Repository: rust
Issue: Cargo can't find std crates using local-rust-root on macOS.
Labels: O-macos,T-bootstrap,requires-custom-config

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**Problem**
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Cargo won't find std or core crates if I try to bootstrap using the following command:

```
cd /opt/mxe/tmp-rustc-x86_64-apple-darwin18.7.0//rustc-1.36.0-src && ./configure --prefix=/opt/mxe/usr --enable-vendor --default-linker=gcc --disable-codegen-tests --disable-docs --release-channel=stable --llvm-root=/opt/mxe/usr/libexec/llvm-8.0.0 --build=x86_64-apple-darwin --local-rust-root=/opt/mxe/tmp-rust-stage0-x86_64-apple-darwin --set=target.x86_64-apple-darwin.cc=gcc --set=target.x86_64-apple-darwin.cxx=g++ --set=target.x86_64-apple-darwin.linker=gcc --set=build.python=python2.7

configure: processing command line
configure: 
configure: rust.default-linker  := gcc
configure: build.vendor         := True
configure: rust.channel         := stable
configure: build.docs           := False
configure: rust.codegen-tests   := False
configure: install.prefix       := /opt/mxe/usr
configure: target.x86_64-apple-darwin.cc := gcc
configure: target.x86_64-apple-darwin.cxx := g++
configure: target.x86_64-apple-darwin.linker := gcc
configure: build.python         := python2.7
configure: build.build          := x86_64-apple-darwin
configure: build.rustc          := /opt/mxe/tmp-rust-stage0-x86_64-apple-darwin/b ...
configure: build.cargo          := /opt/mxe/tmp-rust-stage0-x86_64-apple-darwin/b ...
configure: target.x86_64-apple-darwin.llvm-config := /opt/mxe/usr/libexec/llvm-8. ...
configure: build.configure-args := ['--prefix=/opt/mxe/usr', '--enable-vendor', ' ...
configure: 
configure: writing `config.toml` in current directory
configure: 
configure: run `python /opt/mxe/tmp-rustc-x86_64-apple-darwin18.7.0/rustc-1.36.0-src/x.py --help`
configure: 

cd /opt/mxe/tmp-rustc-x86_64-apple-darwin18.7.0//rustc-1.36.0-src && /opt/Xcode.app/Contents/Developer/usr/bin/make -j6 -w rustc-stage1 VERBOSE=1 BOOTSTRAP_ARGS="-v -j6" 
```

I don't run into this issue if I add `--enable-local-rust`, but then something else happens: configure.py overrides `build.rustc` like so:

```
cd /opt/mxe/tmp-rustc-x86_64-apple-darwin18.7.0//rustc-1.36.0-src && ./configure --prefix=/opt/mxe/usr --enable-vendor --default-linker=gcc --disable-codegen-tests --disable-docs --release-channel=stable --llvm-root=/opt/mxe/usr/libexec/llvm-8.0.0 --build=x86_64-apple-darwin --enable-local-rust --set=target.x86_64-apple-darwin.cc=gcc --set=target.x86_64-apple-darwin.cxx=g++ --set=target.x86_64-apple-darwin.linker=gcc --set=build.python=python2.7 --local-rust-root=/opt/mxe/tmp-rust-stage0-x86_64-apple-darwin
configure: processing command line
configure: 
configure: rust.default-linker  := gcc
configure: build.vendor         := True
configure: rust.channel         := stable
configure: build.docs           := False
configure: rust.codegen-tests   := False
configure: install.prefix       := /opt/mxe/usr
configure: target.x86_64-apple-darwin.cc := gcc
configure: target.x86_64-apple-darwin.cxx := g++
configure: target.x86_64-apple-darwin.linker := gcc
configure: build.python         := python2.7
configure: build.build          := x86_64-apple-darwin
configure: build.rustc          := /opt/mxe/tmp-rust-stage0-x86_64-apple-darwin/b ...
configure: build.cargo          := /opt/mxe/tmp-rust-stage0-x86_64-apple-darwin/b ...
configure: target.x86_64-apple-darwin.llvm-config := /opt/mxe/usr/libexec/llvm-8. ...
configure: build.rustc          := /opt/local/bin/rustc
configure: build.configure-args := ['--prefix=/opt/mxe/usr', '--enable-vendor', ' ...
configure: 
configure: writing `config.toml` in current directory
configure: 
configure: run `python /opt/mxe/tmp-rustc-x86_64-apple-darwin18.7.0/rustc-1.36.0-src/x.py --help`
configure: 
```
The second bit is evidently not related to cargo itself I guess but it's a bit puzzling. Anyway, why won't cargo find the crates in the local-rust-root unless I specifically omit `--enable-local-rust`?


**Notes**

Output of `cargo version`: 0.37.0

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