Confusing error message: dereference of &-pointer

[steveklabnik]

This code:

fn main() {
    let bar = &5;
    foo(&*bar);
}

fn foo(x: &mut int) -> &int {
    x
}

Gives this error message:

hello.rs:3:9: 3:14 error: cannot borrow immutable dereference of `&`-pointer as mutable
hello.rs:3     foo(&*bar);
                   ^~~~~

This error is correct, but is really complicated. First of all, &pointer should probably use 'reference,' the term we've settled on for &T. More importantly, it's just a very dense sentence. This came up in IRC this morning.

[kovek]

I don't know how this one will be handled but I wanted to say that, in the case we discussed, it was even more complicated:

"error: cannot borrow immutable dereference of `&`-pointer `*text` as mutable".

In the error message, *text is the dereference of &-pointer, so I think the error message should have commas around `text:

"error: cannot borrow immutable dereference of `&`-pointer, `*text`, as mutable".

(parenthesizes could do also, I think)

[kornelski]

👍 The error message is 100% jargon, and it doesn't give any hint how this situation can be fixed.

let mut foo: T;
func(&foo);

cannot borrow immutable dereference of &-pointer as mutable

• I thought & are references/borrows and * are pointers, so the phrase "&-pointer" confuses me.
• I thought *foo is a dereference, and &foo is taking a reference, so "dereference" here confuses me.
• What immutable!? I've got mutable variable and function that accepts mutable reference!

Eventually I've figured out that &foo is immutable regardless of mutability of everything everywhere, so how about message like:

Expected mutable reference, but &foo is an immutable reference. Did you mean &mut foo?

[sanxiyn]

I have a change related to this. For the original test case, do you think this is an improvement?

hello.rs:3:9: 3:14 error: mismatched types: expected `&mut isize`, found `&_` (values differ in mutability)
hello.rs:3     foo(&*bar);
                   ^~~~~

[huonw]

@sanxiyn and I were discussing this on IRC and both of us have seen a lot of people tripped up by it, possibly even the most common beginner mistake in Rust.

[steveklabnik]

With #21424 merged, this is the error:

hello.rs:3:17: 3:22 error: cannot borrow immutable borrowed content as mutable
hello.rs:3             foo(&*bar);
                           ^~~~~

Seems good!