Debugging VSCode plugin and the language server

Prerequisites

• Install LLDB and the LLDB Extension.
• Open the root folder in VSCode. Here you can access the preconfigured debug setups.

• Install all TypeScript dependencies

bash cd editors/code npm ci

Common knowledge

• All debug configurations open a new [Extension Development Host] VSCode instance where only the rust-analyzer extension being debugged is enabled.
• To activate the extension you need to open any Rust project folder in [Extension Development Host].

Debug TypeScript VSCode extension

• Run Installed Extension - runs the extension with the globally installed rust-analyzer binary.
• Run Extension (Debug Build) - runs extension with the locally built LSP server (target/debug/rust-analyzer).

TypeScript debugging is configured to watch your source edits and recompile. To apply changes to an already running debug process, press Ctrl+Shift+P and run the following command in your [Extension Development Host]

> Developer: Reload Window

Debug Rust LSP server

• When attaching a debugger to an already running rust-analyzer server on Linux you might need to enable ptrace for unrelated processes by running:

bash echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

• By default, the LSP server is built without debug information. To enable it, you'll need to change Cargo.toml:

toml [profile.dev] debug = 2

• Select Run Extension (Debug Build) to run your locally built target/debug/rust-analyzer.

• In the original VSCode window once again select the Attach To Server debug configuration.

• A list of running processes should appear. Select the rust-analyzer from this repo.

• Navigate to crates/rust-analyzer/src/main_loop.rs and add a breakpoint to the on_request function.

• Go back to the [Extension Development Host] instance and hover over a Rust variable and your breakpoint should hit.

If you need to debug the server from the very beginning, including its initialization code, you can use the --wait-dbg command line argument or RA_WAIT_DBG environment variable. The server will spin at the beginning of the try_main function (see crates/rust-analyzer/src/bin/main.rs)

let mut d = 4;
while d == 4 { // set a breakpoint here and change the value
    d = 4;
}

However for this to work, you will need to enable debug_assertions in your build

RUSTFLAGS='--cfg debug_assertions' cargo build --release

Demo

• Debugging TypeScript VScode extension.
• Debugging Rust LSP server.

Troubleshooting

Can't find the rust-analyzer process

It could be a case of just jumping the gun.

The rust-analyzer is only started once the onLanguage:rust activation.

Make sure you open a rust file in the [Extension Development Host] and try again.

Can't connect to rust-analyzer

Make sure you have run echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope.

By default this should reset back to 1 every time you log in.

Breakpoints are never being hit

Check your version of lldb. If it's version 6 and lower, use the classic adapter type. It's lldb.adapterType in settings file.

If you're running lldb version 7, change the lldb adapter type to bundled or native.