Lib.rs

Cargo (as of 1.37) does not nicely handle the scenario where features of a dependent crate of multiple other crates in the tree vary.

To illustrate the problem consider crates A, B and C arranged as follows:

• Crate A and B are both members of a Cargo virtual manifest
• Crate C provides two features, F1 and F2
• Crate A requests feature F1 of C, crate B requests F2 of C

When crate A and B are built together, cargo builds C with both feature F1 and F2 enabled (the union of all enabled features). However when A or B are built individually, cargo builds C with only feature F1 or F2 enabled.

Unfortunately in all these cases, cargo will build crate C in the same target location and the outputs for C will be recreated every time the features for crate C change.

From a clean workspace, building A individually first will cause C to be built as expected. Then build B individually and C will be re-built because F2 was enabled instead of F1. Now rebuild A and observe that C will be re-built again because F1 was re-enabled.

In practice this problem is much less obvious as both A and B likely to not have a direct dependency on C, indirectly causing rebuilds of numerous other crates as well.

The gemachain-crate-features offers a workaround to this "feature thrashing" problem by explicitly declaring all "C-like crates" with the union of all features that any other crate in the tree (either explicitly or implicitly) enable. All crates in the Gemachain source tree should depend on gemachain-crate-features.

Adding new dependent crates

When unnecessary cargo rebuilds are observed, the first step is to figure what dependent crate is suffering from feature thrashing.

This information is not readily available from the stock cargo program, so use the following steps to produce a custom cargo program that will output the necessary feature information to stderr during a build:

$ git clone git@github.com:rust-lang/cargo.git -b rust-1.38.0
$ cd cargo
$ git apply 0001-Print-package-features.patch
$ cargo build
$ mv ~/.cargo/bin/cargo ~/.cargo/bin/cargo.org
$ cp ./target/debug/cargo ~/.cargo/bin/cargo

Rebuild with the custom cargo and search for indications of crates getting built with different features (repeated runs of ./scripts/cargo-install-all.sh work great for this). When the problematic crate is identified, add it as a dependency of gemachain-crate-features with the union of all observed enabled features for that crate.

Appendix

This command will enable some additional cargo log output that can help debug dependency problems as well:

export CARGO_LOG=cargo::core::compiler::fingerprint=info