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Rollup merge of #82837 - RalfJung:maybe-uninit, r=dtolnay 

tweak MaybeUninit docs

Explain what "(no) fixed value" means.
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Yuki Okushi 2021-03-07 10:41:22 +09:00 committed by GitHub
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library/core/src/mem/maybe_uninit.rs
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@ -39,10 +39,11 @@ use crate::ptr;
/// let b: bool = unsafe { MaybeUninit::uninit().assume_init() }; // undefined behavior! ⚠️
/// ```
///
/// Moreover, uninitialized memory is special in that the compiler knows that
/// it does not have a fixed value. This makes it undefined behavior to have
/// uninitialized data in a variable even if that variable has an integer type,
/// which otherwise can hold any *fixed* bit pattern:
/// Moreover, uninitialized memory is special in that it does not have a fixed value ("fixed"
/// meaning "it won't change without being written to"). Reading the same uninitialized byte
/// multiple times can give different results. This makes it undefined behavior to have
/// uninitialized data in a variable even if that variable has an integer type, which otherwise can
/// hold any *fixed* bit pattern:
///
/// ```rust,no_run
/// # #![allow(invalid_value)]