Don't store locals that have been moved from in generators
This avoids reserving storage in generators for locals that are moved
out of (and not re-initialized) prior to yield points. Fixes#59123.
This adds a new dataflow analysis, `RequiresStorage`, to determine whether the storage of a local can be destroyed without being observed by the program. The rules are:
1. StorageLive(x) => mark x live
2. StorageDead(x) => mark x dead
3. If a local is moved from, _and has never had its address taken_, mark it dead
4. If (any part of) a local is initialized, mark it live'
This is used to determine whether to save a local in the generator object at all, as well as which locals can be overlapped in the generator layout.
Here's the size in bytes of all testcases included in the change, before and after the change:
async fn test |Size before |Size after
-----------------|------------|----------
single | 1028 | 1028
single_with_noop | 2056 | 1032
joined | 5132 | 3084
joined_with_noop | 8208 | 3084
generator test |Size before |Size after
----------------------------|------------|----------
move_before_yield | 1028 | 1028
move_before_yield_with_noop | 2056 | 1032
overlap_move_points | 3080 | 2056
## Future work
Note that there is a possible extension to this optimization, which modifies rule 3 to read: "If a local is moved from, _**and either has never had its address taken, or is Freeze and has never been mutably borrowed**_, mark it dead." This was discussed at length in #59123 and then #61849. Because this would cause some behavior to be UB which was not UB before, it's a step that needs to be taken carefully.
A more immediate priority for me is inlining `std::mem::size_of_val(&x)` so it becomes apparent that the address of `x` is not taken. This way, using `size_of_val` to look at the size of your inner futures does not affect the size of your outer future.
cc @cramertj @eddyb @Matthias247 @nikomatsakis @RalfJung @Zoxc
