Suggest ! return type for functions that never return a value

[rootCircle]

What it does

Rust's ! type, also known as the never type, signifies that a function never returns normally. Clippy currently doesn't have a lint to suggest using ! when a function never returns a value or will result into diverging computation(eg., an infinite loop or panic), which could lead to potential type safety issues and misleading code patterns.

Advantage

• Improved type safety by preventing accidental assignment of panic-only functions to variables expecting a value.
• Clearer code intent, making it more obvious when functions don't return normally.
• Potential for better error handling and code analysis.

Drawbacks

• never type is still in nightly, and still will take time before it's stabilized, so better to wait in that case. (https://doc.rust-lang.org/nightly/std/primitive.never.html)
• It could be challenging to write a foolproof lint that accurately distinguishes functions that always panic from those that may return under some conditions.

Example

fn will_error_out() {
  panic!("It returns nothing, but it's not known to compiler")
}

This function always panics, but its return type is implicitly () (unit), suggesting it might return a value. Adding ! as the return type would clarify its behavior:

fn will_error_out() -> ! {
  panic!("It returns nothing, but it's now explicit")
}

Specifying a never return type will make it easier for its usage in where other functions are dependent on this function.

let some_value: bool = will_error_out(); // will not error out for second case

[Jarcho]

This should ignore both unimplemented! and todo! when checking.

[rootCircle]

Yes, I agree. The linter should only warn when a function either:

• Contains an unconditional panic!()
• Unconditionally exits the program
• Unconditionally calls a method that never returns
• Contains an infinite loop

Checking for infinite loops comprehensively might be difficult, but even basic checks would be helpful.

[J-ZhengLi]

FYI, function contains infinite loop is already linted 😄 infinite_loop

[rootCircle]

FYI, function contains infinite loop is already linted 😄 infinite_loop

Thanks for pointing it out! I think that lint can be used as reference for implementing this one! Also, I think it will be more sane to merge this one in infinite loop one, as both are targeting never return only?(need opinions) 😄