Confusing compiler error when iterating over Option<IntoIterator>

[KeyboardDanni]

I recently wrote a type that implemented IntoIterator with a tuple as the Item type. I then tried to test this by using this iterator in a for loop. I got the following error:

     for (i, j) in foo
         ^^^^^^ expected struct `MyIter`, found tuple

This left me confused, because I had assumed that foo's type was MyIter. Why was the iterator returning itself, instead of the item type I had specified? I began to suspect my implementation of IntoIterator before I realized that foo was actually Option<MyIter>. As it turns out, Option also implements IntoIterator (#27996), so the compiler thought I was trying to iterate over the Option instead of the enclosed type!

It'd be nice if there were a note associated with the compiler error in cases where the iterator is type Option<T : IntoIterator>. Something like this, maybe?

"Note: `iter` is `Option<MyIter>`. Did you forget to unwrap?"

Simple reproduction of the compiler error:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=4432a78517e24ea649a31c7246b34edd

[RustyYato]

@cosmicchipsocket you can use foo.into_iter().flatten() instead of foo.unwrap(). This way you get an iterator that yields nothing when the Option is None, and iterates over the internal iterator when the Option is Some.

[estebank]

Current output:

error[E0308]: mismatched types
  --> src/main.rs:10:9
   |
10 |     for (i, j) in foo_wrapped
   |         ^^^^^^    ----------- this is an iterator with items of type `std::slice::Iter<'_, ({integer}, {integer})>`
   |         |
   |         expected `Iter<'_, ({integer}, {integer})>`, found `(_, _)`
   |
   = note: expected struct `std::slice::Iter<'_, ({integer}, {integer})>`
               found tuple `(_, _)`