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Support for const variable in array init #2

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JoshMcguigan opened this issue Jan 16, 2019 · 5 comments
Open

Support for const variable in array init #2

JoshMcguigan opened this issue Jan 16, 2019 · 5 comments

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@JoshMcguigan
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Currently, arr_macro does not support const variables in array initialization as shown below. This is because the macro needs to know the exact value of SIZE, but the compiler expands macros before doing name resolution.

const SIZE: usize = 3;
arr![None; SIZE];

See additional details at dtolnay/syn#101
@kaimast
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kaimast commented Jun 29, 2020

Is there a rust bug associated with this? All the issues linked seem to be closed or archived.

ColinFinck added a commit to ColinFinck/binread that referenced this issue Mar 27, 2021
@ColinFinck
Implement BinRead for all array lengths using const generics.
fd56089 
This increases the minimum supported Rust version to 1.51.0.
I sadly had to introduce another unsafe statement, because:
* `Default` is only implemented for array lengths up to 32
  (rust-lang/rust#61415)
* arr_macro doesn't support const generics
  (JoshMcguigan/arr_macro#2)
* Direct initialization via [T; N] adds a trait bound
  BinRead: Copy
ColinFinck added a commit to ColinFinck/binread that referenced this issue Mar 27, 2021
@ColinFinck
Implement BinRead for all array lengths using const generics.
6e7860b 
This increases the minimum supported Rust version to 1.51.0.
I sadly had to introduce another unsafe statement, because:
* `Default` is only implemented for array lengths up to 32
  (rust-lang/rust#61415)
* arr_macro doesn't support const generics
  (JoshMcguigan/arr_macro#2)
* Direct initialization via [T; N] adds a trait bound
  `BinRead: Copy`
@ColinFinck ColinFinck mentioned this issue Mar 27, 2021
Merged
jam1garner pushed a commit to jam1garner/binread that referenced this issue Mar 28, 2021
@ColinFinck @jam1garner
Implement BinRead for all array lengths using const generics.
76ed76c 
This increases the minimum supported Rust version to 1.51.0.
I sadly had to introduce another unsafe statement, because:
* `Default` is only implemented for array lengths up to 32
  (rust-lang/rust#61415)
* arr_macro doesn't support const generics
  (JoshMcguigan/arr_macro#2)
* Direct initialization via [T; N] adds a trait bound
  `BinRead: Copy`
@daxpedda
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daxpedda commented Oct 5, 2021

Is there a rust bug associated with this?

rust-lang/rust#52393

@yegor256
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yegor256 commented Mar 1, 2022

@JoshMcguigan what is a possible workaround?

@DrRuhe
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DrRuhe commented Dec 7, 2022 •
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I ran into this while using const generics to specify an array size and trying to fill that array with None without T being Copy or Default. I solved it by using this snippet:

let array: [Option<T>; N] = (0..N).map(|_| None).collect::<Vec<_>>().try_into().ok().unwrap();

The .ok() is needed to relax the Debug trait bound on T.

It is obviously not ideal but for my purposes a good enough workaround.

@PENGUINLIONG
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Is there a plan to support constant generic?

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6 participants
@yegor256 @PENGUINLIONG @daxpedda @kaimast @JoshMcguigan @DrRuhe