`Runfiles::create()` throws error when `parse_repo_mapping` returns an error, instead of returning a default `RepoMapping`

[felipeamp]

Before #2847 was merged Runfiles::create() always called RepoMapping::new() when the Result of parse_repo_mapping was an error. Now, due to the question mark operator after the transpose()

rules_rust/tools/runfiles/runfiles.rs

Lines 143 to 146 in 07dc023

	let repo_mapping = raw_rlocation(&mode, "_repo_mapping")
	.map(parse_repo_mapping)
	.transpose()?
	.unwrap_or_default();

the error is returned to the caller, instead of returning a default RepoMapping as before that PR.

[UebelAndre]

Why is parse_repo_mapping returning an error? Is there a bug in how we parse it?

[felipeamp]

I'm still trying to figure that one out, and it could be to some local patch I'm carrying. I've asked some other folks to help, but since it's (almost) the weekend I guess it will take a while. I'll let you know if we progress on this.

All I know so far: the error I got was a RepoMappingIoError, thrown when reading the file into a string, and the file was obtained (I believe) in a DirectoryBased mode.

[UebelAndre]

I started adding some tests here: #2869

The change to make this non-fatal would be trivial but I would like to know why parsing failed. I'm not so familiar with the _repo_mapping file but would runfiles lookup lead to incorrect results without it? If so my naive approach would be to have this error so users don't get bad results with no warning.

[felipeamp]

I've patched the code from my first comment above by

let repo_mapping = match raw_rlocation(&mode, "_repo_mapping").map(parse_repo_mapping) {
            Some(Ok(r)) => r,
            _ => RepoMapping::new(),
        };

and it fixed the issues we were seeing. Whether there is something else broken by the _repo_mapping logic underneath I don't know, but so far I haven't seen any breakages. Also we don't use bzlmod, so can't say anything about the state of that.

[jblebrun]

We are also encountering this error in a project. We use Bazel 6, without bzlmod, and I think the issue is that the file just doesn't exist? The RepoMappingIoError we see is always with the message "No such file or directory".

There's similar behavior when trying to use the rust-analyzer gen_rust_project rule. WIth the rules_rust version we currently use (0.48.0), after running gen_rust_project the last message emitted is "No repo mapping!"

With the latest tagged release (0.50.1), there's now a failure with the RepoMappingIoError (also with "No such file or directory").