Output collision when using build-std and cc as build-dependency

[tmiasko]

Problem

When using -Zbuild-std there is output collision between build dependencies that are common to the given project and standard library itself.

Steps

One minimal reproducer would be a project with a following Cargo.toml:

[package]
name = "a"
version = "0.1.0"
edition = "2018"
build = "build.rs"

[build-dependencies]
cc = "1.0"

And a build.rs:

extern crate cc;

fn main() {}

During build cargo warns about output collision:

$ cargo -Zbuild-std build --target x86_64-unknown-linux-gnu
warning: output filename collision.
The lib target `cc` in package `cc v1.0.50` has the same output filename as the lib target `cc` in package `cc v1.0.50`.
Colliding filename is: /.../target/debug/deps/libcc-737974c580f2da8c.rlib
The targets should have unique names.
Consider changing their names to be unique or compiling them separately.
This may become a hard error in the future; see <https://github.com/rust-lang/cargo/issues/6313>.
warning: output filename collision.
The lib target `cc` in package `cc v1.0.50` has the same output filename as the lib target `cc` in package `cc v1.0.50`.
Colliding filename is: /.../target/debug/deps/libcc-737974c580f2da8c.rmeta
The targets should have unique names.
Consider changing their names to be unique or compiling them separately.
This may become a hard error in the future; see <https://github.com/rust-lang/cargo/issues/6313>.
...

When building with multiple parallel jobs the conflicts do happen and result in failures observable in a variety of ways:

• SIGBUS: access to undefined memory
• error: failed to build archive: No such file or directory
• error: linking with cc failed: exit code: 1

and potentially more informative:

error[E0658]: use of unstable library feature 'rustc_private': this crate is being loaded from the sysroot, an unstable location; did you mean to load this crate from crates.io via `Cargo.toml` instead?
 --> build.rs:1:1
  |
1 | extern crate cc;
  | ^^^^^^^^^^^^^^^^
  |
  = note: for more information, see https://github.com/rust-lang/rust/issues/27812
  = help: add `#![feature(rustc_private)]` to the crate attributes to enable

Notes

cargo 1.42.0-nightly (9d32b7b01 2020-01-26)

[ehuss]

Thanks for the report, posted a fix at #7860.

[aeubanks]

we're seeing this same issue pop up again in #14253, perhaps need more hashing of target/host?