Gc::from_raw: Rely only on documented guarantees to compute layout #125
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The claim “Because it is ?Sized, it will always be the last field in memory” was not reliable: the ?Sized type may in fact be Sized (which the compiler will know after monomorphization), and in any case, the language provides no guarantees about the layout of repr(Rust) structs. Switch GcBox to repr(C), and rewrite the layout computation based on Layout::extend, which has a documented guarantee to match the layout of repr(C) structs. (RcBox in the standard library is repr(C) for the same reason.) Signed-off-by: Anders Kaseorg <andersk@mit.edu>
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Learnt something new today, thanks! 😀 The fix looks good, but I'm not in a position to test it out right now. I'm also not a contributor so will defer to @Manishearth to approve and merge. |
Technically this can be enforced via |
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Thanks! A lot of this code was written before |
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The claim “Because it is
?Sized, it will always be the last field in memory” was not reliable: the?Sizedtype may in fact beSized(which the compiler will know after monomorphization), and in any case, the language provides no guarantees about the layout ofrepr(Rust)structs.Switch
GcBoxtorepr(C), and rewrite the layout computation based onLayout::extend, which has a documented guarantee to match the layout ofrepr(C)structs.(
RcBoxin the standard library isrepr(C)for the same reason.)Cc @ravern