Unclear error with type inference failure

[jethrogb]

struct S {
    v: Vec<(u32, Vec<u32>)>,
}

impl S {
    pub fn remove(&mut self, i: u32) -> Option<std::vec::Drain<u32>> {
        self.v.get_mut(i as _).map(|&mut (_, ref mut v2)| {
            v2.drain(..)
        })
    }
}

Errors with:

error: the type of this value must be known in this context
 --> src/main.rs:8:16
  |
8 |             v2.drain(..)
  |                ^^^^^

I'm not sure which value the error is pointing at here. The fix here is to explicitly specify the type we want to cast i into.

[estebank]

Current output:

error[E0282]: type annotations needed for `&mut (_, _)`
 --> file8.rs:7:37
  |
7 |         self.v.get_mut(i as _).map(|&mut (_, ref mut v2)| {
  |                                     ^^^^^^^^^^^^^^^^^^^^ consider giving this closure parameter the explicit type `&mut (_, _)`, with the type parameters specified
  |
  = note: type must be known at this point

[estebank]

Output has regressed to the original output on nightly.

[jethrogb]

It's not quite the same as the original:

error[E0282]: type annotations needed
 --> src/lib.rs:8:16
  |
8 |             v2.drain(..)
  |                ^^^^^ cannot infer type
  |
  = note: type must be known at this point

[estebank]

Current output:

error[E0282]: type annotations needed for `&mut (_, _)`
 --> f71.rs:7:37
  |
7 |         self.v.get_mut(i as _).map(|&mut (_, ref mut v2)| {
  |                                     ^^^^^^^^^^^^^^^^^^^^
8 |             v2.drain(..)
  |                ----- type must be known at this point
  |
help: consider giving this closure parameter an explicit type, where the placeholders `_` are specified
  |
7 |         self.v.get_mut(i as _).map(|&mut (_, ref mut v2): &mut (_, _)| {
  |                                                         +++++++++++++