CODEWITHSUNDEEP
rusthashmapborrow-checkermvt
Nino
Nino

Reputation: 441

Changing ownership back and forth

I'm new to Rust and I'm still struggling with borrowing and move concepts.

To set the context, I'm using a library (namely MVT ) that uses following construct to create a layer:

    let mut tile = Tile::new(4096);
    let layer = tile.create_layer("First Layer");
    let b = GeomEncoder::new(GeomType::Linestring, Transform::default())
    // ... add some data to b
    let mut feature = layer.into_feature(b);
    feature.set_id(1);
    feature.add_tag_string("key", "value");
    let layer = feature.into_layer();
    tile.add_layer(layer)?;

The most interesting pieces are layer.into_feature(b) which takes ownership of the layer and later feature.into_layer() which returns ownership of the layer again.

This sounds really strange to me. Is this some known concept/design pattern in Rust? I would expect, that the onwership of layer will remain untouched (i.e. it is not moved but borrowed).

The problem I have with that piece of code is, that I would like to keep a HashMap of Layers to avoid creating layers with the same name twice, i.e. sth. like this:

let mut layers = HashMap::new();
let layer = layers.entry(layer_name).or_insert(tile.create_layer(layer_name));

But this fails with below error, which makes sense b/c layer.into_feature(b) is moving the layer out of the HashMap which it shouldn't (IMO).

error[E0507]: cannot move out of `*layer` which is behind a mutable reference
   --> src/main.rs:39:23
    |
39  |     let mut feature = layer.into_feature(b);
    |                       ^^^^^^---------------
    |                       |     |
    |                       |     `*layer` moved due to this method call
    |                       move occurs because `*layer` has type `Layer`, which does not implement the `Copy` trait

So to summarize my question: Is there a way to get the library code working with a hashmap or do I need to change the library to not do that ownership ping-pong?

Upvotes: 1

Views: 88

Answers (0)

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