[derive] Derive TryFromBytes on unions #800
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joshlf
previously approved these changes
Jan 21, 2024
The implementation itself is very straightforward, but it requires a few prerequisite changes: - The `Ptr::project` method now requires that it only be called on struct and union types - The `AsInitialized` invariant property is changed in a subtle but important way. Previously, a byte offset in a type was required to be initialized if, in every instance of that type, it was initialized. Now, this is required if the byte is initialized in /any/ instance of the type. Without this change, it would be possible to invoke `TryFromBytes::is_bit_valid` on a byte sequence where an uninitialized byte appears which is valid for one union field but not valid for another. Since we check bit validity for all union fields, uninitialized bytes may only appear which are compatible with /all/ union fields.
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joshlf
approved these changes
Jan 21, 2024
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The implementation itself is very straightforward, but it requires a few prerequisite changes:
Ptr::projectmethod now requires that it only be called on struct and union typesAsInitializedinvariant property is changed in a subtle but important way. Previously, a byte offset in a type was required to be initialized if, in every instance of that type, it was initialized. Now, this is required if the byte is initialized in /any/ instance of the type. Without this change, it would be possible to invokeTryFromBytes::is_bit_validon a byte sequence where an uninitialized byte appears which is valid for one union field but not valid for another. Since we check bit validity for all union fields, uninitialized bytes may only appear which are compatible with /all/ union fields.