`target-cpu=native` is broken on latest nightly

[robsmith11]

Reproduce by creating a crate with cargo init and .cargo/config set to

[build]
rustflags = ["-C", "target-cpu=native"]

It produces the following errors:

tmp/test1 $ cargo build --release                                                                            
Compiling test1 v0.1.0 (file:///tmp/test1)                                                              
'native' is not a recognized processor for this target (ignoring processor)                                
'native' is not a recognized processor for this target (ignoring processor)                                
'native' is not a recognized processor for this target (ignoring processor)                                
'native' is not a recognized processor for this target (ignoring processor)
'native' is not a recognized processor for this target (ignoring processor)
'native' is not a recognized processor for this target (ignoring processor)                                
'native' is not a recognized processor for this target (ignoring processor)                                
'native' is not a recognized processor for this target (ignoring processor)                                    
Finished release [optimized] target(s) in 0.25s

It was working fine on nightly-2018-08-06-x86_64-unknown-linux-gnu, but broke with
nightly-2018-08-14-x86_64-unknown-linux-gnu (rustup doesn't show any nightlies available between those dates.

Hardware is Skylake Xeon and correctly detected by rustc --print:

$ rustc --print target-cpus
Available CPUs for this target:
    native         - Select the CPU of the current host (currently skylake).
...

[TheIronBorn]

I'm seeing this with rustc 1.30.0-nightly (d5a448b3f 2018-08-13), host: x86_64-apple-darwin.

$ rustc --print target-cpus
Available CPUs for this target:
    native         - Select the CPU of the current host (currently sandybridge).

[MSxDOS]

host: x86_64-pc-windows-msvc; nehalem 😄

[sanmai-NL]

rustc 1.30.0-nightly (d767ee116 2018-08-15)
binary: rustc
commit-hash: d767ee11616390d128853a06f5addb619e79213f
commit-date: 2018-08-15
host: x86_64-unknown-linux-gnu
release: 1.30.0-nightly
LLVM version: 7.0

Available CPUs for this target:
    native         - Select the CPU of the current host (currently skylake)

[pravic]

And..

What happened and why? Is it a broken LLVM version or what?

[pthariensflame]

Problem still present with rustc 1.30.0-nightly (33b923fd4 2018-08-18) and:

$ rustc --print cfg
debug_assertions
target_arch="x86_64"
target_endian="little"
target_env=""
target_family="unix"
target_feature="fxsr"
target_feature="mmx"
target_feature="sse"
target_feature="sse2"
target_feature="sse3"
target_feature="ssse3"
target_has_atomic="128"
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="cas"
target_has_atomic="ptr"
target_os="macos"
target_pointer_width="64"
target_thread_local
target_vendor="apple"
unix
$ rustc --print cfg -C target-cpu=native -C opt-level=3
target_arch="x86_64"
target_endian="little"
target_env=""
target_family="unix"
target_feature="aes"
target_feature="avx"
target_feature="avx2"
target_feature="bmi1"
target_feature="bmi2"
target_feature="fma"
target_feature="fxsr"
target_feature="lzcnt"
target_feature="mmx"
target_feature="pclmulqdq"
target_feature="popcnt"
target_feature="rdrand"
target_feature="rdseed"
target_feature="sse"
target_feature="sse2"
target_feature="sse3"
target_feature="sse4.1"
target_feature="sse4.2"
target_feature="ssse3"
target_feature="xsave"
target_feature="xsavec"
target_feature="xsaveopt"
target_feature="xsaves"
target_has_atomic="128"
target_has_atomic="16"
target_has_atomic="32"
target_has_atomic="64"
target_has_atomic="8"
target_has_atomic="cas"
target_has_atomic="ptr"
target_os="macos"
target_pointer_width="64"
target_thread_local
target_vendor="apple"
unix

This is especially weird as it accepts it for printing the configuration, but then doesn't when actually compiling. 😫

[TheIronBorn]

Still seeing this with nightly-x86_64-apple-darwin rustc 1.30.0-nightly (33b923fd4 2018-08-18) (sandybridge)

[nikomatsakis]

cc @nagisa @alexcrichton -- anybody have any ideas what's going on here? Could this be related to an LLVM upgrade or something?

[nikomatsakis]

Marking as P-high for now. This is a regression, but on the other hand, if the message is coming from LLVM, there may not be a lot we can do here? Would be nice to get some more insight into what might be happening here.