VSCode debug shows "Locations: 0" for all breakpoints when debugging Rust library

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05-10-2024

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Debugging Rust Libraries in VS Code: Fixing the "Locations: 0" Breakpoint Issue

Debugging your Rust code can be a lifesaver, but sometimes VS Code throws a curveball: "Locations: 0" for all breakpoints, leaving you stranded in a sea of code with no way to step through it. This is especially frustrating when debugging a Rust library, where the code you need to inspect might be in a separate crate.

The Problem:

You've set breakpoints in your Rust library code, but VS Code refuses to recognize them. Instead of the familiar blue dots indicating breakpoints, you see a grayed-out dot with the dreaded "Locations: 0" message. This means VS Code can't find the source code associated with your breakpoints, effectively disabling your debugging capabilities.

Scenario:

Let's imagine you're building a library called my_library that's used by your main application. You set a breakpoint in my_library/src/lib.rs but get the "Locations: 0" message.

// my_library/src/lib.rs
pub fn greet(name: &str) -> String {
    let greeting = format!("Hello, {}!", name);
    println!("{}", greeting); // Breakpoint here
    greeting
}

Why It Happens:

The culprit often lies in how VS Code handles debugging libraries. The debugger needs to know the exact path to the source code to properly set breakpoints. In the case of libraries, this path might not be straightforward, leading to the "Locations: 0" issue.

Troubleshooting:

Here's how to conquer the "Locations: 0" problem and get your Rust library debugging back on track:

1. Verify Your Workspace Configuration: Make sure your VS Code workspace is configured to include both your main application and your library project. This usually involves adding the library's folder as a subfolder within your workspace.

2. Check .vscode/launch.json: The launch.json file in your workspace controls debugging settings. Ensure that the program property points to the correct executable (likely the main application).

3. Inspect Your Cargo.toml: Make sure the path field in the [dependencies] section of your main application's Cargo.toml file correctly points to your library's directory.

4. Force Debugging Mode: You can sometimes trick VS Code into debugging by explicitly setting the debug feature in your Cargo.toml file for your library. This can help overcome issues related to code optimization.

[dependencies]
my_library = { path = "../my_library", features = ["debug"] }

5. Rebuild and Restart: After making any changes to your Cargo.toml files, rebuild your project using cargo build and restart VS Code.

Additional Tips:

• Rebuild and Restart: After any configuration changes, always rebuild your project using cargo build and restart VS Code.
• Project Structure: Keep a clean and organized project structure. This helps VS Code find your source files more easily.
• VS Code Extensions: Ensure you have the necessary Rust extension installed and configured properly.

Key Takeaway:

Debugging Rust libraries requires careful attention to project structure, Cargo dependencies, and VS Code configuration. By understanding the reasons behind the "Locations: 0" issue and implementing the right fixes, you can overcome this hurdle and efficiently debug your Rust libraries.

References:

• Rust Language Book: Debugging
• VS Code Debugging Documentation
• Cargo Documentation