[libc++] Use `[[no_unique_address]]` in place of `__compressed_pair` to reduce compile time and debug info size

[rnk]

As covered in crbug.com/338094922 , __compressed_pair is a major contributor to debug info size. Many libc++ classes use them to store allocators, default deleters, and other things. I believe that no_unique_address was intended to be layout-compatible with the __compressed_pair technique, but I'm not sure it actually is.

Regardless, for unstable ABI users, it would be a major QoI improvement to avoid extra compressed pair template instantiations.

I thought this feature request had already been filed, but I couldn't find it.

Related work:

• https://reviews.llvm.org/D153991
• Implement [[msvc::no_unique_address]] #65675
• Missing [[no_unique_address]] support on Windows #49358
• [libc++] no_unique_address is reserved word in pre-C++20 dialects (non-conforming) #61196

Can we do this yet?

cc @zmodem @cjdb @amykhuang @zygoloid

[philnik777]

See #76756

[dwblaikie]

@Michael137 My understanding was this work was waiting on lldb support for [[no_unique_adress]] - is that correct? How's that work coming along since our last discussion?

[Michael137]

@Michael137 My understanding was this work was waiting on lldb support for [[no_unique_adress]] - is that correct? How's that work coming along since our last discussion?

Yup it's on my list of things to do this week. Got sidetracked with other issues and was OOO for a bit. Will update this thread once I have the PR up