6 ms·

I tried it again in 2021 and it still failed to deal with constant functions. Here's it again in 2022 https://play.rust-lang.org/?version=stable&mode=debug&edit

I tried it again in 2021 and it still failed to deal with constant functions. Here's it again in 2022 https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=e5bcaac465031fdc12323b2e4a8da58a https://play.rust-lang.org/?version=stable&mode=debug&editio...

magicalhippo 4y ago
I don't know any rust but the error description[1] only talks about borrowing mutable from an immutable, which to me seems like an obvious error. However the compiler complains about the reverse condition. Again, I don't know any rust, so it's not clear why that's an error. So is it a documentation bug or a compiler bug? [1]: https://doc.rust-lang.org/stable/error-index.html#E0502 https://doc.rust-lang.org/stable/error-index.html#E0502
- ArrayBoundCheck 4y ago
  I only understand it because I learned how not to shoot myself in the foot with the canon that is C++ I'll sum up "container invalidation". In C or C++ if you append an array you may cause the array to grow which means it might have a different pointer. You basically can't take an address of an array you're planning to grow. All containers are like that except list I think which I never use In rust, it's roughly the same idea. The problem is I borrowed something from vec (you can think of it as taking a reference or creating a pointer). I then call a function that is const (you can see here it's &self and not &mut self https://doc.rust-lang.org/src/alloc/vec/mod.rs.html#1911 https://doc.rust-lang.org/src/alloc/vec/mod.rs.html#1911). But that invalidates the borrow like push_back/resize/reserve and such in C++. Which is absolutely ridiculous because it's doesn't modify anything. It makes me so mad because even the one thing I like about rust is annoying to use. Using an annoying checker, slower compiler, harder to optimize language is too much. I don't shoot myself in the foot in C++ so I'm unable to get a win in rust. For me using rust is more pain than practical
  - pkolaczk 4y ago
    Can you share any compiler benchmarks showing Rust compiles slower than C++? I can see something totally opposite in my projects - particularly recompiling after a change is very quick. And cargo check often runs in less than 1s. I never got such quick incremental recompile times in any of my earlier C++ projects, particularly when using templated or header heavy libraries like STL ;) As for the borrow checker thing, it is not broken but it's simply a limitation. In a strictly statically checked it is usually possible to run into a situation of a valid program that gets rejected by the compiler. Here this limitation comes from the compiler following a pretty simple rule - only one reference is allowed if it is mutable. This simple rule is what makes many of Rust safety guarantees possible, and I guess relaxing it conditionally might be very tricky. NLLs have already made it a bit nicer recently, do maybe it is possible to improve it further.
    - masklinn 4y ago
      > As for the borrow checker thing, it is not broken but it's simply a limitation. Limitations are when the borrow checker is not smart enough to understand things like sub-scope borrows (fixed by NLL) or the interaction of mutable borrows and loops (which hopefully will eventually be fixed by Polonius aka NLLv2). But here they want to acquire a shared reference to an object with an outstanding unique reference. That is quite literally the first of only two rules references have: 1. At any given time you can have either a single unique reference or multiple shared references. 2. References must be valid.
      - pkolaczk 4y ago
        In the given example they are not using the shared reference at all during the whole lifetime of the immutable reference and the operation on the immutable one does not change the object at all, so it would be actually safe. This is the situation where those two rules are a bit too strict and reject a valid program.
        masklinn 4y ago
        > In the given example they are not using the shared reference at all during the whole lifetime of the immutable reference and the operation on the immutable one does not change the object at all, so it would be actually safe. The compiler does not work at that level of resolution. It’s unclear that it should, even for a point query: there is no telling how much a weirdo ISA would bungle that if the two accesses came from different threads, for instance. > This is the situation where those two rules are a bit too strict and reject a valid program. Compilers will always, necessarily, reject valid programs. The alternative is to allow invalid programs.
chlorion 4y ago
There is nothing broken in this example, it's functioning exactly as it's intended to. What makes you think the borrow checker is broken here? It appears like you don't really understand the rule about mutable and immutable references. Asking for the length of the vector requires an immutable reference, but you already have a mutable reference, which is explicitly not allowed by design (that's the whole point!). So, you can only have one mutable reference to something if there are no immutable references, and multiple immutable references if there are no mutable references, you can never have both immutable and mutable references to something at the same time.
- ArrayBoundCheck 4y ago
  I'm taking a reference to the array the vec is pointing to not the vec. Rust needs to be more fine grain because it's obnoxious and C++ container invalidation is less bad https://news.ycombinator.com/item?id=32188672 https://news.ycombinator.com/item?id=32188672
- nlitened 4y ago
  I don’t know Rust, just wondering: why is it forbidden to know a length of vector to which you hold a mutable reference? What could go wrong?
  - scoutt 4y ago
    I imagine you can have one thread modifying the vector while trying to read its length from another thread.
    - ArrayBoundCheck 4y ago
      Noone should imagine that. You definitely don't want two threads accessing the same vec or array at one time. Maybe if there's a lock
      - nickitolas 4y ago
        If it's read only, you can definitely want that, right? As long as the elements are Sync. And it should work (You might have a problem with the cleanup, but Arc or scoped threads should handle that). The problem is when one is a mutable reference, like in this case. AIUI you could have a single mutable reference to an element be used from another thread (For example, a scoped thread) if the elements of the Vec are Sync
fasterthanlime 4y ago
This particular example is trivially restructured into a version that passes the current borrow checker: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1f5aa2489f07829d12be5da9e3bf9a6e https://play.rust-lang.org/?version=stable&mode=debug&editio... It isn't the gotcha you're looking for.

magicalhippo 4y ago

I don't know any rust but the error description[1] only talks about borrowing mutable from an immutable, which to me seems like an obvious error. However the compiler complains about the reverse condition. Again, I don't know any rust, so it's not clear why that's an error. So is it a documentation bug or a compiler bug? [1]: https://doc.rust-lang.org/stable/error-index.html#E0502 https://doc.rust-lang.org/stable/error-index.html#E0502

ArrayBoundCheck 4y ago
I only understand it because I learned how not to shoot myself in the foot with the canon that is C++ I'll sum up "container invalidation". In C or C++ if you append an array you may cause the array to grow which means it might have a different pointer. You basically can't take an address of an array you're planning to grow. All containers are like that except list I think which I never use In rust, it's roughly the same idea. The problem is I borrowed something from vec (you can think of it as taking a reference or creating a pointer). I then call a function that is const (you can see here it's &self and not &mut self https://doc.rust-lang.org/src/alloc/vec/mod.rs.html#1911 https://doc.rust-lang.org/src/alloc/vec/mod.rs.html#1911). But that invalidates the borrow like push_back/resize/reserve and such in C++. Which is absolutely ridiculous because it's doesn't modify anything. It makes me so mad because even the one thing I like about rust is annoying to use. Using an annoying checker, slower compiler, harder to optimize language is too much. I don't shoot myself in the foot in C++ so I'm unable to get a win in rust. For me using rust is more pain than practical
- pkolaczk 4y ago
  Can you share any compiler benchmarks showing Rust compiles slower than C++? I can see something totally opposite in my projects - particularly recompiling after a change is very quick. And cargo check often runs in less than 1s. I never got such quick incremental recompile times in any of my earlier C++ projects, particularly when using templated or header heavy libraries like STL ;) As for the borrow checker thing, it is not broken but it's simply a limitation. In a strictly statically checked it is usually possible to run into a situation of a valid program that gets rejected by the compiler. Here this limitation comes from the compiler following a pretty simple rule - only one reference is allowed if it is mutable. This simple rule is what makes many of Rust safety guarantees possible, and I guess relaxing it conditionally might be very tricky. NLLs have already made it a bit nicer recently, do maybe it is possible to improve it further.
  - masklinn 4y ago
    > As for the borrow checker thing, it is not broken but it's simply a limitation. Limitations are when the borrow checker is not smart enough to understand things like sub-scope borrows (fixed by NLL) or the interaction of mutable borrows and loops (which hopefully will eventually be fixed by Polonius aka NLLv2). But here they want to acquire a shared reference to an object with an outstanding unique reference. That is quite literally the first of only two rules references have: 1. At any given time you can have either a single unique reference or multiple shared references. 2. References must be valid.
    - pkolaczk 4y ago
      In the given example they are not using the shared reference at all during the whole lifetime of the immutable reference and the operation on the immutable one does not change the object at all, so it would be actually safe. This is the situation where those two rules are a bit too strict and reject a valid program.
      - masklinn 4y ago
        > In the given example they are not using the shared reference at all during the whole lifetime of the immutable reference and the operation on the immutable one does not change the object at all, so it would be actually safe. The compiler does not work at that level of resolution. It’s unclear that it should, even for a point query: there is no telling how much a weirdo ISA would bungle that if the two accesses came from different threads, for instance. > This is the situation where those two rules are a bit too strict and reject a valid program. Compilers will always, necessarily, reject valid programs. The alternative is to allow invalid programs.

chlorion 4y ago

There is nothing broken in this example, it's functioning exactly as it's intended to. What makes you think the borrow checker is broken here? It appears like you don't really understand the rule about mutable and immutable references. Asking for the length of the vector requires an immutable reference, but you already have a mutable reference, which is explicitly not allowed by design (that's the whole point!). So, you can only have one mutable reference to something if there are no immutable references, and multiple immutable references if there are no mutable references, you can never have both immutable and mutable references to something at the same time.

ArrayBoundCheck 4y ago
I'm taking a reference to the array the vec is pointing to not the vec. Rust needs to be more fine grain because it's obnoxious and C++ container invalidation is less bad https://news.ycombinator.com/item?id=32188672 https://news.ycombinator.com/item?id=32188672
nlitened 4y ago
I don’t know Rust, just wondering: why is it forbidden to know a length of vector to which you hold a mutable reference? What could go wrong?
- scoutt 4y ago
  I imagine you can have one thread modifying the vector while trying to read its length from another thread.
  - ArrayBoundCheck 4y ago
    Noone should imagine that. You definitely don't want two threads accessing the same vec or array at one time. Maybe if there's a lock
    - nickitolas 4y ago
      If it's read only, you can definitely want that, right? As long as the elements are Sync. And it should work (You might have a problem with the cleanup, but Arc or scoped threads should handle that). The problem is when one is a mutable reference, like in this case. AIUI you could have a single mutable reference to an element be used from another thread (For example, a scoped thread) if the elements of the Vec are Sync

fasterthanlime 4y ago

This particular example is trivially restructured into a version that passes the current borrow checker: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=1f5aa2489f07829d12be5da9e3bf9a6e https://play.rust-lang.org/?version=stable&mode=debug&editio... It isn't the gotcha you're looking for.