Warn that `repr(packed)` and `repr(align)` do not guarantee field order, and as a result do not guarantee size

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
72/100
Issue type
Documentation
Clarity
Clearly specified
Activity status
Stale
Tech stack
rust
Domain
documentation

Research direction

Start with the linked type layout documentation and read the sections on repr(packed) and repr(align). Update the documentation to explain field-order effects on size, the repr(packed(1)) and repr(packed) exception, and when repr(C) is needed; done means all three cases are stated clearly.

Written by the indexing model from the issue text.

Description

While this is already implied by the type layout docs, it's easy to miss since the type layout docs describe reprs as guaranteeing things about the layout, and the special case of repr(packed(1)) does guarantee size.

In particular, call out the following:

  • A repr(packed(N)) struct's size may change depending on field order
    • As a special case, a repr(packed(1)) or repr(packed) struct's size does not depend on field order
  • A repr(align(N)) struct's size may change depending on field order
  • In each of these cases, suggest using repr(C) to guarantee field order if guaranteed size is important
Dominant language
Rust
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Merged PRs (30d)
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