There were a series of unfortunate interactions here. Here's an MCVE of the test this fixes (committed as `tests/ui/meta/no_std-extern-libc.rs`):
```rust
#![crate_type = "lib"]
#![no_std]
#![feature(rustc_private)]
extern crate libc;
```
Before, this would give an error about duplicate versions of libc:
```
error[E0464]: multiple candidates for `rlib` dependency `libc` found
--> fake-test-src-base/allocator/no_std-alloc-error-handler-default.rs:15:1
|
LL | extern crate libc;
| ^^^^^^^^^^^^^^^^^^
|
= note: candidate #1: /home/gh-jyn514/rust/build/aarch64-unknown-linux-gnu/stage2/lib/rustlib/aarch64-unknown-linux-gnu/lib/liblibc-358db1024b7d9957.rlib
= note: candidate #2: /home/gh-jyn514/rust/build/aarch64-unknown-linux-gnu/stage2/lib/rustlib/aarch64-unknown-linux-gnu/lib/liblibc-ebc478710122a279.rmeta
```
Both these versions were downloaded from CI, but one came from the `rust-std` component and one came from `rustc-dev`:
```
; tar -tf build/cache/f2d9a3d0771504f1ae776226a5799dcb4408a91a/rust-std-nightly-x86_64-unknown-linux-gnu.tar.xz | grep liblibc
rust-std-nightly-x86_64-unknown-linux-gnu/rust-std-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-68a2d9e195dd6ed2.rlib
; tar -tf build/cache/f2d9a3d0771504f1ae776226a5799dcb4408a91a/rustc-dev-nightly-x86_64-unknown-linux-gnu.tar.xz | grep liblibc
rustc-dev-nightly-x86_64-unknown-linux-gnu/rustc-dev/lib/rustlib/x86_64-unknown-linux-gnu/lib/liblibc-f226c9fbdd92a0fd.rmeta
```
The fix was to only copy files from `rust-std` unless a Step explicitly requests for the `rustc-dev` components to be available by calling `builder.ensure(compile::Rustc)`.
To avoid having to re-parse the `rustc-dev.tar.xz` tarball every time, which is quite slow, this adds a new `build/host/ci-rustc/.rustc-dev-contents` cache file which stores only the names of files we need to copy into the sysroot.
This also allows reverting the hack in
https://github.com/rust-lang/rust/pull/110121; now that we only copy
rustc-dev on-demand, we can correctly add the `Rustc` check artifacts
into the sysroot, so that this works correctly even when
`download-rustc` is forced to `true`.
---
See https://github.com/rust-lang/rust/issues/108767#issuecomment-1501217657 for why `no_std` is required for the MCVE test to fail; it's complicated and not particularly important.
Fixes https://github.com/rust-lang/rust/issues/108767.
