Suggested code causes error `(*b as usize)` -> `*b`

[giluis]

Summary

Suggested code causes compilation error. See reproducer for details.

Lint Name

clippy::unnecessary_cast

Reproducer

I tried this code:

fn polynomial(nums: &[usize], degree: usize) -> impl FnOnce(usize) -> usize {
                                                                                                                 // suggests *b
    let A: Vec<_> = nums[0..=degree].iter().map(|b|(0..=degree).map(|p| ( *b as usize ).pow(p as u32)).collect::<Vec<_>>()).collect();
    let mut b= nums[0..=degree].to_vec();
    gaussian_elimination(A, &mut b);
    move |x| b.iter().enumerate().fold(0,|acc,(p,b)| acc + b*x.pow(p as u32))
}

I saw this happen:

I expected suggestion to be just b, rather than *b, since *b causes a compilation error, since usize cannot be dereferenced.

Version

rustc 1.75.0-nightly (1c05d50c8 2023-10-21)
binary: rustc
commit-hash: 1c05d50c8403c56d9a8b6fb871f15aaa26fb5d07
commit-date: 2023-10-21
host: x86_64-unknown-linux-gnu
release: 1.75.0-nightly
LLVM version: 17.0.3

Additional Labels

No response

[ericwu17]

Here's an example which I believe demonstrates this same bug:

fn compute_square(x: &usize) -> usize {
    (*x as usize).pow(2)
}

Clippy rightfully determines that the cast is redundant, and wants to remove it. However, the suggested source code is *x.pow(2) which does not compile, since the dereference is evaluated after the pow function, which returns usize.

Clippy should either suggest x.pow(2) or (*x).pow(2)

[ericwu17]

@rustbot label -I-false-positive +I-suggestion-causes-error

[Ethiraric]

@rustbot claim