stage 0 libtests are broken

[RalfJung]

The command ./x.py test --stage 0 src/libstd --no-doc used to be a good way to test libstd without having to build a full compiler. However, it is currently broken:

Testing std stage0 (x86_64-unknown-linux-gnu -> x86_64-unknown-linux-gnu)
   Compiling rand_core v0.2.1                                                                                                                                                                                      
   Compiling libc v0.2.43                                                                                                                                                                                          
   Compiling rand v0.5.5                                                                                                                                                                                           
   Compiling std v0.0.0 (/home/r/src/rust/rustc.2/src/libstd)                                                                                                                                                      
error: the #[global_allocator] in this crate conflicts with global allocator in: std                                                                                                                               
                                                                                                                                                                                                                   
error: aborting due to previous error                                                                                                                                                                              
                                                                                                                                                                                                                   
error: Could not compile `std`.                                                                                                                                                                                    

To learn more, run the command again with --verbose.


command did not execute successfully: "/home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-j" "8" "--release" "--features" "panic-unwind backtrace" "--manifest-path" "/home/r/src/rust/rustc.2/src/libstd/Cargo.toml" "--lib" "--bins" "--examples" "--tests" "--benches" "-p" "std" "--" "--quiet"
expected success, got: exit code: 101

Cc @alexcrichton

[RalfJung]

Now it seems like stage 1 doctests are broken as well...

   Doc-tests std
error: couldn't load codegen backend "/home/r/src/rust/rustc/build/x86_64-unknown-linux-gnu/stage1/lib/rustlib/x86_64-unknown-linux-gnu/codegen-backends/librustc_codegen_llvm-llvm.so": "librustc_codegen_ssa-93008c931eecc4cb.so: cannot open shared object file: No such file or directory"

Cc @eddyb @bjorn3 might be related to the codegen refactoring?

[eddyb]

@RalfJung I don't know why that would be a problem, but maybe rustbuild isn't copying the right files.

[bjorn3]

#56196 may have the same root cause.

[eddyb]

Can we try moving rustc_codegen_ssa to a dependency of rustc_driver?
Maybe it just doesn't work otherwise.
(Maybe librustc_driver.so makes all of its .so dependencies available to dynamic loading?)

[bjorn3]

Can we try moving rustc_codegen_ssa to a dependency of rustc_driver?

I did originally, but @Mark-Simulacrum wanted to avoid rebuilding the driver when cg_ssa changed.

Quoting part of #56048 (comment):

We can maybe move the specific dependency edge to src/rustc to avoid rebuilding driver (or shift rustbuild around to use some other "fake" root crate).

[eddyb]

@bjorn3 Yeah but I think it's just broken if it's not a dependency of rustc_driver.

[bjorn3]

I will make a PR to do it in a few minutes.

[petrochenkov]

Transient stage0 problem should not be tracked by GitHub issues generally.