run rustfmt on generated files. We need to have a version of this for when we are printing to stdout that writes the unformatted bindings to a child rustfmt's stdin, and then pipes the child's stdout to our stdout. We actually have something pretty similar to this in tests/tests.rs as well in the rustfmt function there.