Improve error message for struct element borrow by closure

[BartMassey]

Consider this small example.

The code:

struct S { x: f32, y: f32 }

fn main() {
    let mut s = S { x: 1.0, y: 2.0 };
    let eval = |x| {
        x + s.y
    };
    s.x = eval(s.x);
    s.x = eval(s.x);
    println!("{} {}", s.x, s.y);
}

The compiler reports:

error[E0506]: cannot assign to `s.x` because it is borrowed
 --> src/main.rs:8:5
  |
5 |     let eval = |x| {
  |                --- borrow of `s.x` occurs here
6 |         x + s.y
  |             - borrow occurs due to use in closure
7 |     };
8 |     s.x = eval(s.x);
  |     ^^^^^^^^^^^^^^^ assignment to borrowed `s.x` occurs here
9 |     s.x = eval(s.x);
  |           ---- borrow later used here

error: aborting due to previous error

The compiler is correct in that s is borrowed by the eval closure, so it can't be mutated afterward. However, s.x is never borrowed by anything, so the error message is pretty confusing. In particular, "--- borrow of s.x occurs here" just looks wrong. I'm not sure whether the compiler is just accidentally pointing at the parameter (which is an ordinary f32) or whether it has gotten confused.

[csmoe]

However, s.x is never borrowed by anything

s.x = eval(s.x);
           ^^^

s.x is borrowed by eval() here.

[BartMassey]

Naw, f32 is a Copy type, so s.x actually isn't borrowed. If you make the closure argument |x: f32| you'll get the same error message. That's why I find the error message so confusing: it makes it look like you've borrowed s.x when you actually haven't. Here is the obvious way to fix the program:


    let y = s.y;
    let eval = |x| {
        x + y
    };

Now everything just compiles, so borrowing s.x was never a thing. It was borrowing s.y and later trying to modify s.x that was the issue: Rust doesn't currently track individual struct fields ("partial borrows") separately, because it is hard. IMHO the way forward here is to have the error message report that s cannot be modified because it is borrowed (or maybe that s.y is borrowed).

[kadiwa4]

The output still hasn't changed on Edition 2018, but with E2021 the code compiles

[estebank]

Given the above, explicitly lowering to P-low because people are way less likely to encounter this.