`cargo add`, etc do not preserve `\r\n` line endings

[epage]

Problem

Generally people want line endings to be consistent. When Cargo modifies a Cargo.toml, it may insert \n independent of what line ending type is used

Steps

1. Have a Cargo.toml file with \r\n line endings
2. Run cargo add serde
3. See that \n line endings are mixed in

Possible Solution(s)

Check if \r\n is present and then convert any stray \ns to \r\n

Notes

See also #14857 as cargo-script has another area where this can be hit

Version

[epage]

Somewhat related: #12897

[ranger-ross]

I took a look at how file decides if a file has CRLF line terminators and it also appears to be using the simple approach of just checking the number /r/n in the file is > 0.

I think the proposed solution if checking if the file contains at least one \r\n should work.
A small downside of this is that if a file has mixed line terminates (both \n and \r\n) Cargo would not be able to preserve both.
But this is probably a rare scenario and I don't see a usecase for having both.

Fix no concerns with this approach, I could probably take a stab at implementing this

[weihanglo]

FWIW, #12897 is a similar issue