rfc:calls_in_constant_expressions

PHP RFC: Allow function calls in constant expressions

Introduction

Currently, constant expressions in declarations allow a limited set of expression types such as literals, constants, and operations. This RFC proposes allowing calls of a whitelist of global functions in some constant expressions. Calls would be allowed in declarations of class and global constants, defaults of static properties, static variables, and parameter defaults. This will allow calling functions such as \count(), \array_merge(), \array_keys(), and \in_array(), An Error will be thrown if the call would be invalid to use within a constant (due to references or non-constant return values).

Proposal

This RFC proposes allowing global function calls by name (in any namespace). Method calls in constant expressions continue to be fatal errors (e.g. “MyClass::methodName”() or MyClass::methodName()).

Function calls will be allowed in the following types of constant expressions:

  • Defaults of static properties, but not instance properties, due to changes required to the PHP internals expanding the scope of this RFC too much. PHP currently evaluates all of the default values of instance properties of a class once, and permanently stores them.
    This preserves PHP's current evaluation order (the first time a static property is used, all property default values are evaluated)
  • Parameter defaults (expressions with function calls will always be evaluated each time the parameter is absent)
  • Global constants (evaluated immediately) and class constants (evaluated and cached the first time they are successfully fetched)
  • Defaults of static variables (evaluated and cached the first time the expression succeeds)

Only functions in the following whitelist of functions will be callable in constant expressions. Attempting to use other functions in constant expressions will be a fatal compile error. This whitelist includes functions that are actually deterministic and without side effects, and don't depend on ini settings or locale. The functions must be unambiguously resolved. This has the same implementation as the first option, with additional compile-time restrictions.

For example, allow \count(), \array_merge(), and \in_array(), but don't allow functions such as

  • \strtolower() (different in Turkish locale),
  • \sprintf() (Depends on locale and ini_get('precision'), but so does (“” . EXPR)),
  • \json_encode() (The json extension can be disabled),

or calls which aren't unambiguously resolved with \ or use function.

The function get_defined_functions_allowed_in_constant_expressions() will be added to provide this list of functions the current php version to end users. get_defined_functions_allowed_in_constant_expressions() will be unaffected by the disable_functions ini directive, the same way get_defined_functions() is.

Behaviors in any constant expression

Many of these edge cases aren't possible for functions in the whitelist, but will be checked for in the implementation.

If a function call's result contains anything that is invalid for a constant (e.g. objects or reference cycles), an Error will be thrown and the result will be freed. (same as what define() would accept).

If a function call attempts to modify a parameter by reference, an Error will be thrown.

func_get_args(), func_get_arg(), func_num_args(), extract(), compact(), and get_defined_vars() will all throw an Error if they are called in a function expression (through the same mechanism that already forbids them in dynamic calls).

Argument unpacking is allowed. (e.g. count(…[CONST_ARRAY]))

Function calls must be by name:

// the following are not allowed by this RFC:
// const X1 = ('coun' . 't')([2]);
// const X2 = (MyClass::CALLBACK_NAME)([2]);

Evaluation Order

Global constants will continue to be evaluated immediately.

Class constants, static property defaults, and static variable defaults will be continue to be evaluated and permanently cached the first time they get used.

Parameter defaults of function/method/closure declarations (containing function calls) will get evaluated every time a function gets called without the parameter (unless a Throwable was thrown). Opcache is free to cache the result if the parameter values are known and the function is definitely deterministic.

The behavior of constant expressions that don't contain function calls won't be modified by this RFC.

Handling functions not in the whitelist

A fatal CompileError will be emitted when including a file if any constant expression uses a function name that is outside of the whitelist.

<?php
namespace NS;
use function count;
use function count as c;
class MyClass {
	const VALUES = [1, 0];
	const C1 = count(self::VALUES);  // this would not throw
	const C2 = c(self::VALUES);  // this would not throw
	// const C3 = \my_function();  // this would throw due to being outside of the whitelist
 
	// this would throw due to being allowed to resolve to `NS\in_array()`.
	// const C4 = in_array(0, self::VALUES);  
}
<?php
// in the global namespace
class MyClass {
	const VALUES = [1, 0];
	const C1 = count(self::VALUES);  // this would not throw
	const C2 = \COUNT(self::VALUES);  // this would not throw
	const C3 = namespace\count(self::VALUES);  // this (namespace-relative) would not throw
	// const C4 = OtherNS\count();  // this would throw due to being outside of the whitelist
}

Whitelisted Functions

Functions with the following properties were chosen.

  • Always return values that can be used in constants (or throw)
  • Impossible to disable in unpatched php builds (e.g. json_encode() is excluded because json can be disabled with –disable-json, and because it depends on the ini setting for numeric precision)
  • Reasonably deterministic (e.g. no file/network I/O, not dependent on time zones)
  • Not dependent on locale or ini settings (e.g. strtolower is different in a Turkish locale, (string)$float is different in the locale de_DE due to using a comma instead of a decimal place)
  • Don't expect arbitrary strings as parameters in a way that depends on the way floats implicitly cast to strings without strict_types (due to implicit (string)$float and ini settings for numeric precision)

For this reason, strlen(), sprintf(), and strpos() are omitted from the proposed whitelist. Those may be included in a followup RFC.

The following list of functions is whitelisted.

abs
acosh
acos
array_chunk
array_column
array_count_values
array_diff_assoc
array_diff_key
array_diff
array_fill_keys
array_fill
array_flip
array_intersect_assoc
array_intersect_key
array_intersect
array_key_exists
array_key_first
array_key_last
array_keys
array_merge_recursive
array_merge
array_pad
array_product
array_replace_recursive
array_replace
array_reverse
array_search
array_slice
array_sum
array_unique
array_values
asinh
asin
atan2
atanh
atan
boolval
ceil
checkdate
chr
cosh
cos
count
decbin
dechex
decoct
deg2rad
doubleval
expm1
exp
floatval
floor
fmod
gettype
gmmktime
hash_algos
hypot
in_array
intdiv
intval
is_array
is_bool
is_countable
is_double
is_finite
is_float
is_infinite
is_integer
is_int
is_iterable
is_long
is_nan
is_null
is_numeric
is_object
is_real
is_resource
is_scalar
is_string
log10
log1p
log
max
min
pi
pow
rad2deg
range
round
sinh
sin
sizeof
sqrt
tanh
tan

Backward Incompatible Changes

None

Proposed PHP Version(s)

8.0

RFC Impact

To Opcache

Opcache appears to be unaffected - tests of this RFC are passing. Opcache likely just fails to optimize the constant expressions ahead of time.

In the future, if this gets adopted widely, more aggressive optimizations of calls in constant expressions may be desirable. (e.g. permanently storing parameter defaults if the call is provably deterministic)

Opcache already has the ability to optimize functions such as strlen at compile time, when using other constants from the same class.

New Functions

get_defined_functions_allowed_in_constant_expressions() will be added to PHP.

Future Scope

Future RFCs may expand on this in many ways:

  • Allowing even more expression types in constant expressions, such as static method calls.
  • Adding more groups of functions to the whitelist, or avoiding the whitelist approach.
  • Allowing function calls in the defaults of instance properties.
  • Allowing non-constant defaults for instance properties, such as public $x = new Foo();.

Proposed Voting Choices

Primary vote: Allow calling global functions that are in the described whitelist (Yes/No, Requires 2/3 majority)

Patches and Tests

Implementation

References

rfc/calls_in_constant_expressions.txt · Last modified: 2020/02/17 00:29 by tandre