Use LazyBTreeMap for region constraints.

[nnethercote]

Because a lot of these BTreeMaps don't get anything inserted into them.

In the most extreme case, this halves the number of total bytes
allocated for a debug build of tuple-stress, from 2.2GB to 1.1GB. That
reduction in turn speeds up runs of many benchmarks, the best by 3%.

[rust-highfive]

r? @estebank

(rust_highfive has picked a reviewer for you, use r? to override)

[nnethercote]

Here are the benchmarks that had a run sped up by 1% or more.

tuple-stress-check
	avg: -2.2%	min: -3.3%	max: 0.0%
tuple-stress
	avg: -1.6%	min: -2.5%	max: -0.2%
deep-vector-check
	avg: -1.5%	min: -2.1%	max: 0.0%
coercions-check
	avg: -1.0%	min: -2.0%	max: 0.0%
tuple-stress-opt
	avg: -1.5%	min: -1.9%	max: -0.4%
coercions
	avg: -0.9%	min: -1.5%	max: -0.4%
syn-check
	avg: -0.7%	min: -1.5%	max: -0.1%
deep-vector
	avg: -1.0%	min: -1.5%	max: -0.0%
serde-opt
	avg: -0.6%	min: -1.3%	max: -0.0%
deep-vector-opt
	avg: -0.7%	min: -1.2%	max: -0.0%
coercions-opt
	avg: -0.8%	min: -1.2%	max: -0.2%
regex-check
	avg: -0.4%	min: -1.2%	max: -0.1%
piston-image-check
	avg: -0.8%	min: -1.2%	max: -0.1%
hyper-check
	avg: -0.6%	min: -1.2%	max: -0.1%
encoding-check
	avg: -0.6%	min: -1.2%	max: -0.1%
serde-check
	avg: -0.7%	min: -1.2%	max: -0.1%
html5ever-check
	avg: -0.7%	min: -1.1%	max: -0.2%
style-servo-check
	avg: -0.7%	min: -1.1%	max: -0.1%
encoding
	avg: -0.5%	min: -1.0%	max: -0.1%
serde
	avg: -0.5%	min: -1.0%	max: -0.3%

[michaelwoerister]

Could this even be an FxHashMap?

[nnethercote]

If ConstraintGraph::map is changed to an FxHashMap, then all the other LazyBTreeMap instances in this patch have to change to an FxHashMap as well. Maybe that's what you're suggesting? The various BTreeMaps are iterated over in multiple places. I don't know if the iteration order is important, but I'm inclined to be conservative.

[michaelwoerister]

Yes, Constraint just seems to have a derived Ord implementation, so the order might not be important. I don't know this code though.

[Gankra]

We should possibly close this on the basis that the proper BTreeMap fix is ready and just needs to be profiled?

[nnethercote]

Fixed in a better way by #50352.