[arithmetic_side_effects] false positive with string and derefs

[JP-Ellis]

Summary

Concatenation of a String with &str when the &str is a String deref produces a false positive.

Lint Name

arithmetic_side_effects

Reproducer

Here's a minimal example that produces the error:

let x = String::new();
let y = String::new();
let z = x + &y;

assert_eq!(&z, "");

Playground

Version

rustc 1.84.0 (9fc6b4312 2025-01-07)
binary: rustc
commit-hash: 9fc6b43126469e3858e2fe86cafb4f0fd5068869
commit-date: 2025-01-07
host: aarch64-apple-darwin
release: 1.84.0
LLVM version: 19.1.5

(also reproduced in Rust playground)

Additional Labels

No response