xynovim guide

Debugging Rust in Neovim with nvim-dap and codelldb

Debugging Rust in Neovim works with nvim-dap and the codelldb adapter, but the defaults leave you with raw memory instead of readable values, or with no locals at all. This guide covers the debugger setup in xynovim, a personal Neovim configuration for Rust: the keys, the launch configurations, why strings and options print as text, why a panic stops at the right frame, and the one Cargo setting that decides whether you see local variables.

What you need

• codelldb, which scripts/bootstrap.sh installs through Mason on Linux, and cargo with rustc from rustup.
• Full debuginfo in the dev profile. This is the setting people miss. I keep profile.dev.debug = true in ~/.cargo/config.toml, because with line-tables-only or limited the Locals scope is empty. A user-level profile overrides a project's Cargo.toml, so a project can only change it through its own .cargo/config.toml or the CARGO_PROFILE_DEV_DEBUG environment variable. Do not use split-debuginfo: packed and unpacked both left codelldb without locals. The cost measured on two small fixtures (single, noisy runs): a clean build about 14 percent slower and target/debug about 28 percent larger than with line-tables-only.

Keys

The keys come from LazyVim's dap.core extra plus two additions. Press <leader>d and which-key lists them.

<leader>dr from a lib.rs lists only that file's tests, which is why <leader>dR exists: it reads the binary and example targets from cargo metadata and adds the runnables of the current file. Each entry builds first and then launches.

Launch configurations

Three Rust configurations replace the generic ones. LLDB: Launch and LLDB: Launch (args) ask for the executable, prefilled with <cargo target_directory>/debug/<bin>. The binary comes from cargo metadata: the package that holds the buffer, else the root package, else the first package with a binary. Rust: run current package binary (args prompt) builds with cargo build --package P --bin B without blocking the editor and launches in the package directory. A launch.json still works.

Readable values and panics

Every Rust launch through codelldb gets two additions. First, Rust's own LLDB formatters (lldb_lookup.py from the toolchain's sysroot) are imported, so a String shows as text, a Vec as size=4, an enum as B(7), and a struct as {host:"127.0.0.1", ...} instead of raw fields. Second, a breakpoint on rust_panic is set, so a panic stops at the panicking frame instead of the session ending with exit code 101. A #[should_panic] test stops there too. Both are added in a listener, because rustaceanvim overwrites initCommands on its own launches.

The dap-ui sidebar is wider than the default (35 percent of the columns, clamped between 40 and 60) and wraps, so a type such as alloc::vec::Vec<alloc::string::String, alloc::alloc::Global> stays readable.

A quiet startup

The upstream default loads debug configurations by running cargo test --no-run --all-targets every time rust-analyzer attaches, which competes with your own cargo for the build-directory lock. That autoload is off here. The picker keys ask rust-analyzer on demand, and :RustLsp debuggables still works.

Limits

• Developed and tested on Linux. On Windows and macOS codelldb does not install through Mason, so debugging is not demonstrated there.
• The interactive prompts were stubbed in the tests, and multi-binary workspaces and very narrow terminals were not verified.
• <leader>dR uses rustaceanvim internals that an update could break.

Try it

Install the configuration, open the demo crate, put a breakpoint with <leader>db in main and start it with <leader>dR.

git clone https://github.com/xynorash/xynovim ~/.config/nvim   # or ln -s <checkout> ~/.config/nvim
~/.config/nvim/scripts/bootstrap.sh

xynovim is a personal project: the home page has the tour, the install details and the platform status.

Related guides

• Tests and coverageRunning Rust tests with coverage in Neovim
• Rust diagnosticsInstant Rust diagnostics in Neovim with rust-analyzer and clippy
• Workspaces, features, cratesCargo workspaces, features and dependencies in Neovim

In the repository

• README: Debugging
• DETAILS: Debugger