Mismatched type [E0308] offers Intention for fix but does not actual apply fix

[schoenja]

I am getting an mismatched types [E0308] expected '&mut String', found '&String' error with this code on line 4 (code is from this tutorial)

fn main() {
    let mut s = String::from("hello");

    change(&s);
}

fn change(some_string: &mut String) {
    some_string.push_str(", world");
}

The intention offers to Convert to &mut String using 'BorrowMut' trait.

Expected behaviour:
The line is changed to change(&mut s);.

Actual behaviour:
The line is not changed.

Additional information:
IntelliJ IDEA 2018.1.1 (Ultimate Edition)
Build #IU-181.4445.78, built on April 9, 2018
JRE: 1.8.0_162-b12 amd64
JVM: OpenJDK 64-Bit Server VM by Oracle Corporation
Linux 4.15.15-1-ARCH
Rust plugin: Version: 0.2.0.2092-181

[vlad20012]

c.c. #1730
c.c. @oleg-semenov

[oleg-semenov]

hm it works little bit differently for me, it replaces &s in the line 4 with (&s).borrow_mut() which is still incorrect; i didn't try to match any versions though; I can make a bug-fix that adds a check that the expression is mutable before suggesting applying BorrowMut trait. So BorrowMut won't be suggested in this case.

As for the desired &s => &mut s fix we just currently don't have a quick-fix/intention for that. So far all quick-fixes/intentions for types mismatches don't try to replace or delete anything from the affected source-code; they add stuff before or around it. I don't know if we want to keep this "policy" (or it's just a coincidence). But i think in this case we can add a simple quick-fix/intention that in cases like this can try to check the mutability of s and suggest replacing & with &mut. So, more precisely it would be like if expected type it &mut T actual type is & T and the expression is mutable suggest replacing & with &mut.

@vlad20012 what do you think?

[oleg-semenov]

here is a bug-fix for BorrowMut: #2526