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A lint for taking a mutable reference to a constant #829
Description
i.e. rust-lang/rfcs#885
This is currently legal and prints 0 1 because a and ARR[0] are actually references to temporaries and not to C and ARR, but this is confusing as hell.
const C: u8 = 0;
const ARR: [u8; 3] = [1, 2, 3];
fn main() {
let a = &mut C;
*a = 1;
ARR[0] = 4;
println!("{} {}", C, ARR[0]);
}
Reacted by Sander Maijers and MSxDOS
Activity
mcarton commentedon Apr 2, 2016
😕 Rust
oli-obk commentedon Apr 3, 2016
haha great catch!
oli-obk commentedon Apr 3, 2016
*&mut 1 = 5;is legal, too xDI think the
let a = &mut Ccase should not be caught by linting against taking a mutable reference to a constant, but by improving theunused_assignmentslint. This would also catchARR[0] = 4. But I fear we need to do this analysis in MIR.As an intermediate step, simply linting against having a constant on the lhs of an assignment op or taking a mutable reference to it would be good.
petrochenkov commentedon Apr 3, 2016
@oli-obk
With
&mut 1or&mut (a + b)it's not so bad because it's fairly evident, that the reference is bound to a temporary, in case of&mut CorARR[0]it looks completely like taking a reference to a lvalue.tgnottingham commentedon Jul 19, 2020
This is probably obvious, but just want to point out that this applies to calling methods that take
&mut self. Silly example:Also, I think that this would be more appropriate as a compiler warning or error, FWIW.
camsteffen commentedon Dec 16, 2020
This is resolved by the new rustc lint 👆