[llvm] [IR] Introduce !captures metadata (PR #160913)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> The motivating use case for this is Rust's println!() machinery, which involves storing a reference to the value inside a structure. This means that printing code (including conditional debugging code), can inhibit optimizations because the pointer escapes. With the new metadata we can annotate this as a read-only capture, which has less impact on optimizations.

https://godbolt.org/z/T1479EG43

I am curious about how we annotate this in Rust. Do we provide an intrinsic like `declare ptr @foo(ptr returned captures(address, read_provenance))` in Rust? cc @RalfJung @scottmcm

Anyway, from the LLVM side, I think it is profitable as we can salvage the capture information after inlining.


https://github.com/llvm/llvm-project/pull/160913