Problem

PGO works in two phases:

1. Compile an instrumented binary of your program that enables collecting profile data. This is done via RUSTFLAGS=-Cprofile-generate.
2. Compile an optimized binary that makes use of the profile data collected in step (1). This is done via RUSTFLAGS=-Cprofile-use.

In order for this to work, the symbol names within the two program versions must match up because LLVM generates profiling data in terms of symbol names.

However, since #6503, RUSTFLAGS are fed into the -Cmetadata argument to rustc. This causes symbol names to differ because one version has -Cprofile-generate and the other has -Cprofile-use in their RUSTFLAGS.

I think I would prefer not feeding RUSTFLAGS into -C metadata at all. It does not seem right to me that symbol names are affected by random command line arguments to the compiler.