Can't understand warning when using const_evaluatable_checked

help
1

While developing crate using const_generics I found strange warning.

In method:

pub fn write_struct<T: ConstWriterAdapter>(writer: ConstWriter<T, 10>) -> ConstWriter<T, 0> {
    writer.write_u16_le(34).write_u16_le(2).write_u16_le(3).write_u16_le(4).write_u16_le(5)
}

I got:

warning: cannot use constants which depend on generic parameters in types
   --> src/lib.rs:493:12
    |
    |     writer.write_u32_le(34).write_u16_le(2).write_u16_le(3).write_u16_le(4).write_u16_le(5)
    |            ^^^^^^^^^^^^
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>

There are few questions:

  1. Why do I get this warning only in playground? Does playground uses some rustc flags? Or this is because I use rust on windows?
rustc 1.52.0-nightly (5fa22fe6f 2021-02-14)
binary: rustc
commit-hash: 5fa22fe6f821ac3801d05f624b123dda25fde32c
commit-date: 2021-02-14
host: x86_64-pc-windows-gnu
release: 1.52.0-nightly
LLVM version: 11.0.1
  1. After reading linked issue I didn't get how it's connected with my code.
    You can check linked repo with lib or look at short definition there
pub struct ConstWriter<T: ConstWriterAdapter, const N: usize> {v: T}

impl<T: ConstWriterAdapter, const N: usize> ConstWriter<T, {N}> {
pub fn write_u32_le(self, value: u32) ->ConstWriter<T, {N - std::mem::size_of::<u32>()}> {
    ConstWriter { v: self.v }
}

If this warning really connected with my code, can you please describe me how.

2

Since these features are still under active development, you might also want to ask questions like this on Zulip in the #project-const-generics stream.

3

Could you share a link to that Playground snippet? I haven't been able to reproduce that warning even when testing multiple versions of the nightly toolchain (on macOS though):

  • On old versions (before const_evaluatable_checked) get an error;

  • Starting from 1.48.0 all the versions pass without any warning.

So it may be something that is Windows-specific, or commit specific; in all cases, being able to reproduce the problem would be quite helpful :slightly_smiling_face:

1 Like
5

playground there is a bunch of code

1 Like
6

Minimized this code by dropping unused parts: playground. No ideas what went wrong, though.

2 Likes
7

I have investigated this a bit further, and submitted a report on the associated tracking issue.

8

From a recent comment there, it appears this warning (and the erroring variants) are not intended / a bug :slightly_smiling_face:

9

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