This is a tracking issue for the unstable placement_new_protocol feature in the standard library, and placement_in_syntax/box_syntax in the compiler.

(@pnkfelix adds below:)

Things to decide / finalize before stabilization:

• placement-in syntax, e.g. in PLACE { BLOCK } vs PLACE <- EXPR. (See Place left arrow syntax (place <- expr) rfcs#1228 )
• protocol interface, e.g. passing &mut self vs self for the Placer::make_place (Placement protocol should not consume Placer rfcs#1286).
• Is a desugaring box EXPR part of this? (currently the desugaring doesn't work due to type inference issues).
• Factor a common Place for InPlace and BoxPlace, or just have the InPlace trait independently from any BoxPlace.