Rust Analyzer <-> Maturin interaction

[ion-elgreco]

Bug Description

In delta-rs we have a .cargo/config.toml in the root and .carg/config.toml in our python subcrate.

Both of the following rustflags:

[target.aarch64-apple-darwin]
rustflags = [
  "-C", "link-arg=-undefined",
  "-C", "link-arg=dynamic_lookup",
]

Running plain maturin develop will invalidate the incremental compilation.

Only if I manually provide the RUSTFLAGS it works:

RUSTFLAGS="-C link-arg=-undefined -C link-arg=dynamic_lookup" maturin develop

Shouldn't maturin use the .cargo/config.toml during maturin develop

Your maturin version (maturin --version)

1.8.2

Your Python version (python -V)

3.11

Your pip version (pip -V)

UV is used

What bindings you're using

pyo3

Does cargo build work?

• Yes, it works

If on windows, have you checked that you aren't accidentally using unix path (those with the forward slash /)?

• Yes

Steps to Reproduce

1. Create crate with above config
2. Use VSCode with Rust Analyzer (RA)
3. Let RA run Cargo check
4. Now run maturin develop
5. Change .rs file and save which triggers cargo check
6. Run maturin develop again (notice it starts from beginning)

[messense]

This is likely a known Cargo fingerprinting issue rather than a maturin bug. When maturin sets CARGO_ENCODED_RUSTFLAGS (even with the same flags from .cargo/config.toml), Cargo used to treat any change in rustflags as a cache invalidation, causing full rebuilds.

How maturin handles this today:

• maturin loads existing rustflags from .cargo/config.toml via cargo_config2
• It checks for duplicates before appending its own flags (e.g. -C link-arg=-undefined on macOS)
• It only sets CARGO_ENCODED_RUSTFLAGS when it actually added new flags beyond what config.toml already provides
• So if your config.toml already has the macOS PyO3 flags, maturin should skip setting the env var entirely

The upstream Cargo fix:
This was tracked as rust-lang/cargo#8716 and fixed in rust-lang/cargo#14830 (merged Dec 2024, shipped in Rust 1.85.0). The fix makes Cargo cache artifacts per-RUSTFLAGS using -C extra-filename instead of throwing away the cache when flags change. This means even when maturin does set CARGO_ENCODED_RUSTFLAGS, the old artifacts from rust-analyzer's cargo check are preserved rather than invalidated.

Recommendation: Upgrading to Rust >= 1.85.0 should resolve the incremental compilation invalidation.