The previous message ("cannot assign/mutably borrow immutable field")
when trying to modify a field of an immutable binding gave the
(incorrect) impression that fields can be mutable independently of their
ADT's binding. Slightly reword the message to read "cannot
assign/mutably borrow field of immutable binding".
Point to immutable borrow arguments and fields when trying to use them as
mutable borrows. Add label to primary span on "cannot borrow as mutable"
errors.
Present the following output when trying to access an immutable borrow's
field as mutable:
```
error[E0389]: cannot borrow data mutably in a `&` reference
--> $DIR/issue-38147-1.rs:27:9
|
26 | fn f(&self) {
| ----- use `&mut self` here to make mutable
27 | f.s.push('x');
| ^^^ assignment into an immutable reference
```
And the following when trying to access an immutable struct field as mutable:
```
error: cannot borrow immutable borrowed content `*self.s` as mutable
--> $DIR/issue-38147-3.rs:17:9
|
12 | s: &'a String
| ------------- use `&'a mut String` here to make mutable
...|
16 | fn f(&self) {
| ----- use `&mut self` here to make mutable
17 | self.s.push('x');
| ^^^^^^ cannot borrow as mutable
```
