Detect iterators going out of scope and suggest collecting

[estebank]

Given a situation where an iterator is constructed inside of a scope where the data is being returned, we currently emit:

error[E0515]: cannot return value referencing function parameter `y`
 --> src/main.rs:4:18
  |
4 |         .map(|y| y.iter().map(|x| x + 1))
  |                  -^^^^^^^^^^^^^^^^^^^^^^
  |                  |
  |                  returns a value referencing data owned by the current function
  |                  `y` is borrowed here

We should suggest using .collect() in these cases.

[leonardo-m]

Or flatten/flat_map?

[estebank]

I want for us to detect cases of trying to operate on iterators of iterators because the only option then is to allocate or completely rework the logic. The former will help newcomers, experts will know enough to try the later.

[kper]

@rustbot claim

[the8472]

Couldn't we still recommend both? Recommending bad practices seems suboptimal.