[MIR] Cache drops for early scope exits

[nagisa]

Previously we would rebuild all drops on every early exit from a scope, which for code like:

match x {
    A => return 1,
    B => return 2,
    ...
    C => return 27
}

would produce 27 exactly same chains of drops for each return, basically a O(n*m) explosion. This is such a case for a match on 80-variant enum with 3 droppable variables in scope.

For ::core::iter::Iterator::partial_cmp the CFG looked like this (after initial SimplifyCfg). With this patch the CFG looks like this instead.

Some numbers (overall very small wins, however neither of the crates have many cases which abuse this corner case):

	old time	old rss	new time	new rss
core dump	0.879	224MB	0.871	223MB
core MIR passes	0.759	224MB	0.718	223MB
core MIR codegen passes	1.762	230MB	1.442	228MB
core trans	3.263	279MB	3.116	278MB
core llvm passes	5.611	263MB	5.565	263MB
std dump	0.487	190MB	0.475	192MB
std MIR passes	0.311	190MB	0.288	192MB
std MIR codegen passes	0.753	195MB	0.720	197MB
std trans	2.589	287MB	2.523	287MB
std llvm passes	7.268	245MB	7.447	246MB

[rust-highfive]

r? @arielb1

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

[nagisa]

cc @eddyb (the patch you wanted to see)

[eddyb]

Are the fancy quotes intentional here?

[nagisa]

Yes. These are preferred characters for quotation characters in english prose.

[arielb1]

@bors r+

[bors]

📌 Commit 04d63cc has been approved by arielb1