DerefMut needs to be able to be triggered by implicit coercion to &mut Trait

[lilyball]

Today in IRC, dpc had a problem where he was trying to turn a RefCell<~Trait> into a &mut Trait and couldn't figure it out. The eventual solution that I came up with was

let val: &mut Trait = *&mut *refcell.borrow_mut()

(slightly modified; in his case the expression was being passed to a function that takes &mut Trait).

This was complicated to figure out and looks very confusing to anyone who doesn't understand that &mut *foo is required to trigger DerefMut, which is required to make the resulting ~Trait value (from *&mut *foo) be coercable to &mut Trait.

This should have been able to be written merely as *refcell.borrow_mut().

[eddyb]

Ah, our dear ~T -> &T/&mut T coercions. I hope they won't be part of a post-DST world, as manual reborrows should then work on all types.
The only sane way I know to get a &mut Trait after DST is:

let val: &mut Trait = &mut **refcell.borrow_mut();

[nikomatsakis]

cc me

[nikomatsakis]

I think this is basically a dup of #6308 (DST)

[alexcrichton]

Due to the removal of implicit coercions between Box<T> and &T and friends, I believe this is closed now.