Rust: Improve error message for missing parens in tuple destructure in for loop

Given:

    for foo, bar in [(1, 2), (3, 4)] {}

rustc produces:

error: missing `in` in `for` loop
  --> src/main.rs:44:12
   |
44 |     for foo, bar in [(1, 2)] {}
   |            ^ help: try adding `in` here

error: expected expression, found `,`
  --> src/main.rs:44:12
   |
44 |     for foo, bar in [(1, 2)] {}
   |            ^

…but it would be better if it figured out that the user presumably intended for (foo, bar) in ..., and gave a more specific error message.

This is probably a somewhat common error coming from Python, which has similar syntax for for loops but allows omitting the parentheses.