swift

969d776c - [allocbox-to-stack] When we promote a heap -> stack of noncopyable type, move mark_must_check onto alloc_stack and always

[allocbox-to-stack] When we promote a heap -> stack of noncopyable type, move mark_must_check onto alloc_stack and always ensure that we use consumable_to_assign. What this patch is does is add an extra phase after alloc_box runs where we look at uses of the alloc_stack and if we see any mark_must_check of any kind, we delete them and rewrite a single mark_must_check [consumable_and_assignable] on the alloc_stack and make all uses of the alloc_stack go through the mark_must_check. This has two effects: 1. In a subsequent PR when I add noncopyable semantics for escaping closures, this will cause allocbox to stack to convert such boxes from having escaping semantics to having non-escaping semantics. Escaping semantics means that we always reproject out from the box and use mark_must_check [assignable_but_not_consumable] (since we can't consume from the box, but can assign to it). In contrast, non-escaping semantics means that the box becomes an alloc_stack and we use the traditional var checker semantics. NOTE: We can do this for lets represented as addresses and vars since the typechecker will validate that the let is never actually written to even if at the SIL level we would allow that. 2. In cases where we are implementing simple mark_must_check [consumable_and_assignable] on one of the project_box and capture the box, we used to have a problem where the direct box uses would be on the alloc_stack and not go through the mark_must_check. Now, all uses after allocbox_to_stack occur go through the mark_must_check. This is why I was able to remove instances of the "compiler does not understand this pattern" errors... since the compiler with this change can now understand them.