rustc --print cfg prints features that are invalid for `-Ctarget-feature`

[djc]

I have been investigating how to optimize my binaries for the least common denominator across a number of different CPU types (that is, something between the default CPU and target-cpu=native). In the linked thread, someone pointed me at rustc --print cfg and I found some relevant documentation in the SIMD docs. However, I found that the output of rustc --target x86_64-unknown-linux-gnu --print target-features and rustc --print cfg --target x86_64-unknown-linux-gnu are inconsistent, and the latter outputs features that don't actually work with -Ctarget-feature. So far, the ones that I noticed are:

• bmi (target-feature) vs bmi1 (cfg)
• pclmul vs pclmulqdq
• rdrnd vs rdrand

Should the --print cfg command be updated to print output that's consistent with the target-feature list?

djc-2019 cpu-features rust $ rustc -vV                                                      
rustc 1.49.0 (e1884a8e3 2020-12-29)
binary: rustc
commit-hash: e1884a8e3c3e813aada8254edfa120e85bf5ffca
commit-date: 2020-12-29
host: x86_64-apple-darwin
release: 1.49.0

[nagisa]

-Ctarget-feature accepts rust feature names as of #84991

[djc]

@nagisa the other part of this issue is getting --print target-features aligned with --print cfg, which as far as I can tell is not affected by #84991. Should we keep this issue open until that part is addressed as well?