Cannot borrow `self.foo` as mutable because it is also borrowed as immutable

[ambaxter]

I'm trying to help a colleague, but we're all a bit stumped on this one. Here's the reproducer.

Rust Playground

The original code uses HashMap. They can't use the entry function because the retrieving the value may fail.

[SkiFire13]

You just stumbled on a limitation of the current borrow checker. Polonius, the new borrow checker, should fix these kind of errors (see Polonius update | Inside Rust Blog for a bit of background and some recent updates on the milestones). For now you can emulate it with the polonius-the-crab crate

[nerditation]

I think this is another example for polonius, see the book for details:

https://rust-lang.github.io/polonius/

for a solution on current version, see the crate polonius_the_crab

for this particular example, you can use HashSet::contains() to check the existence, or you can use the nightly HashSet::get_or_insert_with() API as a workaround, see .

if !foo.contains(key) {
    foo.insert(key.to_string());
}
foo.get(key).unwrap()

or

foo.get_or_insert_with(key, || key.to_string())

[quinedot]

Here's the github issue (or perhaps one of the related ones), in case you want to follow it.