Type inference on HashSet

[Yuri6037]

I've recently used HashSet in one of my project and I had to explicitly state the full type when initializing because rust was unable to infer from what I inserted inside. I've replicated the exact problem on a simpler structure on the following playground: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=ca668a190a8de1e47b5505508b8fe419.

Basically the issue is when an if let Some(value) = hashset.get(&anitem) type inference completely falls apart and you must specify let mut set: HashSet<StructureType> = HashSet::new()even if you call set.insert(an_instance_of_structure_type).

The unofficial Rust IRC on Libera.chat hints this is a limitation of accessing items of a HashSet before inserting elements inside and is related to the Borrow trait. It does indeed work if the let binding is moved after the calls to insert.

[stijnfrishert]

Ran into the same issue with a HashMap. This playground doesn't even tell you to type annotate the hashmap/set.

use std::collections::HashMap;

trait Foo {
    type Value;
}

struct Holder<T: Foo>(HashMap<String, T::Value>);

fn foo<T: Foo>(hm: HashMap<String, T::Value>) {
    let holder = Holder(hm);
    if let Some(v) = holder.0.get("abc") {
        let v = v.clone();
    }
}

It just leaves you hanging with:

error[[E0282]](https://doc.rust-lang.org/nightly/error-index.html#E0282): type annotations needed
  --> src/lib.rs:12:19
   |
12 |         let v = v.clone();
   |                   ^^^^^ cannot infer type for type parameter `V`

For more information about this error, try `rustc --explain E0282`.

From the eyes of a beginner:

• Where is V coming from?
• Type annotations needed? On what? Where?