let_unit_value when using constants to limit values of `const usize`

Summary

The pattern let _ = CONSTANT_VALUE is used to ensure that various consts conform to expected values. For example, I have a const that I need to be aligned to 4096 bytes.

This check is done by using the CONSTANT_VALUE during some const constructor and throwing away the result. However, Clippy reports that this code is unused, and needs to be discarded.

Furthermore, clippy appears to ignore #[allow(clippy::let_unit_value)].

Lint Name

let_unit_value

Reproducer

I tried this code:

#[repr(C, align(4096))]
pub struct Buffer<const N: usize = 4096> {
    data: [u8; N],
}

impl<const N: usize> Buffer<N> {
    // Ensure that `N` is a multiple of 4096
    const CHECK_ALIGNED: () = if N & 4095 != 0 {
        panic!("Buffer size must be a multiple of 4096")
    };

    pub const fn new() -> Self {
        #[allow(clippy::let_unit_value)]
        let _ = Self::CHECK_ALIGNED;
        Buffer { data: [0u8; N], }
    }
}

fn main() {
    let buf1 = Buffer::<4096>::new();
    // let buf2 = Buffer::<4097>::new();
    let buf3 = Buffer::<8192>::new();
    println!("Size of buf1: {}", core::mem::size_of_val(&buf1));
    println!("Size of buf3: {}", core::mem::size_of_val(&buf3));
}

I saw this happen:

warning: this let-binding has unit value
  --> src/main.rs:14:9
   |
14 |         let _ = Self::CHECK_ALIGNED;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: omit the `let` binding: `Self::CHECK_ALIGNED;`
   |
   = note: `#[warn(clippy::let_unit_value)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value

I expected to see this happen:

No warning

Version

rustc 1.62.0 (a8314ef7d 2022-06-27)
binary: rustc
commit-hash: a8314ef7d0ec7b75c336af2c9857bfaf43002bfc
commit-date: 2022-06-27
host: x86_64-pc-windows-msvc
release: 1.62.0
LLVM version: 14.0.5

Additional Labels

No response

Start with the let_unit_value lint and reproduce the reported warning using the Rust example in this issue. Investigate why the lint warns on let _ = Self::CHECK_ALIGNED and ignores the local allow attribute; done means the reproducer produces no warning while the lint still handles its other cases correctly.