XHP namespace rules

BC for definitions.php

<?hh

class :foo:bar {
  //   - xhp__foo_bar
  //   + foo\xhp__bar
}

namespaced definition.php

<?hh

namespace foo;

class :bar {
  // - \xhp__bar - this is a BC break
  // + \foo\xhp__bar
  //
  // BC break unlikely to be a practical issue as `class :bar extends :x:element {}` doesn't work (references foo\xhp__x_element)
}

class :herp:derp {
  // - \xhp__herp_derp - BC break
  // + PARSE ERROR
  
  // Arguments for making it an error instead of \foo\herp\xhp__derp:
  //  - allowing two namespace syntaxes is weird. We allow it in the global namespace for backwards compatibility
  //  - it looks like this creates a class in the 'foo' namespace. It would actually make a class in the \foo\herp namespace
  //  - it's currently pretty broken: you can define the class, but you can't reference (or extend) :x:element and friends 
  // Arguments for allowing it:
  //  - creating :foo and :foo:helper is currently common (however, :foo and :foo-helper is also perfectly valid)
  //  - it's a bit weird to allow it in the global namespace, but not others
  //  - possibly more appropriate as a lint error than a parser error
  //
  // We aren't entirely sure what the right approach is. We decided to make it an error as this lets us revisit it later;
  // if we allowed it, we can't ban it in the future without breaking people's code.
}

namespaced usage.php

<?hh

namespace foo;

$x = <x:frag />; // \foo\x\xhp__frag
$x = <:x:frag />; // \x\xhp__frag

use x as x;

$x = <x:frag />; // \x\xhp__frag

use hhvm\xhpbootstrap as bs;

$x = <bs:jumbotron />; // \hhvm\xhpbootstrap\xhp__jumbotron
$x = <:hhvm:xhpbootstrap:jumbotron />; // same

Also need

<?hh
use \hhvm\xhpbootstrap\:jumbotron;

$x = <jumbotron />;

Also

use foo\bar\{
  :herp, 
  :derp
};

$x = <herp><derp /></herp>