CARGO_TARGET_DIR spreads out files in many directories

[soloturn]

Problem

when compiling COSMIC desktop the build is very slow, as it compiles components again. cargo distributes files into a number of directories, despite setting CARGO_TARGET_DIR, and it creates an impressive number of files, just for sendfd/lib.rs :

❯ find . | grep sendfd
./tmp/mytarget/release/.fingerprint/sendfd-5d7732d47158403b
./tmp/mytarget/release/.fingerprint/sendfd-5d7732d47158403b/lib-sendfd.json
./tmp/mytarget/release/.fingerprint/sendfd-5d7732d47158403b/invoked.timestamp
./tmp/mytarget/release/.fingerprint/sendfd-5d7732d47158403b/dep-lib-sendfd
./tmp/mytarget/release/.fingerprint/sendfd-5d7732d47158403b/lib-sendfd
./tmp/mytarget/release/deps/sendfd-5d7732d47158403b.d
./tmp/mytarget/release/deps/libsendfd-5d7732d47158403b.rmeta
./tmp/mytarget/release/deps/libsendfd-5d7732d47158403b.rlib
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/LICENSE-BSD
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/.cargo-checksum.json
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/README.mkd
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/Cargo.toml
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/src
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/src/lib.rs
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/src/changelog.rs
./.cache/yay/cosmic-comp-git/src/cosmic-comp/vendor/sendfd/LICENSE-APACHE
./.cargo/registry/cache/index.crates.io-6f17d22bba15001f/sendfd-0.4.3.crate
./.cargo/registry/index/index.crates.io-6f17d22bba15001f/.cache/se/nd/sendfd
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/LICENSE-BSD
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.github
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.github/dependabot.yml
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.github/workflows
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.github/workflows/sendfd.yml
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/Cargo.toml.orig
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/README.mkd
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.cargo_vcs_info.json
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/Cargo.toml
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.cargo-ok
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/.gitignore
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/src
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/src/lib.rs
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/src/changelog.rs
./.cargo/registry/src/index.crates.io-6f17d22bba15001f/sendfd-0.4.3/LICENSE-APACHE
 

i would have expected AlL files end up in ./tmp/mytarget, and compile results will be reused when compiling next component. it is not so. what am i missing?

Steps

No response

Possible Solution(s)

No response

Notes

No response

Version

No response

[epage]

Cargo has two many locations it operates with

• CARGO_HOME (not yet XDG compliant, see Use OS-native config, cache home directories #1734)
  • This includes global caches
• CARGO_TARGET_DIR: project specific intermediate and final build artifacts

The global cache is what lets us not re-download dependencies again and again (see #14278).

We could reuse intermediate build artifacts between projects but its rare and we need more bookkeeping to handle this. See #5931

As this is expected behavior, I'm going to go ahead and close. If there is a reason we need to keep this open, let us know!