update: use PostCSS 8 API.

[ludofischer]

I've noticed cssnano/cssnano#954 assumes there's a version of this with the PostCSS 8 API, but I could not find it either a release, pull request or branch, so here it it.

I've used the visitor API thinking it should be safe from infinite loops, as once the calc() declarations are gone, the transform() function will exit immediately.

[Semigradsky]

.travis.yml should be updated.

[alexander-akait]

hm, maybe we should remove this from major release? What is use case for at-rule(s)?

[ludofischer]

Since there is a mediaQueries option, I think the use case for at-rules is calc() in media queries, like:

@media (max-width: calc(40em * 2)) { ... }

[alexander-akait]

hm, it sounds reasonable, I think we should enable it by default

[ludofischer]

I would like to add an integration test with other plugins on realistic CSS, to check that processing terminates without errors. But I am unsure what plugins and sample CSS to pick. Maybe autoprefixer?

[ludofischer]

I think this pull request should not be merged in this state, there's a chance of infinite loops apprearing. I've added a test that produces an infinite loop: adding a plugin that changes whitespace inside the calc() property. I guess the 'right' solution would be to skip writing the value if we detect the parsed value AST are equivalent? Or just use Once() :-)?

[alexander-akait]

@ludofischer Let's use Once

[alexander-akait]

@Semigradsky Can you help and do major release due cssnano/cssnano#975 (comment)?

[Semigradsky]

Done https://github.com/postcss/postcss-calc/releases/tag/v8.0.0