add new Rust 1.57 and Rust 1.58 targets

[sunshowers]

There are some idiosyncratic targets in here -- include special code to
handle them.

[sunfishcode]

Thinking about this in the context of #78, #63, and #76, we've been seeing more pressure towards special-case parsing, and it seems that Rust target names have been evolving away from being triples and toward just being arbitrary sequences of identifiers. This suggests that perhaps target-lexicon should start refocusing on supporting rustc json target identifiers, which are explicitly split into arch/os/vendor/env categories, and not try to support all the new rust target names.

I'm curious if you have a perspective on this. Do you need all these new targets? Do you use target-lexicon for parsing target names?

[sunshowers]

Thanks! That's a great question. In my own uses in target-spec I use cfg-expr first. That library has a key-value map of builtin targets, similar to what you're suggesting.

I then fall back to target-lexicon if cfg-expr doesn't have a record of that target, typically if the target is new or somehow custom. The heuristic parsing target-lexicon does is still useful for some new targets (eg in this PR, m68k-unknown-linux-gnu, armv7-unknown-linux-uclibceabihf).

So I think the "key-value map queried from the latest rustc json targets" use case is covered well by cfg-expr. I can see target-lexicon still serving a few purposes:

• historical record of old rustc targets
• heuristic processing as a fallback to the key-value map approach

Let me know what you think!

[sunfishcode]

Makes sense. So the thing about this PR that makes me the most nervous is TripleField. Would it be difficult to organize this code in a way that allows special-case rules to be factored out into helper functions, so that they can be applied one at a time and be clearly separated?

[sunshowers]

Hi -- I completely apologize for dropping the ball here. I'm going to get to it now.

[sunshowers]

This should address your concerns -- thanks @sunfishcode!

[sunfishcode]

Looks good, thanks!