PHP: rfc:attributes-on-constants

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====== PHP RFC: Attributes on Constants ======
  * Version: 0.2
  * Date: 2024-11-30
  * Author: Daniel Scherzer, daniel.e.scherzer@gmail.com
  * Status: Pending Implementation
  * First Published at: https://wiki.php.net/rfc/attributes-on-constants
  * Implementation: https://github.com/php/php-src/pull/16952

===== Introduction =====
Attributes were first introduced in [[rfc:attributes_v2]], with the current syntax adopted [[rfc:shorter_attribute_syntax_change]]. This RFC would add support for attributes on

  * Compile-time non-class constants

To the list of current valid targets (see [[rfc:attributes_v2#attribute_syntax]]):

  * functions (including closures and short closures)
  * classes (including anonymous classes), interfaces, traits
  * class constants
  * class properties
  * class methods
  * function/method parameters

Additionally, taking advantage of this new feature, the <php>#[\Deprecated]</php> attribute is updated to also target compile-time non-class constants.

"Compile-time non-class constants" refers to constants defined via <php>const MY_CONST = 5;</php> (for example) rather than those via <php>define( 'MY_CONST', 5 );</php>. Unless clear from context, in this RFC "constant" refers to a compile-time non-class constant.

===== Proposal =====

Attributes are allowed to target constants, with the introduction of a <php>Attribute::TARGET_CONSTANT</php>. Additionally, <php>Attribute::TARGET_ALL</php> will now include constants.

To prevent confusing code, attributes are only allowed to be applied to constants that are declared in their own statement:

<PHP>
// This is okay
#[\MyAttribute]
const Example1 = 1;

// This is an error
#[\MyAttribute]
const Example2 = 2,
    Example3 = 3;
</PHP>

To expose information about these attributes to runtime, <php>ReflectionConstant::getAttributes()</php> is introduced.

Additionally, the <php>#[\Deprecated]</php> attribute is updated to allow targeting constants; when applied, the constant is marked with ''CONST_DEPRECATED''.

===== Backward Incompatible Changes =====
Given that <php>Attribute</php> is <php>final</php>, the added constant shouldn't cause any conflicts. Likewise, <php>ReflectionConstant</php> is final, so the added method shouldn't cause any conflicts.

Existing attributes that use <php>Attribute::TARGET_ALL</php> may need to be updated if they do not apply to constants.

Furthermore, as with all RFCs that make code that previously had a compile-time error become valid, this RFC requires updates to static analyzers and IDEs to understand the semantics of adding attributes to constants and not erroneously report errors.

===== Proposed PHP Version(s) =====
Next version of PHP (PHP 8.5 or PHP 9.0)

===== RFC Impact =====
==== To SAPIs ====
Just noting for completion: the current implementation passes the attribute information to runtime via a new opcode, ''ZEND_DECLARE_ATTRIBUTED_CONST'', that phpdbg will ignore the same way it ignores ''ZEND_DECLARE_CONST''.

==== To Opcache ====
The JIT currently has logic to emit deprecation messages for constants that are deprecated; to avoid duplicating the new logic to check a constant's attributes for a <php>#[\Deprecated]</php> attribute, that logic is centralized in a new <php>zend_deprecated_constant()</php> function.

==== To Reflection ====
New method <php>ReflectionConstant::getAttributes()</php> is added, with a signature to match the other such methods for retrieving attributes:

<PHP>
final class ReflectionConstant implements Reflector
{
    // Everything that currently exists
    // ...

    public function getAttributes(?string $name = null, int $flags = 0): array {}
}
</PHP>

Additionally, code that expects <php>ReflectionConstant::isDeprecated()</php> to always return <php>false</php> for userland constants will need to be updated, since it can now be true.

==== New Constants ====
<php>Attribute::TARGET_CONSTANT</php> indicates that an attribute can be applied to compile-time non-class constants.

===== Unaffected PHP Functionality =====
Existing uses of attributes or constants - previously, trying to add an attribute to a constant would have generated an error, so any existing working current code should continue to work as expected.

===== Future Scope =====
Adding support to non-compile-time non-class constants, via <php>define()</php>.

===== Proposed Voting Choices =====
Voting started on 2024-12-16 and will end on 2025-01-06 00:00:00 UTC.

This vote requires a 2/3 majority to adopt.

<doodle title="Support attributes on constants" auth="danielescherzer" voteType="single" closed="true" closeon="2025-01-06T00:00:00Z">
   * Yes
   * No
</doodle>

===== Patches and Tests =====
https://github.com/php/php-src/pull/16952

===== Implementation =====
After the project is implemented, this section should contain 
  - the version(s) it was merged into
  - a link to the git commit(s)
  - a link to the PHP manual entry for the feature
  - a link to the language specification section (if any)

===== References =====
  * [[rfc:attributes_v2]]
  * [[rfc:shorter_attribute_syntax_change]]
  * https://github.com/php/php-src/pull/16952

===== Changelog =====

0.2:

  * Improved discussion of backward incompatible changes

0.1:

  * Initial proposal
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