unnecessary_cast can silently change behavior of numeric code #11882
Description
Summary
The unnecessary_cast lint, which is marked machine-applicable, can give suggestions that make code no longer compile—or even worse, silently change its behavior.
Reproducer
I tried this code:
fn main() { let k: u64 = (!0 as u64).overflowing_shr(1_u32).0; println!("{k}"); }I expected to see this happen: Suggest replacing (!0 as u64) with (!0_u64).
Instead, this happened:
warning: casting to the same type is unnecessary (`u64` -> `u64`) --> src/main.rs:2:18 | 2 | let k: u64 = (!0 as u64).overflowing_shr(1_u32).0; | ^^^^^^^^^^^ help: try: `!0` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast = note: `#[warn(clippy::unnecessary_cast)]` on by default Applying the machine-applicable suggestion results in:
fn main() { let k: u64 = !0.overflowing_shr(1_u32).0; println!("{k}"); }error[E0689]: can't call method `overflowing_shr` on ambiguous numeric type `{integer}` --> src/main.rs:2:21 | 2 | let k: u64 = !0.overflowing_shr(1_u32).0; | ^^^^^^^^^^^^^^^ | help: you must specify a concrete type for this numeric value, like `i32` | 2 | let k: u64 = !0_i32.overflowing_shr(1_u32).0; | ~~~~~I tried this code:
fn main() { let k: u64 = (!0_u64 as u64).overflowing_shr(1_u32).0; println!("{k}"); }I expected to see this happen: Suggest replacing (!0_u64 as u64) with (!0_u64).
Instead, this happened:
warning: casting to the same type is unnecessary (`u64` -> `u64`) --> src/main.rs:2:18 | 2 | let k: u64 = (!0_u64 as u64).overflowing_shr(1_u32).0; | ^^^^^^^^^^^^^^^ help: try: `!0_u64` | = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_cast = note: `#[warn(clippy::unnecessary_cast)]` on by default Applying the machine-applicable suggestion results in:
fn main() { let k: u64 = !0_u64.overflowing_shr(1_u32).0; println!("{k}"); }Which compiles without warning, but prints a different value than the original, because ! has lower precedence than ..
Version
0.1.76 (2023-11-26 6cf0888) Additional Labels
@rustbot label I-suggestion-causes-error