This hot function dominates the flamegraphs for the completions thread, and any
optimizations are worthwhile.
A variety of different approaches were tested and benchmarked against real-world
fish-history file inputs and this is the one that won out across all rustc
target-cpu variations tried.
Benchmarks and code at https://github.com/mqudsi/fish-yaml-unescape-benchmark
It's reasonable since this is only checking to see that the history file
contains the expected format and if it's corrupted but we at least got what we
expect to be the correct key/value pairs, then that's all we can do.
Of course the real motivation is to speed up this very hot function in any way
possible!
On the completions and history thread, the parent function
HistoryFileContents::decode_item() is responsible for ~60% of the CPU time, and
extract_prefix_and_unescape_yaml() alone comprising 14% (of the total).
This change removes allocations in the event that the history item is either
fully or partially plain yaml with no escapes to begin with, and brings down the
execution time of this function to only 7% of the total execution time.
The bulk of the remaining time is spent in wcs2string(), which is called
unconditionally and is naturally alloc-heavy.
This is a step towards converting `wopen_cloexec()` to return `File` instead of
`OwnedFd`/`AutocloseFd`.¹
In addition to letting us use native standard library functions instead of
unsafe libc calls, we gain additional semantic safety because `File` operations
that manipulate the state of the fd (e.g. `File::seek()`) require a `&mut`
reference to the `File`, whereas using `RawFd` or `OwnedFd` everywhere leaves us
in a position where it's not clear whether or not other references to the same
fd will manipulate its underlying state.
¹ We actually wouldn't even need `wopen_cloexec()` at all (just a widechar
wrapper) as Rust's native `File::open()`/`File::create()` functionality uses
`FD_CLOEXEC` internally.
