Cherry-pick "AK+LibJS: Performance improvements for resolving large rope strings" #25131
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This feature is unused in Ladybird and will complicate an upcoming patch to hand-off StringBuilder's memory to String. (cherry picked from commit af220af8bf15f9eb297a165e6d2ae3bf5bbc49fc)
Opening StringData.h in any clangd-enabled editor previously resulted in a sea of clangd errors. (cherry picked from commit 77eef8a8f6723b1e2fb5107aac7cf39bc1b40dad)
Currently, invoking StringBuilder::to_string will re-allocate the string data to construct the String. This is wasteful both in terms of memory and speed. The goal here is to simply hand the string buffer over to String, and let String take ownership of that buffer. To do this, StringBuilder must have the same memory layout as Detail::StringData. This layout is just the members of the StringData class followed by the string itself. So when a StringBuilder is created, we reserve sizeof(StringData) bytes at the front of the buffer. StringData can then construct itself into the buffer with placement new. Things to note: * StringData must now be aware of the actual capacity of its buffer, as that can be larger than the string size. * We must take care not to pass ownership of inlined string buffers, as these live on the stack. (cherry picked from commit 29879a69a4b2eda4f0315027cb1e86964d333221; amended minor conflict in AK/String.h due to us not having String::from_utf16() from LadybirdBrowser/ladybird#674, last commit)
For performance, rather than slowly incrementing the capacity of the rope string's buffer, compute an approximate length for that buffer to be reserved up front. (cherry picked from commit e8f4ae487d228dac491a446ed548400176331ae9)
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LadybirdBrowser/ladybird#724