a {
  &[href^="mailto:"] {
    white-space: nowrap;
  }
}

lexer: scss
stack: [:root]
stream: "\n  &[href^=\"mailto:\""
  trying #<Rule /\s+/>
    got "\n  "
    yielding Text, "\n  "
lexer: scss
stack: [:root]
stream: "&[href^=\"mailto:\"] {"
  trying #<Rule /\s+/>
  trying #<Rule /\/\/.*?$/>
  trying #<Rule /\/[*].*?[*]\//m>
  trying #<Rule /@import\b/>
  entering mixin content_common
  trying #<Rule /@for\b/>
  trying #<Rule /@(debug|warn|if|each|while|else|return|media)/>
  trying #<Rule /(@mixin)(\s+)((?-mix:[\w-]+))/>
  trying #<Rule /(@function)(\s+)((?-mix:[\w-]+))/>
  trying #<Rule /@extend\b/>
  trying #<Rule /(@include)(\s+)((?-mix:[\w-]+))/>
  trying #<Rule /@(?-mix:[\w-]+)/>
  trying #<Rule /([$](?-mix:[\w-]+))([ \t]*)(:)/>
  exiting  mixin content_common
  trying #<Rule /(?=[^;{}][;}])/>
  trying #<Rule /(?=[^;{}:]+:[^a-z])/>
    got ""
    pushing :attribute

$ diff -u debug-scss{,-after}.txt 
--- debug-scss.txt      2017-12-20 12:26:42.903885511 +0900
+++ debug-scss-after.txt        2017-12-20 12:39:44.493408945 +0900
@@ -22,7 +22,7 @@
   trying #<Rule /([$](?-mix:[\w-]+))([ \t]*)(:)/>
   exiting  mixin content_common
   trying #<Rule /(?=[^;{}][;}])/>
-  trying #<Rule /(?=[^;{}:]+:[^a-z])/>
+  trying #<Rule /(?=[^;{}:\[]+:[^a-z])/>
+  trying #<Rule //>
     got ""
-    pushing :attribute
-
+    pushing :selector