gistfile1.txt

kotlin code: 
  val formatter = HtmlFormatter(usersLocale).useLongDates()
  "foo $a bar $b".toHtml(appender, formatter)

by default is turned into...

  StringTemplate(array("foo ", " bar"), array(a, b)).toHtml(appender, formatter)

if a custom compiler plugin knew you wanted a Html template, say, it could change that code to

  MyHtmlTemplate(a, b).toHtml(appender, formatter)

with this class being generated:

class MyHtmlTemplate(a: String, b: Foo) : HtmlTemplate {
  fun toString() = "foo " + a + " bar " + b
 
  fun toHtml(out: Appender, format: HtmlFormat): Unit {
    out.append("foo ")
    format.append(out, a)
    out.append(" bar ")
    format.append(out, b)
    out.append(" bar ")
  }
}

A really smart compiler plugin could just do this to avoid any object construction :) 

    appender.append("foo ")
    format.append(appender, a)
    appender.append(" bar ")
    format.append(appender, b)
    appender.append(" bar ")

though its a bit more complex - e.g. there may be a few toHtml() methods - whether or not you pass an Appender or just want a String being returned