OsStr::starts_with API to support clap?

libs
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Hi. I was redirected here from https://github.com/rust-lang/rust/issues/new/choose since this is a feature request.

clap is a popular command line argument parsing crate. Clap requires the ability to peek into an &OsStr, for example, to see if the arg starts with - or --. It is desirable for Clap to operate on &OsStr instead of &str because native platforms do not limit command line arguments to &str.

I believe parsing arguments from the command line to be a common use case, of which determining switches and flags is part of the task. Is adding an OsStr::starts_with API something the libs team would be interested in?

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I am not a team member but I have seen that suggestions like this seem to normally be encouraged to be created as a straight PR where the function is unstable behind a gate. Given that this method is already on &str it seems very reasonable that it would be accepted.

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It's part of RFC 2500, which has been sadly stalled for quite some time. Perhaps it can still be added now in a way that would be forwards compatible with the RFC though.

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There was a conversation recently about using a different approach for the Pattern API here: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/New.20Pattern.20API.3F/near/209492625

Unfortunately I think recent attempts to move it forward (https://github.com/rust-lang/rust/pull/76901) are stalled for concerns about the current design.

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I've dealt with the same problem in my configure_me crate which aims to be a replacement for clap in some situations. The methods for parsing &OsStr arguments and detecting if they contain -- are actually separated into a small parse_arg crate, so clap is free to use them without waiting for addition to std. See parse_arg::match_arg

LMK if you need some help using it.

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