`suspicious_arithmetic_impl` flags on size addition in multiplication

Summary

I maintain a library that supports symbolic multiplication on bitvectors. The API exposes how many bits to keep in the output, i.e. multiply(lhs, rhs, n). For the std::ops::Mul impl I just use the maximum size needed: multiply(lhs, rhs, lhs.len() + rhs.len()). Because the lint sees the + as the sole binary operator in Mul, it flags it as suspicious.

As has been noted in other issues (#2268, #3215), this lint is hard to get right, but would be great if it didn't flag here.

Lint Name

suspicious_arithmetic_impl

Reproducer

I tried this code:

pub struct BitVec {
    bytes: Vec<u8>,
}

impl BitVec {
    pub fn multiply(self, _rhs: Self, _n: usize) -> Self {
        unimplemented!()
    }
}

impl std::ops::Mul for BitVec {
    type Output = Self;

    fn mul(self, rhs: Self) -> Self::Output {
        // False positive due to +
        let n = self.bytes.len() + rhs.bytes.len();
        self.multiply(rhs, n)
    }
}

I saw this happen:

warning: suspicious use of `+` in `Mul` impl
   --> test.rs:16:34
    |
16 |         let n = self.bytes.len() + rhs.bytes.len();
    |                                  ^
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/rust-1.91.0/index.html#suspicious_arithmetic_impl
    = note: `#[warn(clippy::suspicious_arithmetic_impl)]` on by default

I expected to see this happen: Lint not to fire.

Version

rustc 1.91.1 (ed61e7d7e 2025-11-07)
binary: rustc
commit-hash: ed61e7d7e242494fb7057f2657300d9e77bb4fcb
commit-date: 2025-11-07
host: x86_64-unknown-linux-gnu
release: 1.91.1
LLVM version: 21.1.2

Additional Labels

No response

Start by reproducing the suspicious_arithmetic_impl warning with the BitVec example, then locate the lint's implementation and existing tests using the lint name. Done means the shown size addition in a Mul implementation no longer triggers the warning without weakening detection of genuinely suspicious arithmetic.