custom_build.rs regression: Dependency not found in dependencies

[dtolnay]

Problem

Since #16436, cargo is not able to build https://github.com/dtolnay/cxx/tree/master/demo.

@ranger-ross @epage

Steps

cargo check in that directory:

thread 'main' (1857167) panicked at src/cargo/core/compiler/custom_build.rs:477:21:
Dependency `cxx` not found in `demo`s dependencies
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Possible Solution(s)

No response

Notes

No response

Version

cargo 1.94.0-nightly (8c133afcd 2026-01-09)
release: 1.94.0-nightly
commit-hash: 8c133afcd5e0d69932fe11f5907683723f8d361d
commit-date: 2026-01-09
host: x86_64-unknown-linux-gnu
libgit2: 1.9.2 (sys:0.20.3 vendored)
libcurl: 8.17.0-DEV (sys:0.4.84+curl-8.17.0 vendored ssl:OpenSSL/3.5.4)
ssl: OpenSSL 3.5.4 30 Sep 2025
os: Ubuntu 24.4.0 (noble) [64-bit]

[dtolnay]

# Cargo.toml

[package]
name = "cxx"
edition = "2024"
links = "cxx"
publish = false

[workspace]
members = ["repro"]

[patch.crates-io]
cxx = { path = "." }

// src/lib.rs (empty)

# repro/Cargo.toml

[package]
name = "repro"
edition = "2024"
publish = false

[dependencies]
cxx = "0"

// repro/src/lib.rs (empty)

cargo check --manifest-path repro/Cargo.toml

[ranger-ross]

Thanks for the report! we had a similar issue for build std dependencies that was fixed in #16489.

I am traveling over the weekend so I won't be able to look into the issue until next week. I'll look into this when I get back.

For now I'd recommend pinning to a previous version of nightly. (Sorry)

[weihanglo]

Ah, that doesn't consider patches. And just realized the buggy implementation looks pretty similar to Dependency::matches_id.

[weihanglo]

The fix will be available in tonight's nightly: rust-lang/rust#150932

Thanks for the issue report again!