Optimize PCRE for StaticStrings

[paulbiss]

When we statically know the regex string passed to PCRE we can avoid a cache lookup on each call by burning the handle for the compiled regex into the TC.

[wjywbs]

Is it the right path to add a service request to compile the static string regex and write the handle in TC for the first time? The pcre caches for these regex can't be freed.

[paulbiss]

Well, it probably wouldn't be a service request, but the idea behind doing this with static strings is that they live forever anyway so it should be safe to burn the pcre handle in.

[lexidor]

HH\Lib\Regex\Pattern<T> literals are easy to spot in code.

re"/ab?cdc/"

In an ideal world, these would be compiled once and only once when this file is loaded (or even as a repo auth step, provided pcre handles can be serialized to disk). Hack can currently observe that re"" strings are just strings. A opaque value would be in order if changing this to a pcre handle expression.