Rollup merge of #40445 - estebank:issue-18150, r=jonathandturner

Point to let when modifying field of immutable variable

Point at the immutable local variable when trying to modify one of its
fields.

Given a file:

```rust
struct Foo {
    pub v: Vec<String>
}

fn main() {
    let f = Foo { v: Vec::new() };
    f.v.push("cat".to_string());
}
```

present the following output:

```
error: cannot borrow immutable field `f.v` as mutable
 --> file.rs:7:13
  |
6 |    let f = Foo { v: Vec::new() };
  |        - this should be `mut`
7 |    f.v.push("cat".to_string());
  |    ^^^

error: aborting due to previous error
```

Fix #27593.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/test/ui/did_you_mean/issue-39544.stderr b/src/test/ui/did_you_mean/issue-39544.stderr
index c0088f39ad3..7f124e6d34d 100644
--- a/src/test/ui/did_you_mean/issue-39544.stderr
+++ b/src/test/ui/did_you_mean/issue-39544.stderr
@@ -1,8 +1,10 @@
 error: cannot borrow immutable field `z.x` as mutable
   --> $DIR/issue-39544.rs:21:18
    |
+20 |     let z = Z { x: X::Y };
+   |         - consider changing this to `mut z`
 21 |     let _ = &mut z.x;
-   |                  ^^^
+   |                  ^^^ cannot mutably borrow immutable field
 
 error: aborting due to previous error