nix build: replace fetchCargoTarball with importCargoLock

[mukilan]

importCargoLock allows us to use the existing Cargo.lock file. This means we no longer need to update the sha256 hash whenever the dependencies are upgraded. It also integrates with nix's rustToolchain support via cargoSetupHooks and automatically vendors the dependencies, allowing us to simplify the logic for filterlock derivation.

• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes do not require tests because the only modify files related to build support for Nix.

[sagudev]

This is also written on provided link:

If a Cargo.lock file is available, you can alternatively use the importCargoLock function. In contrast to fetchCargoTarball, this function does not require a hash (unless git dependencies are used)

but this might not apply to crown?

[mukilan]

@sagudev, I'm not sure I follow, are you suggesting that we use importCargoLock for crown as well? IIUC, buildRustPackage internally uses importCargoLock , passing it the lockFileContents. The reason we don't directly pass the lockfile is because we don't want all Servo's dependencies ending up in nix store.

Let me know if I misunderstood your comment.

[sagudev]

I was just asking, but now I see that hash was only used for filterlock, while I thought it was used for crown. My bad.

[delan]

Thanks!