Function std::intrinsics::is_val_statically_known
source · pub const fn is_val_statically_known<T>(_arg: T) -> boolwhere
T: Copy,
core_intrinsics
)Expand description
Returns whether the argument’s value is statically known at compile-time.
This is useful when there is a way of writing the code that will
be faster when some variables have known values, but slower
in the general case: an if is_val_statically_known(var)
can be used
to select between these two variants. The if
will be optimized away
and only the desired branch remains.
Formally speaking, this function non-deterministically returns true
or false
, and the caller has to ensure sound behavior for both cases.
In other words, the following code has Undefined Behavior:
#![feature(is_val_statically_known)]
#![feature(core_intrinsics)]
use std::hint::unreachable_unchecked;
use std::intrinsics::is_val_statically_known;
if !is_val_statically_known(0) { unsafe { unreachable_unchecked(); } }
This also means that the following code’s behavior is unspecified; it may panic, or it may not:
#![feature(is_val_statically_known)]
#![feature(core_intrinsics)]
use std::intrinsics::is_val_statically_known;
assert_eq!(is_val_statically_known(0), is_val_statically_known(0));
Unsafe code may not rely on is_val_statically_known
returning any
particular value, ever. However, the compiler will generally make it
return true
only if the value of the argument is actually known.
§Stability concerns
While it is safe to call, this intrinsic may behave differently in
a const
context than otherwise. See the const_eval_select
documentation for an explanation of the issues this can cause. Unlike
const_eval_select
, this intrinsic isn’t guaranteed to behave
deterministically even in a const
context.
§Type Requirements
T
must be either a bool
, a char
, a primitive numeric type (e.g. f32
,
but not NonZeroISize
), or any thin pointer (e.g. *mut String
).
Any other argument types may cause a compiler error.
§Pointers
When the input is a pointer, only the pointer itself is ever considered. The pointee has no effect. Currently, these functions behave identically: