1.18 doesn't build with 1.17

[kyrias]

Trying to build 1.18.0 with the Arch Linux 1.17.0 package results in the following error:

info: looks like you are running this command under `sudo`
      and so in order to preserve your $HOME this will now
      use vendored sources by default. Note that if this
      does not work you should run a normal build first
      before running a command like `sudo make install`
   Compiling rustc-serialize v0.3.23
   Compiling libc v0.2.21
   Compiling getopts v0.2.14
   Compiling gcc v0.3.45
   Compiling filetime v0.1.10
   Compiling num_cpus v0.2.13
   Compiling build_helper v0.1.0 (file:///build/rust/src/rustc-1.18.0-src/src/build_helper)
   Compiling cmake v0.1.22
   Compiling toml v0.1.30
   Compiling bootstrap v0.0.0 (file:///build/rust/src/rustc-1.18.0-src/src/bootstrap)
    Finished dev [unoptimized] target(s) in 13.49 secs


failed to execute command: "/build/rust/src/rustc-1.18.0-src/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "metadata" "--manifest-path" "/build/rust/src/rustc-1.18.0-src/src/libstd/Cargo.toml"
error: No such file or directory (os error 2)


Build completed unsuccessfully in 0:00:13

(It's built in a clean chroot.)

[leahneukirchen]

Same on Void Linux, cargo is in $PATH and set in rustc-1.18.0-src/config.toml. This used to work fine before.

[steveklabnik]

tagging with both infra and compiler because i don't know whose fault it is here.

[leahneukirchen]

src/bootstrap/bootstrap.py reads config.toml to find cargo, but src/bootstrap/lib.rs doesn't seem to... it only finds cargo in CFG_LOCAL_RUST_ROOT. Passing --enable-local-rust --local-rust-root=.../stage0 (both are needed!) adds the setting to config.mk, but then bootstrap doesn't seem to load it.

[aclemons]

Same on Slackware linux. Our build was using x.py build --verbose. Removing --verbose allows the build to succeed. Perhaps that helps to track down the root cause.

[leahneukirchen]

FWIW, we currently use this hack to make it use the cargo from $PATH, which then works fine: https://github.com/voidlinux/void-packages/blob/master/srcpkgs/rust/patches/cargo-hack.patch

[Mark-Simulacrum]

I suspect at least some of the problem here is from #41779, which will hopefully be fixed soon. I plan to look into it more today, though I wasn't able to reproduce yet.

[kyrias]

1.18 doesn't build using 1.17 with #42695 applied either.