bootstrap: stage 0/1 doctests fail because of missing codegen backend

[RalfJung]

To reproduce, make sure you have an entirely clean checkout -- I did rm -rf build/. Then run

./x.py --incremental --stage 0 test src/libcore

This will go on for a while and ultimately fail with

    Finished release [optimized] target(s) in 40.58s
     Running build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/coretests-c81f2af9b50a498c

running 81 tests
.................................................................................
test result: ok. 81 passed; 0 failed; 0 ignored; 0 measured; 795 filtered out

   Doc-tests core
error: failed to find a `codegen-backends` folder in the sysroot candidates:
* /home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage0-tools/x86_64-unknown-linux-gnu
* /home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage0-sysroot/lib/rustlib/x86_64-unknown-linux-gnu
* /home/r/src/rust/rustc.2/build/x86_64-unknown-linux-gnu/stage0-sysroot

error: test failed, to rerun pass '--doc'


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 jemalloc backtrace" "--manifest-path" "/home/r/src/rust/rustc.2/src/libstd/Cargo.toml" "-p" "core" "--" "slice" "--quiet"
expected success, got: exit code: 101

Notably, it doesn't seem to build LLVM at all -- which may be why it can't find a codegen-backend...

[RalfJung]

Turns out that ./x.py --incremental --stage 0 test src/libcore --no-doc works fine.

However, stage 0 tests with doctests still seem pretty much broken. Can these even reasonably work without having a "bootstrap rustdoc"?

[RalfJung]

Cc @Mark-Simulacrum

[RalfJung]

Note that it fails in stage 1 as well -- both of these fail:

./x.py --stage 0 test src/libcore

./x.py --stage 1 test src/libcore