False positive with suspicious use of operator when using const generics

Lint name:

• suspicious_op_assign_impl
• suspicious_arithmetic_impl

I tried this code, and related other ops implementations:

Playground

use std::ops::Add;

#[derive(Debug, Copy, Clone)]
struct Matrix<T, const M: usize, const N: usize> {
    data: [[T; M]; N],
}

impl<T, const M: usize, const N: usize> Add<Matrix<T, M, N>> for Matrix<T, M, N>
where
    T: Copy + Add<Output = T>,
{
    type Output = Matrix<T, M, N>;

    fn add(mut self, other: Matrix<T, M, N>) -> Self::Output {
        for idx in 0..(M * N) {
            self[idx] = self[idx].add(other[idx]);
        }
        self
    }
}

I expected to see this happen: compile without errors

Instead, this happened:

    Checking playground v0.0.1 (/playground)
error: suspicious use of binary operator in `Add` impl
  --> src/main.rs:15:26
   |
15 |         for idx in 0..(M * N) {
   |                          ^
   |
   = note: `#[deny(clippy::suspicious_arithmetic_impl)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#suspicious_arithmetic_impl

error: aborting due to previous error

error: could not compile `playground`

To learn more, run the command again with --verbose.

Meta

• clippy 0.0.212 (44e3daf5 2020-12-31)

•  rustc 1.51.0-nightly (44e3daf5e 2020-12-31)
   binary: rustc
   commit-hash: 44e3daf5eee8263dfc3a2509e78ddd1f6f783a0e
   commit-date: 2020-12-31
   host: x86_64-apple-darwin
   release: 1.51.0-nightly
  

Start with the Rust Playground reproducer in the issue and run Clippy with suspicious_op_assign_impl and suspicious_arithmetic_impl enabled. Check why the M * N expression in the const-generic Matrix Add implementation is reported, then verify the example compiles without a false-positive diagnostic.