mixin inside at-root not working correctly

[xzyfer]

This code was recently added to foundation.

@mixin main() {
  bar {
    baz: 1;
  }
}

foo {
  @at-root {
    @include main();
  }
}

Ruby Sass

bar {
  baz: 1; }

LibSass

foo bar {
  baz: 1; }

I've done some digging and looks like this has never worked. The issue appears to be due to parentize not respecting at-root.

Spec added sass/sass-spec#769

[xzyfer]

I'm pretty close on this. Should have something this weekend.

[xzyfer]

I thought this may be fixed a side effect of correcting our cssizg'ing of at-root in #2089 but that wasn't the case.

[xzyfer]

I've compared our at-root implementation against Ruby Sass. Our implementation is correct in this case.
The problem here is that the nested selector is being resolved (parentised) when it shouldn't have been.
This appears to related to #1890.

This is the AST when entering CSSize.

At_Root_Block 0x7fcdc0d139a0 (0@[7:2]-[7:10]) 0
 :At_Root_Query 0x7fcdc0d12cb0 (0@[7:2]-[7:10])
  Block 0x7fcdc0d12d70 (0@[7:11]-[9:3]) [root] 0
   Trace 0x7fcdc0d12ff0 (0@[8:13]-[8:17]) [name:main]
    Block 0x7fcdc0d12f40 (0@[8:13]-[8:17]) 0
     Ruleset 0x7fcdc0d13900 (0@[1:2]-[3:3]) [indent: 0]
     >Selector_List 0x7fcdc0d130d0 (0@[1:2]-[1:5]) <8166671770794171514> [@media:0x0] - - - -
     > Complex_Selector 0x7fcdc0d13310 (0@[6:0]-[6:3]) <8166671593091081176> [weight:200] [@media:0x0] - - - - --  <>
     >  Compound_Selector 0x7fcdc0d133b0 (0@[6:0]-[6:0]) <8166660432830525886> [weight:100] [@media:0x0] - - - - - <>
     >   Type_Selector 0x7fcdc0d01610 (0@[6:0]-[6:3]) <8166660258955216540> <<foo>> - - - - <>
     > { }Complex_Selector 0x7fcdc0d135c0 (0@[1:2]-[1:5]) <8275265288974811151> [weight:100] [@media:0x0] - - - - --  <>
     > { } Compound_Selector 0x7fcdc0d00a70 (0@[1:2]-[1:2]) <8275253853970583923> [weight:100] [@media:0x0] - - - - - <>
     > { }  Type_Selector 0x7fcdc0d00b40 (0@[1:2]-[1:5]) <8275253679961073361> <<bar>> - - - - <>
      Block 0x7fcdc0d137b0 (0@[1:6]-[3:3]) 0
       Declaration 0x7fcdc0d13860 (0@[2:4]-[2:7]) 0
        prop: String_Constant 0x7fcdc0d01020 4 (0@[2:4]-[2:7]) [baz] <>
        value: String_Constant 0x7fcdc0d01150 4 (0@[2:9]-[2:11]) [1] <>