Use intermediate variable in slice()

[markfairbanks]

See #377

[markfairbanks]

Good for review.

Also does anyone have an idea what this check failure is caused by? 🤷‍♂️

[eutwt]

My guess is that since the output of expr({...}) has a "srcref" attribute, covr treats it as a separate source file to check. You can either use call2 instead (whose output doesn't have that attribute) or use i <- rlang::zap_srcref(i) after to get rid of the attribute.

[markfairbanks]

Thanks for figuring out that test @eutwt 😄