false positive: unstable const fn

[Nugine]

I tried this code:

#![deny(clippy::missing_const_for_fn)]

/*
error: this could be a `const fn`
*/

fn is_ascii_digit(byte: u8) -> bool {
    byte.is_ascii_digit()
}

/*
error: `core::num::<impl u8>::is_ascii_digit` is not yet stable as a const fn
*/

// const fn is_ascii_digit(byte: u8) -> bool {
//     byte.is_ascii_digit()
// }

fn main() {
    dbg!(is_ascii_digit(b'1'));
}

Meta

• cargo clippy -V: clippy 0.0.212 (04488af 2020-08-24)
• rustc -Vv:

  rustc 1.46.0 (04488afe3 2020-08-24)
  binary: rustc
  commit-hash: 04488afe34512aa4c33566eb16d8c912a3ae04f9
  commit-date: 2020-08-24
  host: x86_64-unknown-linux-gnu
  release: 1.46.0
  LLVM version: 10.0
  

[rail-rain]

The underlying cause is that rustc_mir::transform::qualify_min_const_fn::is_min_const_fn, which is the lint's core, is not responsible to check whether const fns in stable crates call unstable const fns, rustc_mir::transform::check_consts::validation::Validator is; but Validator panics given a non-const fn. Unless the rework (rust-lang/rust#76618) leaves something useful for the lint, fixing this false positive requires fair amount of rewriting; probably copying some parts of qualify_min_const_fn.

[ecstatic-morse]

It would be fairly easy to add a "speculative" mode to const-checking. I'll look into this once rust-lang/rust#7661 is complete.