Newline style changes to Unix-style when file is modified

On Windows, using the default newline style (either without rustfmt.toml or by explicitly setting newline_style="Auto":

• leaves the newlines intact if the file does not have any formatting issues (ok, expected)
• saves modified files with Unix-style endings, even when all line-breaks in the input file are CRLF.

I would expect that in such cases the "Auto" value would use the line-break style inferred from examining the file.

Version used: rustfmt 1.4.11-stable (9eb4b564 2020-01-29)
as invoked via the cargo fmt command, version cargo 1.42.0 (86334295e 2020-01-31)

It's possible to work around this by forcing "Native" style in the rustfmt.toml but the default behaviour results in a mixture of line-ending styles throughout the project and triggers git line-ending conversion warnings.

Start by reproducing the issue on Windows through the mentioned cargo fmt entry point, using a CRLF file with a formatting change and the default Auto setting. Trace how Auto chooses newline style and add coverage for preserving the input style; done means modified CRLF files remain CRLF without requiring Native, with the relevant tests passing.