streamline a[begin] lowering via firstindex(a, n)

[stevengj]

Suggested by @mbauman in #33946 (comment)

[mbauman]

Thanks! I do fear this is a "minor change" as it requires non-AbstractArrays that define axes to now also implement firstindex... but they also need to implement lastindex so I don't think it's that big a deal.

[StefanKarpinski]

Would it make sense to include in 1.5 since we haven't branched yet?

[mbauman]

The A[begin] lowering was added in 1.4 so it's a change either way. (edit: whoops, wrong button)

[mbauman]

I think this is a worthwhile change (and I'd bet will be unobjectionable to triage), just could use NEWS.md.

[stevengj]

Should be good to merge?

[stevengj]

linuxaarch64 is the usual unrelated NaN == Inf error that we've been seeing recently on CI, so this should be good to merge once the other tests are green.

[nalimilan]

FWIW this technically breaks some packages like DataFrames which implemented axes but not firstindex. That's probably OK for since we can fix it soon, but it might be worth adding a fallback firstindex definition that calls axes to minimize breakage.

Cc: @bkamins

[stevengj]

For that matter, a fallback lastindex seems worthwhile too.

[StefanKarpinski]

Better to be consistent here and there's no harm since it's a fallback anyway.

[bkamins]

I think it would be best to provide a fallback in Julia Base.

The problem is that the firstindex and lastindex interface https://docs.julialang.org/en/v1/manual/interfaces/ is now specified only for firstindex(collection) and lastindex(collection) (without the dimension).

If you agree I would recommend that the interface specification should be updated in the manual in a PR that implements the fallback.

[stevengj]

Should be fixed by #38742.