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amluto • today at 6:51 PM • 1 reply • view on HN

In my mind, a no-access reference would have exactly the same lifetime rules (except for the exclusivity part) as any other reference. And they'd have the same size and lifetime rules.

Unsafe Rust could promote a no-access reference to a shared or a mutable reference, and the unsafe code would be responsible for not violating exclusivity rules but would have a guarantee that the referent actually exists. Using unsafe code to create a no-access reference to a nonexistent object or to a misaligned object would be UB.

Safe code could convert the other way:

    let a: &mut u32 = ...;
    let b: &noaccess u32 = a;
    let c: &u32 = ...;
    let d: &noaccess u32 = c;
This is not an entirely serious proposal.

Replies

AlotOfReading • today at 7:44 PM

Under NLL, reference lifetime is defined by the places where the reference is used, and noaccess types can't be used. So then reference lifetime has to devolve to the spans where the reference is live, which is only defined lexically.

So for example:

    let mut data = vec!['a', 'b', 'c'];
    let b: &noaccess [char] = &data[..];
    data.push('d'); // Does this error?
Hence the question
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