Improve parent resolution

[maciej-ka]

Places pseudo class selectors at the end, removes duplicate selectors
and removes universal selector if possible during resolution of parent
selector (&)

Pseudo at the end

div:before { &[id] { color: red; }}

Before:

div:before[id] {
  color: red; }

After:

div[id]:before {
  color: red; }

Remove duplicates

.foo { &.foo { color: red; }}

Before:

.foo.foo {
  color: red; }

After:

.foo {
  color: red; }

Remove universal

* { &.foo { color: red; }}

Before:

*.foo {
  color: red; }

After:

.foo {
  color: red; }

Related to: #490
Tests: sass/sass-spec#931

[long-lazuli]

Beware of removing duplicate selector.
It should not be done automatically. Sass never controlled what you are trying to achieve with CSS.

There is a lot of cases, for specificity purpose, where you exactly expect this :

.foo.foo {
  color: red; }

from this :

.foo { &.foo { color: red; }}

It's not a Sass/SCSS problem.
People want to do that, they should be able to.

[maciej-ka]

Reason for this implementation was @nex3 suggestion in #360. @long-lazuli Could you give any example where .foo.foo would be desirable?

[long-lazuli]

Let's say we use a plugin which use the class .foo to describe the width property to 10em.
We want to surcharge this width, to make this 'plugin/widget/whatever' responsive, to 100%.
We can achieve that to make a specificity higher using twice the class. As in the selector chain .foo.foo.

This is not a good practice, but I deal with that kind of tricks enough. That pull request would make some old-fashioned Sass code obsolete.

[maciej-ka]

@long-lazuli Right, it is possible to use .foo.foo to rise selector specifity. I'm perfectly fine to remove this part of pull request. @nex3, @chriseppstein, what's your opinion on this?

[nex3]

Thanks for the patch, and sorry for the long response time! Unless we can find a new maintainer, Ruby Sass is on track to be deprecated, so we aren't accepting non-critical new features at this time.

I'm marking this as "on ice", which means that if/when we find a new maintainer, they can find it and revive it.