auto merge of #15399 : kballard/rust/rewrite_local_data, r=alexcrichton
This was motivated by a desire to remove allocation in the common pattern of let old = key.replace(None) do_something(); key.replace(old); This also switched the map representation from a Vec to a TreeMap. A Vec may be reasonable if there's only a couple TLD keys, but a TreeMap provides better behavior as the number of keys increases. Like the Vec, this TreeMap implementation does not shrink the container when a value is removed. Unlike Vec, this TreeMap implementation cannot reuse an empty node for a different key. Therefore any key that has been inserted into the TLD at least once will continue to take up space in the Map until the task ends. The expectation is that the majority of keys that are inserted into TLD will be expected to have a value for most of the rest of the task's lifetime. If this assumption is wrong, there are two reasonable ways to fix this that could be implemented in the future: 1. Provide an API call to either remove a specific key from the TLD and destruct its node (e.g. `remove()`), or instead to explicitly clean up all currently-empty nodes in the map (e.g. `compact()`). This is simple, but requires the user to explicitly call it. 2. Keep track of the number of empty nodes in the map and when the map is mutated (via `replace()`), if the number of empty nodes passes some threshold, compact it automatically. Alternatively, whenever a new key is inserted that hasn't been used before, compact the map at that point. --- Benchmarks: I ran 3 benchmarks. tld_replace_none just replaces the tld key with None repeatedly. tld_replace_some replaces it with Some repeatedly. And tld_replace_none_some simulates the common behavior of replacing with None, then replacing with the previous value again (which was a Some). Old implementation: test tld_replace_none ... bench: 20 ns/iter (+/- 0) test tld_replace_none_some ... bench: 77 ns/iter (+/- 4) test tld_replace_some ... bench: 57 ns/iter (+/- 2) New implementation: test tld_replace_none ... bench: 11 ns/iter (+/- 0) test tld_replace_none_some ... bench: 23 ns/iter (+/- 0) test tld_replace_some ... bench: 12 ns/iter (+/- 0)
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