Externally Implementable Items: `error: undefined symbol` when opt-level >= 1

[AsakuraMizu]

Cross-crate EII fails to link with error: undefined symbol when opt-level >= 1.

Reproduction

Multi-crate workspace — cannot be reduced to a single file.

Repo with a test script: https://github.com/AsakuraMizu/eii-playground

decl1/src/lib.rs:

#![feature(extern_item_impls)]

#[eii]
pub fn decl1();

impl1/src/lib.rs:

#[decl1::decl1]
pub fn impl1() {
    println!("Hello, world!");
}

src/main.rs:

extern crate impl1;

fn main() {
    decl1::decl1();
}

$ CARGO_PROFILE_DEV_OPT_LEVEL=1 cargo run
error: linking with `cc` failed: exit status: 1
  = note: rust-lld: error: undefined symbol: decl1::decl1

Works with any of:

• opt-level = 0
• codegen-units = 1
• lto = "off"

So the EII shim function appears to be eliminated during thin-local LTO.

Meta

rustc --version --verbose:

rustc 1.96.0-nightly (2d76d9bc7 2026-03-09)
binary: rustc
commit-hash: 2d76d9bc76f27b03b4899e72ce561c7ac2c5cf6b
commit-date: 2026-03-09
host: x86_64-unknown-linux-gnu
release: 1.96.0-nightly
LLVM version: 22.1.0

Possibly related: #151271

Tracking issue: #125418

[eggyal]

@rustbot label +A-linkage +A-LTO +F-extern_item_impls +T-compiler -needs-triage