As a Rust developer using Visual Studio Code and rust-analyzer, you might have encountered a common problem: unnecessary recompilation of dependencies upon saving a file. Even changes to files seemingly unrelated to dependencies can trigger a cargo check, causing delays in your development workflow.

This article examines the reasons behind this behavior and offers practical solutions to mitigate the issue.

Why Does rust-analyzer Trigger Dependency Recompilation?

rust-analyzer aims to provide a comprehensive and accurate understanding of your project, constantly updating its internal model as you work. Whenever you save a file, rust-analyzer assumes the change might impact dependencies, prompting it to run a cargo check to validate the project's consistency. This validation involves recompiling dependencies.

Understanding the Challenge

Predicting the precise impact of a change, even on seemingly unrelated dependencies, is difficult. This difficulty arises from factors such as macros, where a change can cascade through your code. rust-analyzer, in its effort to provide the most reliable feedback and code completion, takes a more cautious approach, triggering recompilation for potentially impacted dependencies.

Scenarios Where Recompilation is More Noticeable

The recompilation issue becomes particularly noticeable in scenarios with a high number of dependencies.

• Projects with a Large Dependency Tree: Large, interconnected dependency trees result in longer compilation times. rust-analyzer analyzes each dependency, adding significant overhead.

• Dependencies with Build Scripts: Dependencies involving build scripts can drastically increase compilation time. Build scripts generate code, download external resources, or configure build settings, contributing to the complexity and execution time.

• Native Dependencies: Dependencies with native code (like C or C++) add an additional layer of complexity. Native libraries must be compiled and linked, further delaying the build process.

Mitigating the Issue

• cargo check is faster than cargo test or cargo build. So, these aren't more effective alternatives to checking your code on save.

• Disabling rust-analyzer.checkOnSave. While you can disable code checks on save by setting rust-analyzer.checkOnSave to false in your project workspace settings, it effectively turns the editor into a less efficient development environment by completely disabling checks.

{
    "rust-analyzer.checkOnSave": false,
}

• rust-analyzer.check.extraArgs. The most effective solution is to use rust-analyzer.check.extraArgs to configure a separate target directory for cargo check operations:

{
    "rust-analyzer.checkOnSave": true,
    "rust-analyzer.check.extraArgs": [
        "--target-dir", "${workspaceFolder}/target/check"
    ]
}

Conclusion

While dependency recompilation is a common challenge faced by Rust developers, the right configuration can significantly improve the development cycle by reducing unnecessary delays. Understanding the reasoning behind rust-analyzer's approach is essential for effectively managing these issues. You can optimize your workspace configuration to achieve a faster, more responsive coding experience, maximizing your productivity and streamlining your development workflow.