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/// Returns whether the argument's value is statically known at |
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/// compile-time. |
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/// |
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/// This is useful when there is a way of writing the code that will |
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/// be *faster* when some variables have known values, but *slower* |
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/// in the general case: an `if is_val_statically_known(var)` can be used |
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/// to select between these two variants. The `if` will be optimized away |
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/// and only the desired branch remains. |
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/// |
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/// Formally speaking, this function non-deterministically returns `true` |
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/// or `false`, and the caller has to ensure sound behavior for both cases. |
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/// In other words, the following code has *Undefined Behavior*: |
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/// |
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/// ```no_run |
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/// #![feature(is_val_statically_known)] |
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/// #![feature(core_intrinsics)] |
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/// # #![allow(internal_features)] |
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/// use std::hint::unreachable_unchecked; |
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/// use std::intrinsics::is_val_statically_known; |
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/// |
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/// unsafe { |
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/// if !is_val_statically_known(0) { unreachable_unchecked(); } |
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/// } |
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/// ``` |
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/// |
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/// This also means that the following code's behavior is unspecified; it |
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/// may panic, or it may not: |
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/// |
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/// ```no_run |
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/// #![feature(is_val_statically_known)] |
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/// #![feature(core_intrinsics)] |
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/// # #![allow(internal_features)] |
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/// use std::intrinsics::is_val_statically_known; |
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/// |
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/// unsafe { |
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/// assert_eq!(is_val_statically_known(0), is_val_statically_known(0)); |
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/// } |
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/// ``` |
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/// |
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/// Unsafe code may not rely on `is_val_statically_known` returning any |
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/// particular value, ever. However, the compiler will generally make it |
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/// return `true` only if the value of the argument is actually known. |
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/// |
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/// When calling this in a `const fn`, both paths must be semantically |
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/// equivalent, that is, the result of the `true` branch and the `false` |
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/// branch must return the same value and have the same side-effects *no |
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/// matter what*. |
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#[rustc_const_unstable(feature = "is_val_statically_known", issue = "none")] |
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#[rustc_nounwind] |
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pub fn is_val_statically_known<T: Copy>(arg: T) -> bool; |
