Unhelpful "constants cannot evaluate destructors" with loops

[RalfJung]

This code:

#![feature(const_if_match)]
#![feature(const_loop)]

const _: Option<Vec<i32>> = {
    let mut never_returned = Some(Vec::new());
    let mut always_returned = None;

    let mut i = 0;
    loop {
        always_returned = never_returned;
        never_returned = None;

        i += 1;
        if i == 10 {
            break always_returned;
        }
    
    }
};

yields the following error:

error[E0493]: destructors cannot be evaluated at compile-time
 --> src/lib.rs:7:9
  |
7 |     let mut always_returned = None;
  |         ^^^^^^^^^^^^^^^^^^^ constants cannot evaluate destructors

error: aborting due to previous error

That is not very helpful as there is nothing with a destructor on that line. The actual line where something could get dropped is line 11, and that is also the line where the drop-requiring thing is assigned into always_returned.

Cc @rust-lang/wg-const-eval

[ecstatic-morse]

This isn't specific to loops and conditionals.

Historically, we have emitted live drop errors at the initial definition of a local because the span of the offending Drop terminator is often a single closing brace. It would probably be better to emit a two-part error message that has the place where the local is defined as well as where it is dropped. Perhaps we omit the "where it was dropped" part when the span is a single byte long?

[RalfJung]

It would probably be better to emit a two-part error message that has the place where the local is defined as well as where it is dropped.

Yeah that sounds good.