Copy stage0 binaries into stage0-sysroot

[jyn514]

Currently, the beta binaries are only present in build/stage0/bin; build/stage0-sysroot has only the standard library, not any binaries. That causes two issues:

1. rustup toolchain link doesn't work with stage0-sysroot, meaning there's no way to change the standard library and use those changes at runtime without doing a full stage 1 build
2. x dist --stage 0 doesn't work. This is very rare to want to do, but can be useful when modifying the dist process itself, to avoid long compile times.

thread 'main' panicked at 'could not read dir "/home/dawn/projects/rust/build/x86_64-unknown-linux-gnu/stage0-sysroot/bin": Os { code: 2, kind: NotFound, message: "No such file or directory" }', lib.rs:1573:25

Copying the binaries into stage0-sysroot is an easy way to fix both problems at once.

Mentoring instructions: Add a builder.copy call in impl Step for Sysroot

rust/src/bootstrap/compile.rs

Line 1146 in 6f0c4a6

INTERNER.intern_path(sysroot)

which copies the whole bin directory from build/stage0.

[jyn514]

We should also consider renaming stage0-sysroot to something else; the name is very confusing.

[chenyukang]

@rustbot claim

[bjorn3]

We should also consider renaming stage0-sysroot to something else; the name is very confusing.

The name is correct though. It is a directory you can pass in as --sysroot argument to rustc.

[jyn514]

@bjorn3 so is stage0. I'm not saying the name is wrong, just that it's ambiguous and not intuitive.

Honestly I would rather rename all the stageN directories to stageN-sysroot and change stage0-sysroot to stage0-sysroot-with-libstd or something.

[bjorn3]

stage0-sysroot-with-libstd is confusing too as stage0 already has a libstd. Just one from the bootstrap compiler rather than compiled from the current source.

[jyn514]

Ok, then maybe stage0-sysroot-with-recompiled-libstd 🤷 nearly anything would be less confusing than just appending -sysroot IMO

[bjorn3]

Maybe rename stage0 to bootstrap and stage0-sysroot to stage0, matching stage1-...?

[jyn514]

bootstrap is confusing with the artifacts for src/bootstrap, but maybe bootstrap-sysroot would work, keeping stage0-sysroot as is and renaming stage1 to stage1-sysroot