Error message about immutable binding is not actually caused by a binding

[gdesmott]

See the following code:

use std::collections::HashMap;

struct Test {
    v: u32,
}

fn main() {
    let mut map = HashMap::new();
    map.insert("a", Test { v: 0 });

    let values = map.values();
    for mut t in values {
        t.v += 1;
    }
}

It raises this compiler error:

error[E0594]: cannot assign to field t.v of immutable binding
--> /tmp/test.rs:14:9
|
14 | t.v += 1;
| ^^^^^^^^ cannot mutably borrow field of immutable binding

The fix is to use values_mut() instead of values().
But the error message is confusing as the problem here is not the binding as t itself is mut making it harder to debug for newcomers.

[mbrubeck]

In Rust 1.23 and earlier, the error was:

error[E0594]: cannot assign to immutable field `t.v`
  --> diag.rs:13:9
   |
13 |         t.v += 1;
   |         ^^^^^^^^ cannot mutably borrow immutable field

error: aborting due to previous error

[estebank]

Current output is slightly more misleading, but I think improving on this will be hard:

error[E0594]: cannot assign to `t.v` which is behind a `&` reference
  --> src/main.rs:13:9
   |
12 |     for mut t in values {
   |         ----- help: consider changing this to be a mutable reference: `&mut Test`
13 |         t.v += 1;
   |         ^^^^^^^^ `t` is a `&` reference, so the data it refers to cannot be written

[estebank]

This overlaps slightly with #18150, but is more targeted, leaving open.

[steveklabnik]

Triage: no change

[sasurau4]

I'll work on it. @rustbot calim

current output is following.

error[E0594]: cannot assign to `t.v` which is behind a `&` reference
  --> src/main.rs:13:9
   |
12 |     for mut t in values {
   |                  ------ this iterator yields `&` references
13 |         t.v += 1;
   |         ^^^^^^^^ `t` is a `&` reference, so the data it refers to cannot be written

[estebank]

@sasurau4 you might want to first focus on the case where the values() method has been called in the for loop as it will be easier to find the needed expressions and types to verify that values_mut is available. It would also be interesting to account for .iter() vs .iter_mut() as well as part of this work (it just adds another method to check for). For the exact case that is presented in the original report, you'll have to find where the binding that is in the iterator expression of the for loop comes from, which as things are today might be had (but not impossible for the common case) to do. I don't want to discourage you from tackling it if you desire to do so, but I do want you to know that so you don't get discouraged if you find it takes to long or becomes too hard. Handling the simpler cases has a lot of value on their own.

[sasurau4]

@estebank Thanks for your advice! I was stuck on how to get the binding rhs expression for.
I'll focus on the case you suggest 👍