Github Allow unused variables with todo! by charles-r-earp · Pull Request #79850...

Allow unused variables with todo! by charles-r-earp · Pull Request #79850 · rust-lang/rust · GitHub

Fixes #78136

As stated in the issue, todo! should imply that the function will be implemented at some point but isn't yet, and therefore the user shouldn't have to either apply the #[allow(unused_variables)] themselves or prefix the arguments with underscores, since they will at some point replace the todo! with an implementation. Unlike unimplemented, todo! is a placeholder, so the suggested prefixing of the arguments with underscores does not make sense because that would imply that those arguments are not going to be used, when in fact they may be used when the function is implemented. Once the todo! is removed, the lint will return if applicable.

The lint error is emitted in rustc_passes::liveness::Liveness::report_unused. This can be traced back to the <IrMaps as Visiter>::visit_body method. Unfortunately the Body argument here has been fully expanded, so we cannot match on the todo! macro directly. Instead, the idea is to detect a trailing expression, ie the last expression without a semicolon, that is core::panicking::panic (also std). If the panic is called with "not yet implemented", which is what is emitted by todo!, then we return early and don't do any checking for unused variables.

For example:

fn foo(x: i32) -> i32 {
    // some code
    todo!()
}