Spurious rebuilds of ring: the env variable CARGO_MANIFEST_DIR changed

[kryptan]

Problem

I have noticed that cargo likes to sometimes rebuild ring for no apparent reason. It happens randomly and is very hard to reproduce. I managed to catch it once with verbose output and it says that variable CARGO_MANIFEST_DIR has changed. I do not set any env variables, this is just a regular cargo project with no unusual build settings.

       ...
       Fresh toml v0.9.8
       Fresh mio v1.0.4
       Dirty ring v0.17.14: the env variable CARGO_MANIFEST_DIR changed
   Compiling ring v0.17.14
       Fresh futures-task v0.3.31
       Fresh slab v0.4.11
       ...

Looks like ring build script is outputting rerun-if-env-changed=CARGO_MANIFEST_DIR which is invalid usage but I don't think it should cause random rebuilds.

Steps

1. Have a large project with many dependencies, including ring.
2. Perform many incremental rebuilds of your project. Sometimes ring might get rebuild.

Possible Solution(s)

No response

Notes

No response

Version

> cargo version --verbose
cargo 1.92.0-nightly (367fd9f21 2025-10-15)
release: 1.92.0-nightly
commit-hash: 367fd9f213750cd40317803dd0a5a3ce3f0c676d
commit-date: 2025-10-15
host: x86_64-pc-windows-msvc
libgit2: 1.9.1 (sys:0.20.2 vendored)
libcurl: 8.15.0-DEV (sys:0.4.83+curl-8.15.0 vendored ssl:Schannel)
os: Windows 10.0.26100 (Windows 11 Enterprise) [64-bit]

[weihanglo]

Would you mind providing a minimal reproducible example that can show that there is excessive rebuild bug in Cargo?

From my quick reading of briansmith/ring#1962 and briansmith/ring#1975, it is not clear to me how it would affect your build.

[kryptan]

I figured out what is causing this. I have a helper script written in Rust to build my project. Alternating invocations of cargo build from terminal and Command::new("cargo").arg("build").status() from Rust is causing ring to be rebuilt each time.

Here is an MRE: https://github.com/kryptan/cargo-rebuilds

If you make alternating calls to:

cargo build

and

cargo run --package builder # execute Command::new("cargo").arg("build").status()

ring will be rebuilt each time.

The issue here is that cargo sees that CARGO_MANIFEST_DIR has changed and thinks that ring needs to be rebuilt even though the value of CARGO_MANIFEST_DIR seen by ring would still be the same and there is no reason to rebuild it.