Function transform_async_context 

fn transform_async_context<'tcx>(
    tcx: TyCtxt<'tcx>,
    body: &mut Body<'tcx>,
) -> Ty<'tcx>

Transforms the body of the coroutine applying the following transforms:

• Eliminates all the get_context calls that async lowering created.
• Replace all Local ResumeTy types with &mut Context<'_> (context_mut_ref).

The Locals that have their types replaced are:

• The resume argument itself.
• The argument to get_context.
• The yielded value of a yield.

The ResumeTy hides a &mut Context<'_> behind an unsafe raw pointer, and the get_context function is being used to convert that back to a &mut Context<'_>.

Ideally the async lowering would not use the ResumeTy/get_context indirection, but rather directly use &mut Context<'_>, however that would currently lead to higher-kinded lifetime errors. See https://github.com/rust-lang/rust/issues/105501.

The async lowering step and the type / lifetime inference / checking are still using the ResumeTy indirection for the time being, and that indirection is removed here. After this transform, the coroutine body only knows about &mut Context<'_>.