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This post is auto-generated from RSS feed The Rust Programming Language Forum - Latest topics. Source: Help with miri error

Hi.

I want to transmute &[u8] to &A where A is a DST and just lays out some fixed prefix of the original slice:

/// `T` here is for `CoerceUnsized`
struct A<T: ?Sized = [u8]> {
    prefix: [u8; 2],
    tail: T,
}

My approach was quite strait forward — inspecting the pointer data of identical *const [u8] and *const A I realized that the *const A have it's length smaller by the length of the prefix field,
so to transmute &[u8] to &A I did the same — made slice length smaller by the length of the prefix and called transmute.

Some basic assertions shows that the resulting &A does indeed represents the original &[u8], but miri complains with "Undefined Behavior: trying to retag ..." (see the full error by invoking miri on playground).

The error seem cryptic to me. Is it actually UB to transmute?

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