Merge `Promoted` and `Static` variants of `mir::Place`

[oli-obk]

This would allow merging most of the code in

rust/src/librustc_mir/interpret/place.rs

Line 470 in 1114ab6

Promoted(ref promoted) => {

I suggest to create a single variant (Maybe just leave the Static variant), but modify the Static struct fields for the DefId (update docs that the DefId is the one of the containing function in case of a promoted) and additionally add an Option<Promoted> field.

[csmoe]

I'm working on changing the representation of mir::Place, since that may take some time to be merged, so block this for easing rebase-stuff at first and this issue will be unblocked as soon as the PR merged.

[wesleywiser]

I'm interested on working on this when @csmoe's changes land.

[saleemjaffer]

@oli-obk I would like to take this up.

[oli-obk]

Cool! So I think the changes necessary are

1. Remove the Promoted variant

   rust/src/librustc/mir/mod.rs

   Line 1914 in aa97448

   Promoted(Box<(Promoted, Ty<'tcx>)>),

2. Add an Option<Promoted> field to https://github.com/rust-lang/rust/blob/master/src/librustc/mir/mod.rs#L1920-L1923
3. Recompile and follow the fallout to always create a Static variant
   • Everywhere where we used to create Promoted, fill in the Option<Promoted> with Some
   • Everywhere where we used to create Static, use None
4. Merge the two arms in

   rust/src/librustc_mir/interpret/place.rs

   Line 586 in aa97448

   Base(PlaceBase::Promoted(ref promoted)) => {

   (this might be a little finicky, so maybe start out by just doing what the current code is doing by differentiating between the Some and the None case)

[saleemjaffer]

@oli-obk

rust/src/librustc_mir/transform/promote_consts.rs

Lines 289 to 298 in aa97448

	fn promote_candidate(mut self, candidate: Candidate) {
	let mut operand = {
	let promoted = &mut self.promoted;
	let promoted_id = Promoted::new(self.source.promoted.len());
	let mut promoted_place = |ty, span| {
	promoted.span = span;
	promoted.local_decls[RETURN_PLACE] =
	LocalDecl::new_return_place(ty, span);
	Place::Base(PlaceBase::Promoted(box (promoted_id, ty)))
	};

If i have to replace the Place::Base(PlaceBase::Promoted(box (promoted_id, ty))) with the PlaceBase::Static variant, what do I do for the def_id for PlaceBase::Static?

[oli-obk]

the DefId should be the one of the current function. So for now you'll need to give

rust/src/librustc_mir/transform/promote_consts.rs

Line 146 in aa97448

struct Promoter<'a, 'tcx: 'a> {

a DefId field that refers to the current function. Then you can access the field here