4 ms·

Congratulations! The highlight for me is stable conversion from Vec to array: let my_arr: [u32; 3] = my_vec.try_into().expect("msg");

by jph 6y ago

simias 6y ago
Indeed, I can't wait for const generics to become available outside of std, I've been bumping into that limitation since pre-rust-1.0 days, it'll be amazing to finally be able to rewrite all that hacky code correctly.
- nicoburns 6y ago
  min_const_generics is targeted for Rust 1.50 :)
  - matt_kantor 6y ago
    For anyone who wants to learn more: https://github.com/rust-lang/rust/pull/79135 https://github.com/rust-lang/rust/pull/79135
wyldfire 6y ago
Hmm. Could you do this kind of code for a static allocation at compile-time too?
- efnx 6y ago
  I don’t think so - you can only use static functions to declare static variables. But you could do it with a macro. It’s debatable whether that would be readable and predictable, though.
- steveklabnik 6y ago
  I believe https://github.com/rust-lang/const-eval/issues/20 https://github.com/rust-lang/const-eval/issues/20 is the entry point into this kind of thing. As you can see, there's a lot of discussion. If it ever lands it'll be farther out.
mark4 6y ago
Newbie question. Why is this useful? Wouldn't slices be enough to fulfill the use-cases this would see?
- steveklabnik 6y ago
  Some things want an array of a specific size. If you have a function like https://doc.rust-lang.org/stable/std/primitive.u16.html#method.from_be_bytes https://doc.rust-lang.org/stable/std/primitive.u16.html#meth... pub const fn from_be_bytes(bytes: [u8; 2]) -> u16 you can't pass it a slice. This would let you pass a vector to this function. Now, because this wasn't possible previously, it means many things take a slice, and then check that the length is the length they expect, because that ends up being easier for folks. Stuff like this helps it be able to be done properly. (This specific function is one example that is done in this style, and it's a pain. I have real world code that looks like let foo = u16::from_be_bytes([some_slice[0], some_slice[1]]); This gets even worse with say, u128::from_be_bytes.)
  - antonf 6y ago
    TryInto is already implemented for slices in 1.47, so `let foo = u16::from_be_bytes(some_slice.try_into().unwrap());` would work before 1.48. This trait wasn't implemented for vectors though, so you would have to write `let foo = u16::from_be_bytes(some_vec.to_slice().try_into().unwrap());`. With new release you can drop `to_slice()` part. Nothing earth-shattering, but makes sense.
    - steveklabnik 6y ago
      Ah fun! I missed that. Time to clean up some code...
    - est31 6y ago
      > TryInto is already implemented for slices in 1.47, so `let foo = u16::from_be_bytes(some_slice.try_into().unwrap());` would work before 1.48. Rustdocs say it was 1.36 even? https://doc.rust-lang.org/nightly/std/primitive.array.html#impl-TryFrom%3C%26%27a%20%5BT%5D%3E https://doc.rust-lang.org/nightly/std/primitive.array.html#i...

Congratulations! The highlight for me is stable conversion from Vec to array: let my_arr: [u32; 3] = my_vec.try_into().expect("msg");

by jph 6y ago

Congratulations! The highlight for me is stable conversion from Vec to array: let my_arr: [u32; 3] = my_vec.try_into().expect("msg");

simias 6y ago

Indeed, I can't wait for const generics to become available outside of std, I've been bumping into that limitation since pre-rust-1.0 days, it'll be amazing to finally be able to rewrite all that hacky code correctly.

nicoburns 6y ago
min_const_generics is targeted for Rust 1.50 :)
- matt_kantor 6y ago
  For anyone who wants to learn more: https://github.com/rust-lang/rust/pull/79135 https://github.com/rust-lang/rust/pull/79135

wyldfire 6y ago

Hmm. Could you do this kind of code for a static allocation at compile-time too?

efnx 6y ago
I don’t think so - you can only use static functions to declare static variables. But you could do it with a macro. It’s debatable whether that would be readable and predictable, though.
steveklabnik 6y ago
I believe https://github.com/rust-lang/const-eval/issues/20 https://github.com/rust-lang/const-eval/issues/20 is the entry point into this kind of thing. As you can see, there's a lot of discussion. If it ever lands it'll be farther out.

mark4 6y ago

Newbie question. Why is this useful? Wouldn't slices be enough to fulfill the use-cases this would see?

steveklabnik 6y ago
Some things want an array of a specific size. If you have a function like https://doc.rust-lang.org/stable/std/primitive.u16.html#method.from_be_bytes https://doc.rust-lang.org/stable/std/primitive.u16.html#meth... pub const fn from_be_bytes(bytes: [u8; 2]) -> u16 you can't pass it a slice. This would let you pass a vector to this function. Now, because this wasn't possible previously, it means many things take a slice, and then check that the length is the length they expect, because that ends up being easier for folks. Stuff like this helps it be able to be done properly. (This specific function is one example that is done in this style, and it's a pain. I have real world code that looks like let foo = u16::from_be_bytes([some_slice[0], some_slice[1]]); This gets even worse with say, u128::from_be_bytes.)
- antonf 6y ago
  TryInto is already implemented for slices in 1.47, so `let foo = u16::from_be_bytes(some_slice.try_into().unwrap());` would work before 1.48. This trait wasn't implemented for vectors though, so you would have to write `let foo = u16::from_be_bytes(some_vec.to_slice().try_into().unwrap());`. With new release you can drop `to_slice()` part. Nothing earth-shattering, but makes sense.
  - steveklabnik 6y ago
    Ah fun! I missed that. Time to clean up some code...
  - est31 6y ago
    > TryInto is already implemented for slices in 1.47, so `let foo = u16::from_be_bytes(some_slice.try_into().unwrap());` would work before 1.48. Rustdocs say it was 1.36 even? https://doc.rust-lang.org/nightly/std/primitive.array.html#impl-TryFrom%3C%26%27a%20%5BT%5D%3E https://doc.rust-lang.org/nightly/std/primitive.array.html#i...