bson.rs

// ...

type SizeMap = HashMap<Vec<String>, u32>;

fn document_size(bytes: &[u8]) -> SizeMap {
   // ...
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_sizes() {
        let doc: Vec<u8> = vec![
            0x0b, 0x00, 0x00, 0x00,             // Document size
            0x08, 0x66, 0x6f, 0x6f, 0x00, 0x01, // "foo" => true
            0x00                                // Document end
        ];
        let mut expct = SizeMap::new();
        expct.insert(vec!["root"], 0x0b);
        expct.insert(vec!["root", "foo"], 0x01);

        assert_eq!(expct, document_size(&doc[..]));
    }
}

error.txt

error[E0308]: mismatched types
  --> src/bson.rs:36:27
   |
36 |         expct.insert(vec!["root"], 0x0b);
   |                           ^^^^^^ expected struct `std::string::String`, found reference
   |
   = note: expected type `_`
   = note:    found type `&'static str`