Nils had an excellent idea the other day: the same way that rustdoc is
able to load `rustc_driver` from the sysroot, ui-fulldeps tests should
also be able to load it from the sysroot. That allows us to run fulldeps
tests with stage1, without having to fully rebuild the compiler twice.
It does unfortunately have the downside that we're running the tests on
the *bootstrap* compiler, not the in-tree sources, but since most of the
fulldeps tests are for the *API* of the compiler, that seems ok.
I think it's possible to extend this to `run-make-fulldeps`, but I've
run out of energy for tonight.
- Move `plugin` tests into a subdirectory.
Plugins are loaded at runtime with `dlopen` and so require the ABI of
the running compile to match the ABI of the compiler linked with
`rustc_driver`. As a result they can't be supported in stage 1 and have
to use `// ignore-stage1`.
- Remove `ignore-stage1` from most non-plugin tests
- Ignore diagnostic tests in stage 1. Even though this requires a stage
2 build to load rustc_driver, it's primarily testing the error message
that the *running* compiler emits when the diagnostic struct is malformed.
- Pass `-Zforce-unstable-if-unmarked` in stage1, not just stage2. That
allows running `hash-stable-is-unstable` in stage1, since it now
suggests adding `rustc_private` to enable loading the crates.
- Add libLLVM.so to the stage0 target sysroot, to allow fulldeps tests
that act as custom drivers to load it at runtime.
- Pass `--sysroot stage0-sysroot` in compiletest so that we use the
correct version of std.
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