`./x.py doc --stage 0` fails: "crate `core` compiled by an incompatible version of rustc"

[QuietMisdreavus]

From a clone i'd just run ./x.py clean on, simply running ./x.py doc --stage 0 builds all of rustc and all the tools, then when it tries to document std_unicode (the first thing after core) it fails with the following:

Documenting stage0 std (x86_64-unknown-linux-gnu)
 Documenting core v0.0.0 (file:///home/misdreavus/git/rust/src/libcore)
 Documenting std_unicode v0.0.0 (file:///home/misdreavus/git/rust/src/libstd_unicode)
error[E0514]: found crate `core` compiled by an incompatible version of rustc
  |
  = help: please recompile that crate using this compiler (rustc 1.21.0-dev (5cc1baa29 2017-07-27))
  = note: crate `core` path #1: /home/misdreavus/git/rust/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/libcore-c40a1e0952d5bad5.rlib compiled by "rustc 1.20.0-beta.1 (e93aa3aa8 2017-07-18)"

error: aborting due to previous error

error: Could not document `std_unicode`.

Caused by:
  process didn't exit successfully: `/home/misdreavus/git/rust/build/bootstrap/debug/rustdoc --crate-name std_unicode src/libstd_unicode/lib.rs --target x86_64-unknown-linux-gnu -o /home/misdreavus/git/rust/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/doc -L dependency=/home/misdreavus/git/rust/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps -L dependency=/home/misdreavus/git/rust/build/x86_64-unknown-linux-gnu/stage0-std/release/deps --extern core=/home/misdreavus/git/rust/build/x86_64-unknown-linux-gnu/stage0-std/x86_64-unknown-linux-gnu/release/deps/libcore-c40a1e0952d5bad5.rlib` (exit code: 101)

command did not execute successfully: "/home/misdreavus/git/rust/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "doc" "-j" "1" "--target" "x86_64-unknown-linux-gnu" "--release" "--features" "panic-unwind jemalloc backtrace" "--manifest-path" "/home/misdreavus/git/rust/src/libstd/Cargo.toml" "--no-deps" "-p" "alloc" "-p" "collections" "-p" "core" "-p" "std" "-p" "std_unicode"
expected success, got: exit code: 101

My HEAD is commit 5cc1baa.

cc @Mark-Simulacrum

[est31]

Maybe #39751 ?

[kennytm]

Well #43482 does say "./x.py doc --stage 0 will now fail".

[Mark-Simulacrum]

Yeah, I asked @QuietMisdreavus to file this so I could keep track of it. We should at least provide a better error message here, though I'm hopeful it can be made to work.

[Mark-Simulacrum]

Closing. I don't think this will be possible to make work seamlessly currently, and isn't something worth tracking.