Regression: E0152 duplicate lang item 'sized' when using -Z build-std with Apple iOS targets

[alycda]

Code

I tried this code:

// Any Rust project with Cargo.toml
[package]
name = "test-lang-item"
version = "0.1.0"
edition = "2021"

[lib]
crate-type = ["staticlib"]

cargo +nightly build --target aarch64-apple-ios-sim -Z build-std=std,panic_abort -Z build-std-features=panic_immediate_abort

I expected to see this happen: The project should compile successfully for Apple iOS targets when using -Z build-std.

Instead, this happened: Compilation fails with duplicate lang item error during std_detect crate compilation:

error[E0152]: duplicate lang item in crate `core`: `sized`
  |
  = note: the lang item is first defined in crate `core` (which `std_detect` depends on)
  = note: first definition in `core` loaded from /target/aarch64-apple-ios-sim/release/deps/libcore-4c40bdd97baee939.rmeta
  = note: second definition in `core` loaded from /.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/aarch64-apple-ios-sim/lib/libcore-215222db00f59ba6.rlib

error: could not compile `std_detect` (lib) due to 1 previous error

Version it worked on

It most recently worked on: nightly-2025-08-15 (rustc 1.91.0-nightly (898aff7 2025-08-14))

Version with regression

rustc --version --verbose:

rustc 1.91.0-nightly (9eb4a2652 2025-08-18)
binary: rustc
commit-hash: 9eb4a2652031ed5ba97c29ef21c79db1645f7883
commit-date: 2025-08-18
host: aarch64-apple-darwin
release: 1.91.0-nightly
LLVM version: 21.1.0

Additional Information

• Affected targets: aarch64-apple-ios, aarch64-apple-ios-sim, x86_64-apple-ios
• Command variations: Issue occurs with various -Z build-std flag combinations
• Impact: Blocking iOS development workflows and CI/CD pipelines
• Environment: macOS with aarch64-apple-darwin host

The error suggests that there are conflicting definitions of the sized lang item between the locally built core crate (via -Z build-std) and the pre-compiled core crate from the toolchain.

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged

[alycda]

still present: (I assume it would be but if it fixes itself I will update this issue)

rustc 1.91.0-nightly (05f5a58e8 2025-08-19)
binary: rustc
commit-hash: 05f5a58e84a9c3a68586d70bf3d7442c571e379e
commit-date: 2025-08-19
host: aarch64-apple-darwin
release: 1.91.0-nightly
LLVM version: 21.1.0

[Noratrieb]

@Kobzol seems related to your std_detect changes

[Kobzol]

I think that #145619 might fix this.

[lqd]

#145619 would fix it if this issue was a dupe of #145594, but the last known good version is apparently nightly-2025-08-15, but wasn't #145594 introduced by #145489 which landed yesterday?

I can't even repro this issue locally (thus can't bisect), const eval fails for me on cfg errors earlier than the OP.

[lqd]

@alycda could you bisect using https://github.com/rust-lang/cargo-bisect-rustc/?

The description in the OP is not enough to reproduce this issue:

• -Z build-std works with panic=unwind on master
• -Z build-std-features=panic_immediate_abort needs panic=abort in Cargo.toml
• I'm only getting error: can't find crate for compiler_builtins #145594 locally as I said above, and this issue just appeared yesterday, not around the 15th, so I can't bisect.

edit: now that #145619 has landed, you could also check whether it fixed your issue, https://github.com/kennytm/rustup-toolchain-install-master/ allows installing toolchains for its artifacts 38a19df4d6694ac96eb949f2d835cfccf461af4c (or the artifacts on master, 22a86f8280becb12c34ee3efd952baf5cf086fa0, or wait for tomorrow's nightly).