libcore fails to compile for thumbv6m-none-eabi

[gnzlbg]

The libc crate cron job has started failing to build on master for the target thumbv6m-none-eabi. To reproduce:

cargo install xargo
rustup target add thumbv6m-none-eabi
git clone git@github.com:rust-lang/libc.git
cd libc
xargo +nightly build -vv --no-default-features --target thumbv6m-none-eabi

errors with

Compiling core v0.0.0 (/home/travis/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore)
     Running `rustc --edition=2018 --crate-name core /home/travis/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/lib.rs --color never --crate-type lib --emit=dep-info,metadata,link -C opt-level=3 -C metadata=60fcc652106674f6 -C extra-filename=-60fcc652106674f6 --out-dir /tmp/xargo.t1IUdReqtxLM/target/thumbv6m-none-eabi/release/deps --target thumbv6m-none-eabi -L dependency=/tmp/xargo.t1IUdReqtxLM/target/thumbv6m-none-eabi/release/deps -L dependency=/tmp/xargo.t1IUdReqtxLM/target/release/deps --sysroot /home/travis/.xargo -Z force-unstable-if-unmarked`
warning: the feature `const_generics` is incomplete and may cause the compiler to crash
  --> /home/travis/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/lib.rs:77:37
   |
77 | #![cfg_attr(not(bootstrap), feature(const_generics))]
   |                                     ^^^^^^^^^^^^^^
LLVM ERROR: invalid symbol redefinition
error: Could not compile `core`.

libcore failing to build for the target is bad - that it prevents to build the libc crate for the target is a sideeffect of that.

[nikic]

I wasn't able to reproduce this based on your instructions. Though for me there is no attempt to even build core, it just builds libc.

[gnzlbg]

Though for me there is no attempt to even build core, it just builds libc.

Weird. The first thing xargo does is build a sysroot for the target, and the first step of doing that is building libcore.

[gnzlbg]

See the exact build failure in libc's CI which mentions the exact same xargo invocation and how the first thing xargo does is building libcore: https://travis-ci.com/rust-lang/libc/jobs/219031083#L5403

[nikic]

I suspect that it doesn't build libcore because it's already distributed by rustup (and removing the rustup target makes compilation fail with libcore not found). Maybe something needs to be done to force building of libcore even for a non-custom target?

[gnzlbg]

I don't really follow. xargo and cargo-xbuild ignore anything distributed by rustup, except for the rust-src component, which include libcore sources, and as can be seen in the logs, the first thing they do is compiling the core crate, which fails with an LLVM error since the LLVM upgrade.

[gnzlbg]

AFAIK, we do not ship a rust-std component for this particular target.