Support Question Mark Operator in Control Flow Highlighting

[CryZe]

rust-analyzer can nicely highlight which control flow relevant pieces of the code relate to each other, such as shown here:

This currently does not extend to the question mark operator:

However, while writing this issue, I did notice that it actually works perfectly fine already if you are on the fn itself:

[ChayimFriedman2]

That is really really weird... We have code for highlighting when on -> or ?, we have tests for them, the tests pass, yet this does not work! Could it be that VSCode is not sending highlight requests for symbols?

[CryZe]

That sounds plausible at least.

[Veykril]

This is a known VSCode issue

[Veykril]

Duplicate of #9395

[CryZe]

A case that definitely does not work atm is the following:

The cursor is on the async but only the return is marked, not the implicit return or the question mark operator.