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Update type-layout.md

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Joshua Liebow-Feeser 2023-08-19 13:41:35 -07:00 committed by GitHub
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@ -56,12 +56,16 @@ Most primitives are generally aligned to their size, although this is
platform-specific behavior. In particular, on x86 u64 and f64 are only
aligned to 32 bits.
For the primitive numeric types (`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`,
`i64`, `u128`, `i128`, `usize`, `isize`, `f32`, and `f64`), every bit pattern
represents a valid instance of the type (in other words, for every primitive numeric
type, `T`, `transmute::<[u8; size_of::<T>()], T>(...)` is always sound). For the
primitive numeric types and also for `bool` and `char`, every byte is guaranteed to be
initialized (in other words, for every such type, `T`,
For every primitive numeric type (`u8`, `i8`, `u16`, `i16`, `u32`, `i32`, `u64`,
`i64`, `u128`, `i128`, `usize`, `isize`, `f32`, and `f64`), `T`, the bit validity
of `T` is equivalent to the bit validity of `[u8; size_of::<T>()]`. `u8` has 256
valid representations (namely, every 8-bit sequence). An uninitialized byte is not
a valid u8. A byte at any offset in a reference or pointer type may not be a valid
u8 (the semantics of transmuting a reference or pointer to a non-pointer type is
currently undecided).
For the primitive numeric types and also for `bool` and `char`, every byte is
guaranteed to be initialized (in other words, for every such type, `T`,
`transmute::<T, [u8; size_of::<T>()]>(...)` is always sound).
## Pointers and References Layout
@ -69,10 +73,7 @@ initialized (in other words, for every such type, `T`,
Pointers and references have the same layout. Mutability of the pointer or
reference does not change the layout.
Pointers to sized types have the same size and alignment as `usize`. Every
byte of a pointer to a sized type and of a reference to a sized type is
initialized (in other words, for such a pointer or reference type, `P`,
`transmute::<P, [u8; size_of::<P>()]>(...)` is always sound).
Pointers to sized types have the same size and alignment as `usize`.
Pointers to unsized types are sized. The size and alignment is guaranteed to be
at least equal to the size and alignment of a pointer.