Custom MIR: bad span for assignments

[RalfJung]

Consider this code:

#![feature(custom_mir, core_intrinsics)]
use std::intrinsics::mir::*;

pub struct S(i32);

#[custom_mir(dialect = "runtime", phase = "optimized")]
fn main() {
    mir! {
        let x: i32;
        {
            let _observe = x;
            Return()
        }

    }
}

When running it in Miri, I wold expect this to point at the assignment. Instead it points at the entire mir! block:

error: Undefined Behavior: constructing invalid value: encountered uninitialized memory, but expected an integer
  --> src/main.rs:8:5
   |
8  | /     mir! {
9  | |         let x: i32;
10 | |         {
11 | |             let _observe = x;
...  |
14 | |
15 | |     }
   | |_____^ constructing invalid value: encountered uninitialized memory, but expected an integer

I've seen it use proper spans for Call terminators inside mir! blocks so I hope this is just an oversight and easy to fix. :)

Cc @JakobDegen

[JakobDegen]

So if you change this to

#![feature(custom_mir, core_intrinsics)]
use std::intrinsics::mir::*;

pub struct S(i32);

#[custom_mir(dialect = "runtime", phase = "optimized")]
fn main() {
    mir! {
        let x: i32;
        let _observe: i32;
        {
            _observe = x;
            Return()
        }

    }
}

It does the right thing. The reason for this is that the let statement results in the assignment being taken apart and put back together in the mir! macro, which loses the span info. This can possibly be fixed with some cleverness, not sure