[RFC] Readonly Variables

Hi Joshua

https://wiki.php.net/rfc/readonly-variables

First of all, thank you for your proposal and the effort you've put into
the implementation.

I'm skeptical about the usefulness of local immutability as a whole. I
personally don't recognize local mutability as a significant source of
bugs, contrary to action at a distance. Putting that aside:

I understand your implementation is a proof-of-concept, but the approach
looks wrong to me. I think if local immutability is added, it should
mostly be a compile-time concept. The implementation currently add a new
opcode for readonly assignment, and touches all existing assignments to
add immutability checks. This seems mostly unnecessary with few
exceptions (those being var-vars, globals, extract, pass-by-reference
reference promotion, etc.), as the compiler can assert that the variable
being assigned to isn't marked as readonly. The implementation shows an
instruction count increase of 5.2%, 1.3% and 2.1% for Zend/bench.php,
Symfony Demo and Wordpress, respectively. That should mostly go away
with a compile time approach.

There are likely also some limitations to the compile-time approach:

• Without runtime tracking of readonly (and the concept of block
  scoping) every variable name could only be declared once in a function.
• Disallowing $readonly['foo'] = 'bar'; at compile-time will
  incorrectly restrict ArrayOffset, which I personally find acceptable.
• Similar to the above, compile-time checks are insufficient for future
  value types. [1] $readonly->foo = 'bar'; should fail if $readonly is a
  value type, which will necessarily require additional runtime checks.
  This is further complicated by $readonly->foo->bar = 'baz';, which
  should fail only if $readonly->foo itself is also a value type. Given
  this issue applies to arbitrarily long chains, I don't know how this
  could be solved.

For the reasons mentioned, I don't think I'd support this RFC in any
form. Nonetheless, I appreciate the care that has gone both into the RFC
and implementation.

Ilija

[1] https://wiki.php.net/rfc/structs-v2