This constructor is a great pitfall, because it's very tempting to write code like this: int len = ...; ... nsCAutoString str(len); SomeMethod(str.BeginWriting()); str(len) doesn't even show a warning, which I don't quite understand, since it's a narrower type. Would be nice to run an analysis to see where the (char) form is used and if we can just get rid of them, then just get rid of this constructor -- or perhaps make it do the more useful thing (capacity len, length 0).
I just tried commenting this ctor out, as well as making it private, and both compile fine on debug Linux. Want to just do a tryserver run with it private or removed, and if that passes push it?
It's used by xpcom/tests/TestTArray.cpp but that appears to only be included in the Makefile in a disable libxul build.
So if it's not actually used -- what about actually making it do the more useful thing, making it equivalent to str.SetCapacity(len) ? Though you'd still need to remember to call SetLength after writing to it, so maybe just getting rid of it is best. I'll make a patch and fix up TestTArray.cpp.
Created attachment 574912 [details] [diff] [review] Patch v1 Removing the culprit ctor. I didn't get any failure so I am assuming TestTArray.cpp file has changed. https://tbpl.mozilla.org/?tree=Try&rev=2d768e29de54
Comment on attachment 574912 [details] [diff] [review] Patch v1 r=dbaron