Treat Assigning Coroutine to `_` the Same as Unused Coroutine

Feature

Treat assigning a function's coroutine return value to the _ variable the same as an unused coroutine.

Pitch

Mypy attempts to catch the bug of forgetting to await a coroutine by detecting unused coroutines. For example, with mypy 1.19.1:

async def foo() -> int:
    return 42

def bar() -> None:
    foo()  # error: Value of type "Coroutine[Any, Any, int]" must be used  [unused-coroutine]

It's the style convention of some organizations to assign the unused return value of a function to the _ variable. This signals to readers that the return value was intentionally ignored and not a possible bug. However, this then silences the unused-coroutine issue.

async def foo() -> int:
    return 42

def bar() -> None:
    _ = foo()  # no mypy error

I'm not sure if there is precedent of mypy treating _ differently, but I believe that practically this would aim to catch the same category of bugs as unused-coroutine.

Start by locating mypy's existing unused-coroutine diagnostic and the handling of assignments to _. Trace how the example _=foo() is analyzed, then verify that assigning a coroutine to _ produces the same diagnostic as leaving it unused, using focused type-checking examples to confirm the behavior.