Re:Writing Pythonic Rust
by Animats on Tuesday May 25, 2021 @02:49AM
(#27273656)
We're talking about Vec's iterator, not simple slice access. That "SAFETY" stuff in slice.rs is just a subscript check. Vec's iterator is in [1]. The iterator which returns a mutable reference is unsafe code, because you can't express that concept in safe code.
To recap: the interesting issue is the thread pool example which spins off a new thread with a mutable reference to each element of an array. That's a rather unusual thing to do. It is only safe if each iteration returns a reference disjoint from all other references. Because Rust at the language level does not understand disjointness within an array, it needs a hack using "unsafe" to make this work. The borrow checker would not allow this in safe code.
chunks_mut uses the same trick, with more unsafe code.
My point in all this is that because the language doesn't have syntax for talking about disjointness of parts of an array, each time this comes up, another hack in unsafe code is required. But enough here; I may say more on the Rust forums.
To recap: the interesting issue is the thread pool example which spins off a new thread with a mutable reference to each element of an array. That's a rather unusual thing to do. It is only safe if each iteration returns a reference disjoint from all other references. Because Rust at the language level does not understand disjointness within an array, it needs a hack using "unsafe" to make this work. The borrow checker would not allow this in safe code.
chunks_mut uses the same trick, with more unsafe code.
My point in all this is that because the language doesn't have syntax for talking about disjointness of parts of an array, each time this comes up, another hack in unsafe code is required. But enough here; I may say more on the Rust forums.
[1] https://doc.rust-lang.org/src/core/slice/iter.rs.html