ensure BuildError impls std::error::Error

[scottlamb]

Some usage in tracing expects this impl, as mentioned here: tokio-rs/tracing#3033 (review) It's provided when regex-automata has its std feature enabled. As matchers uses std itself, no reason not to allow regex-automata to do so also. Seems like the most straightforward way to address the problem:

• avoids the need to create a wrapper type.
• less fragile than bumping tracing-subscriber's regex dependency to 1.9.0 and hoping that all semver-compatible versions will continue depending on regex-automata with this std feature enabled.

[w4]

@hawkw light bump on this, seems to be blocking moving tracing to matchers-0.2. thanks 🙏

[w4]

@scottlamb to make this more digestible, it might be better to add a std = ["regex-automata/std"] feature instead?

[scottlamb]

@scottlamb to make this more digestible, it might be better to add a std = ["regex-automata/std"] feature instead?

Why? Are you under the impression matchers was previously not using std or did so conditionally? Take a look at the very first non-comment line in the crate:

matchers/src/lib.rs

Line 29 in a73b203

use std::{fmt, io, str::FromStr};

Making the crate no_std is not necessary to solve this problem, and I'm not interested in doing it.