Nonzeroing Move Hints (take3 branch) #26173

…oCopy.

…hInput` rather than `ValueRef`. (already thumbs-upped pre-rebase by nikomatsakis) The refactoring here is trivial because `trans::datum::Lvalue` currently carries no payload. However, future commits will start adding a payload to `Lvalue`, and thus will force us either 1. to thread the payload through the `_match` code (a long term goal), or 2. to ensure the payload has some reasonable default value.

(already thumbs-upped pre-rebase by nikomatsakis)

…hint. Instrumented calls sites that construct Lvalues to ease tracking down cases that we might need to change whether or not they carry a hint. Note that this commit does not do anything to actually *construct* the `lldropflag_hints` map, nor does it change anything about codegen itself. Those parts are in follow-on commits.

…rans. Added code to maintain these hints at runtime, and to conditionalize drop-filling and calls to destructors. In this early stage, we are using hints, so we are always free to leave out a flag for a path -- then we just pass `None` as the dropflag hint in the corresponding schedule cleanup call. But, once a path has a hint, we must at least maintain it: i.e. if the hint exists, we must ensure it is never set to "moved" if the data in question might actually have been initialized. It remains sound to conservatively set the hint to "initialized" as long as the true drop-flag embedded in the value itself is up-to-date. ---- Here are some high-level details I want to point out: * We maintain the hint in Lvalue::post_store, marking the lvalue as moved. (But also continue drop-filling if necessary.) * We update the hint on ExprAssign. * We pass along the hint in once closures that capture-by-move. * You only call `drop_ty` for state that does not have an associated hint. If you have a hint, you must call `drop_ty_core` instead. (Originally I passed the hint into `drop_ty` as well, to make the connection to a hint more apparent, but the vast majority of current calls to `drop_ty` are in contexts where no hint is available, so it just seemed like noise in the resulting diff.)

Testing indicates bug would have been caught, albeit later than one might hope, during `sync::mpsc::tests::smoke_shared_port_gone2` test.

…lling drop is in use.

Updated all call sites that used the other contructors to uniformly call `Lvalue::new_with_hint`, passing along the appropriate kind of hint for each context. Placated tidy in a few other places in datum.rs.

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Nonzeroing Move Hints (take3 branch) #26173

Nonzeroing Move Hints (take3 branch) #26173

Commits on Jul 28, 2015