A mystery of unnecessary Rust crate recompilation

Thomas Karpiniec discusses a frustrating issue he encountered while working with Rust, specifically related to unnecessary crate recompilation. The problem stemmed from a misconfiguration in the Cargo.toml file of a crate named "time," which was part of a larger project with multiple Rust workspaces. The workspaces shared a common target directory for build artifacts, leading to conflicts when different workspaces had varying resolved dependencies for the same crate. This situation caused the "time" crate to be rebuilt frequently, even when unrelated changes were made, resulting in wasted compilation time.

Karpiniec discovered that the issue was linked to a copy-paste error in the Cargo.toml file, where the crate-type was incorrectly specified to include both "cdylib" and "rlib." This configuration prevented the caching of the crate due to the lack of metadata, causing it to be rebuilt whenever switching between workspaces. By removing the unnecessary crate-type specification, he restored the metadata, which resolved the recompilation issue. This experience highlights the importance of proper configuration in Rust projects, especially when dealing with multiple workspaces and shared dependencies, to avoid inefficiencies in the build process.