[rfc] Introduce allow broken symlinks for walk builder

[17rohith29]

Context: Hi, Our team uses ignore crate's walk builder to traverse directories to build a packages that we upload. When traversing directories, we do so with the follow links options to resolve symlinks.
However, we are now building certain packages that are known to have broken symlinks. For these packages, we would like to follow_links when possible, and skip following links when the symlinks are broken.

Proposed changes: introduce a follow_valid_links option, where we only follow links when valid.

Starting a discussion to see if this seems like an ok addition to the crate, if so will follow up with a pr :)

[BurntSushi]

I'm pretty sure this is already possible with the current API. You should just get an error, and then you can skip or log it or whatever.

If this isn't working, please provide an MRE.

[17rohith29]

you do get an error, but currently the error propagation can't be easily mapped to a file that exists but only has a broken symlink.

For example if a broken symlink exists we get data via fs::metadata call https://github.com/BurntSushi/ripgrep/blob/master/crates/ignore/src/walk.rs#L409, which if the symlinks is broken, will error out with: No such file or directory (or error 2).

This isn't distinguishable when compared to when the file is removed while this code is running, which will produce the exact same error and in this case we wanna handle it differently.

Of course, for our case we can totally by pass this by calling a stat call on the path after the fact, but it would be nice if the crate can handle it directly.

As you mentioned before, this doesn't completely block us, but i think that it would be nice if the crate can handle this better either via a different error message or by allowing this option. But if this is an anti pattern for this crate, i totally understand :)

[BurntSushi]

I think for this niche of a use case, if you can get what you want with an extra stat call, then that's what you should do.