Registry crates with a build script are never extracted: build-script-rustc fails hashing source/build.rs

What happened

Building any target that depends on a registry crate with a build script fails: Once declares <crate>/source/build.rs as an input to the build-script-rustc action, but never extracts the crate source, so the file does not exist when the action hashes its inputs.

The output directory for the crate is created and left completely empty.

Reproduction

Cargo.toml:

[package]
name = "icu-demo"
version = "0.1.0"
edition = "2021"

[dependencies]
url = "2.5.8"

src/main.rs:

fn main() {
    let _ = url::Url::parse("https://example.com");
}

Then:

cargo generate-lockfile
once build cargo_icu_demo_bin_icu_demo

Actual

ERROR once: session failed error=building action 1 for icu_normalizer_data-2.2.0 (icu_normalizer_data-2.2.0:build-script-rustc)
once: building action 1 for icu_normalizer_data-2.2.0 (icu_normalizer_data-2.2.0:build-script-rustc): hashing declared input `.once/out/icu_normalizer_data-2.2.0/source/build.rs`: No such file or directory (os error 2)

The output directory exists but is empty — no source/ subdirectory was ever created:

$ ls -A .once/out/icu_normalizer_data-2.2.0/ | wc -l
0

By contrast, a dependency without a build script is extracted correctly:

$ ls .once/out/percent-encoding-2.3.2/source
Cargo.lock  Cargo.toml  Cargo.toml.orig  LICENSE-APACHE  LICENSE-MIT  src

The upstream .crate archive does contain the file, and its manifest declares it, so the declared input itself is correct:

$ tar tzf ~/.cargo/registry/cache/*/icu_normalizer_data-2.2.0.crate | grep build.rs
icu_normalizer_data-2.2.0/build.rs

$ grep '^build' ~/.cargo/registry/src/*/icu_normalizer_data-2.2.0/Cargo.toml
build = "build.rs"

In this reproduction none of the extracted crates has a build.rs — every crate that has one fails at this step.

Expected

The crate source is extracted before its build-script-rustc action hashes inputs, so source/build.rs is present.

Notes

url is only the smallest trigger I found; it reaches build.rs-bearing crates through idna → idna_adapter → icu_properties → icu_properties_data. The same failure occurs via other paths — e.g. sqlx-core depends on url, so any sqlx user hits it as well, reported as icu_properties_data-2.2.0 there.

Environment

• once 0.42.0 (x86_64-unknown-linux-gnu release build)
• Linux 6.8, x86_64

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