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.

Debug options view

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

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.