Improve allocation of escape_into

[hhirtz]

In regex-syntax,
Make escape_into allocate at most twice (all characters are escaped)
instead of more if used on an unallocated String. This way, escape
doesn't have to allocate.

[hhirtz]

Sorry for the second push, added syntax: to the commit title.

[BurntSushi]

LGTM

[pickfire]

@hhirtz doesn't it still needs to be allocated later from escape_into since escape uses it, why should we allocate twice?

[hhirtz]

hi @pickfire, I don't really understand the first part of your question, but the patch here made escape_into performs less allocations on average.

For example, if you called escape_into with String::new, you'd likely end up doing more allocations than necessary, because String allocates on push when there's not enough space. There's an example here: https://doc.rust-lang.org/std/string/struct.String.html#representation.

Since s = String::with_capacity(cap) is semantically the same as s = String::new(); s.reserve(cap), I removed the with_capacity in escape.

Also escape_into(text, &mut buf) writes between text.len() (no character needs an escape) and text.len() * 2 (all characters are 1-byte codepoints, and they all need to be escaped*) characters in buf. Here it reserves text.len() bytes in buf because we know it needs at least text.len() more bytes in buf.

*Though I don't know any regex meta character that uses more than 1 byte in UTF-8.

[pickfire]

@hhirtz Ah, thanks for the explaination, now I understand it. By the way, if we count first for exactly how many allocations we need, will allocating / reserving once be faster than potentially allocating multiple times?

[hhirtz]

Yes, because in most cases reallocation is performed by allocating a larger buffer and copying the previous one into it. If you know how much you want to push, it's better to call reserve before.

[BurntSushi]

will allocating / reserving once be faster than potentially allocating multiple times?

Generally, yes. This is generally referred to as "amortizing allocation" and it greatly influences the structure of a lot of performance critical code. The entire architecture of the regex crate is designed around doing just that. :-)