add overload add(a: var string, b: openArray[char])

[timotheecour]

2 goals:

• it's currently hard to convert openArray[char] to a string (unless you do it manually, essentially inlining the code in this PR)
• avoid allocations when possible if user already has a y openArray to begin with

future work

• after merged, fix this comment:
  https://github.com/nim-lang/Nim/pull/15421/files#r522703485
• see also addCstringN
• see add overload add(a: var string, b: openArray[char]) #15951 (comment) (see also follow up #15951 #17478)

[metagn]

While the intention here is good there are so many things in the stdlib that could benefit from using a more generic char sequence type (like in strutils) and openarray[char] is not the best candidate, lest we snowball into doing this everywhere. This change is however small and mirrors add(var seq[T], openarray[T]), so it's fine, except that that overload doesn't copy memory directly and uses move/sink instead

[juancarlospaco]

A snowball has to start somewhere... 😼

[timotheecour]

openarray[char] is not the best candidate

string/seq/array implicitly converts to openArray[char], which is the main point; I'm not sure what your other "best candidate" is (apart from std/views see #14869 which has the advantage of being 1st class but doesn't implicitly convert from string/seq/array; only explicitly)

mirrors add(var seq[T], openarray[T]), so it's fine, except that that overload doesn't copy memory directly and uses move/sink instead

for openArray[char] (this PR) memcpy (or memmove if memory overlaps) is the correct thing to do; for the other existing overload with seq[T], we could/should do the same depending on T

[Araq]

Can we add this to the new strbasics instead?

[timotheecour]

Can we add this to the new strbasics instead?

done
@Araq PTAL

I'll followup with a PR to address this:

xxx use nimCopyMem(x[n].addr, y[0].addr, y.len) after some refactoring

with some refactoring from compiler/strutils2