Unhelpful suggestion for "cannot infer type" error

[Sherlock-Holo]

I tried this code:

fn main() {
    let map: HashMap<i32, i32, _> = HashMap::from_iter(vec![(1, 1), (2, 2)].into_iter());
}

I expected to see this happen:

when run cargo check, it should says

  --> src/main.rs:30:14
   |
30 |     let map: HashMap<i32, i32, _> = HashMap::from_iter(vec![(1, 1), (2, 2)].into_iter());
   |         ---  ^^^^^^^^^^^^^^^^^^^^ cannot infer type
   |         |
   |         consider change `HashMap<i32, i32, _>` to `HashMap<i32, i32>` because it has a default type, or specify a detail type.

Instead, this happened:

  --> src/main.rs:30:14
   |
30 |     let map: HashMap<i32, i32, _> = HashMap::from_iter(vec![(1, 1), (2, 2)].into_iter());
   |         ---  ^^^^^^^^^^^^^^^^^^^^ cannot infer type
   |         |
   |         consider giving `map` the explicit type `std::collections::HashMap<i32, i32, _>`, with the type parameters specified

Meta

rustc --version --verbose:

ustc 1.43.0-nightly (a1912f2e8 2020-02-12)
binary: rustc
commit-hash: a1912f2e89b77cfe2a0e64b96f444848fe4e2d49
commit-date: 2020-02-12
host: x86_64-unknown-linux-gnu
release: 1.43.0-nightly
LLVM version: 9.0

[compiler-errors]

This actually suggests nothing now:

Compiling playground v0.0.1 (/playground)
error[[E0282]](https://doc.rust-lang.org/stable/error-index.html#E0282): type annotations needed
 --> src/main.rs:4:14
  |
4 |     let map: HashMap<i32, i32, _> = HashMap::from_iter(vec![(1, 1), (2, 2)].into_iter());
  |              ^^^^^^^^^^^^^^^^^^^^ cannot infer type

For more information about this error, try `rustc --explain E0282`.
error: could not compile `playground` due to previous error