Format markdown documents containing Rust

I was wanting to do something similar to #1695, although in my case I've got a bunch of markdown files containing Rust snippets which I'd like to format.

I knocked up a quick Python script which almost does this (gist).

However it looks like rustfmt will skip any file not ending in *.rs... Would it be possible to allow non-Rust files if they are explicitly specified using the --file-lines argument?

Output of running rustfmt from Python:

python rustfmt.py 

b"Warning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/codemap.md'\nWarning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/lib.md'\nWarning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/parse.md'\nWarning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/lowering.md'\nWarning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/analysis.md'\nWarning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/errors.md'\nWarning: Extra file listed in file_lines option '/home/michael/Documents/playground/static-analyser-in-rust/src/lex.md'\n\n"
b''

Start by reproducing the reported behavior with the linked Python script and the --file-lines argument. Inspect how rustfmt handles explicitly listed files that do not end in .rs. Done means Rust snippets in explicitly specified Markdown files can be formatted without the reported warnings, while unrelated files remain unaffected.