nixpkgs/pkgs/build-support/rust

Updated fetchCargo behavior

Changes to the fetchcargo.nix behavior that cause changes to the cargoSha256 are somewhat disruptive, so historically we've added conditionals to provide backwards compatibility. We've now accumulated enough of these that it makes sense to do a clean sweep updating hashes, and delete the conditionals in the fetcher to simplify maintenance and implementation complexity. These conditionals are:

1. When cargo vendors dependencies, it generates a config. Previously, we were hard-coding our own config, but this fails if there are git dependencies. We have conditional logic to sometimes copy the vendored cargo config in, and sometimes not.

2. When a user updates the src package, they may forget to update the cargoSha256. We have an opt-in conditional flag to add the Cargo.lock into the vendor dir for inspection and compare at build-time, but it defaults to false.

3. We were previously vendoring into a directory with a recursive hash, but would like to vendor into a compressed tar.gz file instead, for the reasons specified in the git commit message adding this feature.

Migration plan

1. (DONE in this PR) Implement fetchCargoTarball as a separate, clean fetcher implementation along-side fetchcargo. Rename verifyCargoDeps (default false) to legacyCargoFetcher (default true), which switches the fetcher implementation used. Replace verifyCargoDeps = true; with legacyCargoFetcher = false; in Rust applications.

2. Send a treewide Rust PR that sets legacyCargoFetcher = true; in all Rust applications not using this (which is ~200 of them), with a note to maintainers to delete if updating the package. Change the default in buildRustPackage to false.

3. Go through all Rust src packages deleting the legacyCargoFetcher = false; line and re-computing the cargoSha256, merging as we go.

4. Delete the fetchcargo.nix implementation entirely and also remove:

• All overrides in application-level packages
• The fetchcargo-default-config.toml and conditionals around using it when no $CARGO_CONFIG exists
• This README.md file