rust-lang / rust-lang/rust-clippy

const assert in struct is not always evaluated

Open
#16,004 0 comments 1 reaction 0 assignees View on GitHub

Nobody has claimed this yet.

A-lint
Dominant language
Rust
Stars
13.5k
Forks
2.2k
Avg merge
2d 10h
Merged PRs (30d)
32

Description

What it does

I would expect this not to compile because of the const assert, But it actually compiles fine, leading me to believe that I have a compile-time check in place, when actually I don't.

Advantage

Protects users from assuming that some const code is evaluated when it actually isnt.

Drawbacks

No response

Example
struct Foo<const C: char>;

impl<const C: char> Foo<C> {
    // This const check should fail for 'x', but it isn't evaluated.
    const _CHECK: () = assert!(C != 'x');

    fn new() -> Self {
        // Uncommenting the next line triggers the compile-time error as expected:
        // let _ = Self::CHECK;
        Foo
    }
}

fn main() {
    // This should fail (C == 'x'), but compiles fine.
    let _ = Foo::<'x'>::new();
    println!("it compiled!")
}

This also does not work:

struct Foo<const C: char>;

impl<const C: char> Foo<C> {
    const fn _check() {
        assert!(C != 'x');
    }
    
    fn new() -> Self {
        Foo
    }
}

fn main() {
    // This should fail (C == 'x'), but compiles fine.
    let _ = Foo::<'x'>::new();
    println!("it compiled!")
}
Comparison with existing lints

No response

Additional Context

I searched for duplicate issues: https://github.com/rust-lang/rust-clippy/issues?q=sort%3Aupdated-desc%20is%3Aissue%20is%3Aopen%20const%20struct%20eval

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Reproduce both examples and compare them with the explicitly referenced Self::CHECK access, confirming when const evaluation occurs. Then determine the intended Clippy diagnostic and its scope for const assertions in generic structs and associated const functions; done requires an agreed behavior and regression coverage.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
devtools
Issue type
Feature
Difficulty
5/5
Estimated time
Over a week
Activity status
Stale
Clarity
Needs clarification
Newbie friendliness
30/100

Get new issues in your inbox

A short digest of beginner-friendly GitHub issues.