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"AI" audit of rust stdlib (github.com)
submitted 6 months ago by to c/rust@programming.dev
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I checked some of the issues out, and this looks legit. Although a good chuck relates to obscure platform abstractions.

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[–] 2 points 6 months ago (2 children)

Regarding your question about the cast from const* T to UserRef<T>:

This should be legal as long as UserRef<T> is a #[repr(transparent)] wrapper over a const* T and does most of the magic through its implementation.

There are some other requirements iirc, but I am not entirely sure about them and some or all of them should be given here automatically through the architecture and/or a proper smart pointer implementation as a transparent wrapper...

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  • [–] 1 point 6 months ago (1 child)

    Oh, a bit like inheritance in pure C? Where as long as inheriting struct puts the parent struct as first field, you can pass it to functions that use parent struct because the memory layout rules ensure that this part of memory will be exactly the same in both?

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  • [–] 2 points 6 months ago* (last edited 6 months ago)

    Yes, except rust has even stricter requirements - namely that your struct must only contain the "inherited" field and you still need to tell the rust compiler to use the special #[repr(transparent)]!

    This is because the compiler is allowed - under "normal" representation rules - to rearrange basically everything about the memory layout of a struct to better suit its needs. And as far as I know this includes rearranging the location of a field which is arbitrarily deep inside other structures in your struct! As such

    struct A {
  foo: u8,
  bar: u8,
}

struct B {
  test: u8,
  nested: A,
}

    could theoretically be laid out as bar test foo - as opposed to e.g. foo bar for just A - in memory if the compiler determined that accessing bar at the start of the struct was overall the "best".

    If, on the other hand, you use #[repr(c)] you get exactly what you just said, although the direct casting may or may not be undefined still. (I currently do not remember the relevant parts of the nomicon or other treaties I have read about this... I really need to get back into programming rust at some point!)

    Quick edit: Expanded the possible memory layout of just the struct A.

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