Constant rebuilds with checksum-freshness when using bindgen

[svix-jplatte]

Problem

When using -Zchecksum-freshness in a project that uses the current version of bindgen, every cargo check or cargo build starts at clang-sys, always considering it in need of a rebuild. I could not reproduce the issue with just clang-sys as a dependency.

Steps

1. Create a new Cargo package
2. cargo add bindgen@0.72.1
3. cargo c -Zchecksum-freshness
4. cargo c -Zchecksum-freshness

Version

cargo 1.93.0-nightly (2d4fa1395 2025-11-12)
release: 1.93.0-nightly
commit-hash: 2d4fa139552ebdd5f091a1401ed03f7dc62cb43f
commit-date: 2025-11-12
host: x86_64-unknown-linux-gnu
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.15.0-DEV (sys:0.4.83+curl-8.15.0 vendored ssl:OpenSSL/3.5.4)
ssl: OpenSSL 3.5.4 30 Sep 2025
os: Arch Linux [64-bit]

@rustbot label +Z-checksum-freshness +A-rebuild-detection

[epage]

Using -v

       Dirty clang-sys v1.8.1: file size changed (1380 != 1429) for `/home/epage/.cargo/build/01/76261f5cc2e6cd/debug/
build/clang-sys-b51426cc266d3245/out/macros.rs`

[epage]

I could not reproduce the issue with just clang-sys as a dependency.

Huh, I'm able to reproduce with a direct dependency on clang-sys by mimicking the features bindgen enables

[package]
name = "cargo-16253"
version = "0.1.0"
edition = "2024"

[dependencies]
#bindgen = "0.72.1"
clang-sys = { version = "1", features = ["clang_11_0", "runtime"] }

[svix-jplatte]

Sorry, I didn't try very hard 😅

[epage]

Looks like rustc is calculating file sizes in a different way than cargo

[epage]

Trying to create a test for this but not working so far:

#[cargo_test(nightly, reason = "requires -Zchecksum-hash-algorithm")]
fn include_dep_info_matches() {
    let p = project()
        .file("Cargo.toml", &basic_lib_manifest("foo"))
        .file(
            "build.rs",
            r#"
fn main() {
    let out = std::path::PathBuf::from(std::env::var("OUT_DIR").unwrap());
    std::fs::write(out.join("written.txt"), "10").unwrap();
    copy("src/copied.txt", &out.join("copied.txt"));
}

fn copy(source: &str, destination: &std::path::Path) {
    use std::fs::File;
    use std::io::{Read, Write};

    let mut string = String::new();
    File::open(source)
        .unwrap()
        .read_to_string(&mut string)
        .unwrap();
    File::create(destination)
        .unwrap()
        .write_all(string.as_bytes())
        .unwrap();
}
"#,
        )
        .file(
            "src/lib.rs",
            r#"
pub const STATIC: usize = include!("static.txt");
pub const WRITTEN: usize = include!(concat!(env!("OUT_DIR"), "/written.txt"));
pub const COPIED: usize = include!(concat!(env!("OUT_DIR"), "/copied.txt"));
"#,
        )
        .file("src/static.txt", "5")
        .file("src/copied.txt", "15")
        .build();
    p.cargo("check -Zchecksum-freshness -v")
        .masquerade_as_nightly_cargo(&["checksum-freshness"])
        .with_stderr_data(str![[r#"
[COMPILING] foo v0.5.0 ([ROOT]/foo)
[RUNNING] `rustc --crate-name build_script_build --edition=2015 build.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type bin --emit=dep-info,link -C embed-bitcode=no --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=717be6ce15238bbd -C extra-filename=-108e821ed10eead9 --out-dir [ROOT]/foo/target/debug/build/foo-[HASH] -L dependency=[ROOT]/foo/target/debug/deps -Z checksum-hash-algorithm=blake3`
[RUNNING] `[ROOT]/foo/target/debug/build/foo-[HASH]/build-script-build`
[RUNNING] `rustc --crate-name foo --edition=2015 src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata -C embed-bitcode=no -C debuginfo=2 --check-cfg 'cfg(docsrs,test)' --check-cfg 'cfg(feature, values())' -C metadata=b33563f562248144 -C extra-filename=-c8429bad05cb35f8 --out-dir [ROOT]/foo/target/debug/deps -L dependency=[ROOT]/foo/target/debug/deps -Z checksum-hash-algorithm=blake3`
[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s

"#]])
        .run();
    p.cargo("check -Zchecksum-freshness -v")
        .masquerade_as_nightly_cargo(&["checksum-freshness"])
        .with_stderr_data(str![[r#"
[FRESH] foo v0.5.0 ([ROOT]/foo)
[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s

"#]])
        .run();
}

[epage]

I switched the test so that we include content the same size as the manual reproduction case and still not getting the test to reproduce.

[epage]

Moving all of the logic into a published package that is depended on still doesn't allow the test to reproduce the problem.

[epage]

If I'm reading this right, when rustc generates the dep info files, it is using File::metadata to get the length.
https://github.com/rust-lang/rust/blob/af5c5b74c8f2c0b1dfd9a201453bf2df538e93e0/compiler/rustc_interface/src/passes.rs#L616-L641

I assume that should give the same results as std::fs::metaata which is what we call

cargo/src/cargo/core/compiler/fingerprint/mod.rs

Lines 2042 to 2053 in cd90026

	let Ok(current_file_len) = fs::metadata(&path).map(|m| m.len()) else {
	return Some(StaleItem::FailedToReadMetadata {
	path: path.to_path_buf(),
	});
	};
	if current_file_len != file_len {
	return Some(StaleItem::FileSizeChanged {
	path: path.to_path_buf(),
	new_size: current_file_len,
	old_size: file_len,
	});
	}

[epage]

Removed the file length comparison and confirmed that we both agree on the checksum.

One mitigation might be to remove the file length check. Hate to not understand the root cause but the file length check is meant to optimize the files-changed case which will usually be in the minority so it doesn't seem like much of an optimization.