duplicate lang item if `#![feature(test)]` is enabled

[skyzh]

Minimum reproducible step:

cargo new --lib test-build-std

lib.rs:

#![feature(test)]
extern crate test;

#[cfg(test)]
mod tests {
    #[test]
    fn it_works() {
        let result = 2 + 2;
        assert_eq!(result, 4);
    }
}

cargo build -Zbuild-std --target aarch64-apple-darwin

Then there are a lot of errors in console...

error: duplicate lang item in crate `core` (which `std` depends on): `const_ptr`.
  |
  = note: the lang item is first defined in crate `core` (which `std` depends on)
  = note: first definition in `core` loaded from /Users/skyzh/Work/test-build-std/target/aarch64-apple-darwin/debug/deps/libcore-ba459098a3dc74d8.rmeta
  = note: second definition in `core` loaded from /Users/skyzh/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcore-dd6de681850a671c.rlib

error: duplicate lang item in crate `core` (which `std` depends on): `mut_ptr`.
  |
  = note: the lang item is first defined in crate `core` (which `std` depends on)
  = note: first definition in `core` loaded from /Users/skyzh/Work/test-build-std/target/aarch64-apple-darwin/debug/deps/libcore-ba459098a3dc74d8.rmeta
  = note: second definition in `core` loaded from /Users/skyzh/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/aarch64-apple-darwin/lib/libcore-dd6de681850a671c.rlib

Once feature test is removed, everything compiles...

Thanks for investigating into this!

[repnop]

hit this on a personal project and found this issue. seems pretty trivial to reproduce, all I did was cargo new --lib foo and add in #![no_std], #[cfg(test)] extern crate std;, and the .cargo/config.toml for the flags

lib.rs

#![no_std]

#[cfg(test)]
extern crate std;

pub fn add(left: u64, right: u64) -> u64 {
    left + right
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = add(2, 2);
        assert_eq!(result, 4);
    }
}

.cargo/config.toml

[unstable]
build-std = ["core", "alloc", "compiler_builtins", "test"]
build-std-features = ["compiler-builtins-mem"]

repnop@chaos /t/f/foo (master)> cargo test --target x86_64-unknown-linux-gnu
  Downloaded compiler_builtins v0.1.157
  Downloaded 1 crate (239.4KiB) in 0.85s
   Compiling compiler_builtins v0.1.157
   Compiling core v0.0.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core)
   Compiling libc v0.2.172
   Compiling std v0.0.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std)
   Compiling rustc-std-workspace-core v1.99.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-core)
   Compiling alloc v0.0.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc)
   Compiling cfg-if v1.0.0
   Compiling rustc-demangle v0.1.24
   Compiling unwind v0.0.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/unwind)
   Compiling rustc-std-workspace-alloc v1.99.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-alloc)
   Compiling panic_abort v0.0.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/panic_abort)
   Compiling hashbrown v0.15.3
   Compiling std_detect v0.1.5 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/stdarch/crates/std_detect)
   Compiling rustc-std-workspace-std v1.99.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-std)
   Compiling unicode-width v0.1.14
   Compiling getopts v0.2.21
   Compiling test v0.0.0 (/home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/test)
   Compiling foo v0.1.0 (/tmp/foo/foo)
error: the `#[alloc_error_handler]` in std conflicts with allocation error handler in: std

error[E0152]: duplicate lang item in crate `core` (which `std` depends on): `sized`
  |
  = note: the lang item is first defined in crate `core` (which `foo` depends on)
  = note: first definition in `core` loaded from /tmp/foo/foo/target/x86_64-unknown-linux-gnu/debug/deps/libcore-09a9b117b1bf70a4.rlib
  = note: second definition in `core` loaded from /home/repnop/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/x86_64-unknown-linux-gnu/lib/libcore-11d9a250f9da47d5.rlib

For more information about this error, try `rustc --explain E0152`.
error: could not compile `foo` (lib test) due to 2 previous errors

if you remove the "test" in the build-std config, it errors out earlier on talking about sized being duplicated, but if you keep it, you get the above. something is definitely funky given this line makes no sense and nothing defines a #[alloc_error_handler]...

error: the `#[alloc_error_handler]` in std conflicts with allocation error handler in: std

[adamgemmell]

I believe in both reproductions having both std and test in the list of crates for build-std fixes things.

With the implementation of -Zbuild-std right now test is not built by Cargo unless manually specified so rustc brings in the test (and by extension std) from the sysroot causing duplicate item errors.

If std is not specified then cargo does not pass std and other needed dependencies via --extern to the top level crate and so rustc brings in std from the sysroot again.