Recognize upper bounds values '0' and '$' in slices.

[ibuclaw]

Both can never result in a bounds error, and so don't generate code to check for it.

Follow-up of #7682.

This means that code like:

auto sl = arr[n .. $])

Will only check that n <= $.

For smart optimizers, the conditions 0 <= $ and $ <= $ would be removed as dead code anyway. This just means that the glue/backend never generates the test in the first place.

[dlang-bot]

Thanks for your pull request, @ibuclaw!

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[d-random-contributor]

Do we have xfail tests for

a[1..0]
a[$..0]

to not overdo with shortcuts?

[ibuclaw]

Do we have xfail tests for

There's plenty. Anyway that's a different slicing flag to the one being tweaked here. Array bounds consists of two checks, is upper is in bounds, and is lower less than or equal to upper.

This only elides the first test. But again, a smart backend optimizer is already doing this, and so this patch is only removing that extra work done to get the exact same result.