Editor support

Visual Studio Code

By default, rust-analyzer tries to run cargo without mach, which will cause issues due to the special configuration that Servo currently needs to build. It's recommend that you add the following to your project specific settings in .vscode/settings.json:

{
    "rust-analyzer.rustfmt.overrideCommand": [ "./mach", "fmt" ],
    "rust-analyzer.check.overrideCommand": [
        "./mach",
        "clippy",
        "--message-format=json",
        "--target-dir",
        "target/lsp",
        "--features",
        "tracing,tracing-perfetto"
    ],
    "rust-analyzer.cargo.buildScripts.overrideCommand": [
        "./mach",
        "clippy",
        "--message-format=json",
        "--target-dir",
        "target/lsp",
        "--features",
        "tracing,tracing-perfetto"    ],
}

Notes:

• In the above excerpt, the language server is building into its own target directory, target/lsp, in order to avoid unwanted rebuilds. If you would like to save disk space you can remove the --target-dir and target/lsp arguments and the default target directory will be used.
• To enable optional build settings, simply add them to the build argument list in your configuration file.
• Windows: If you are on Windows, you will need to use ./mach.bat instead of just ./mach. If you do not, you may receive an error saying that the command executed is not a valid Win32 application.
• Cross-compilation: If you are cross-compiling, you can override the target used for cargo by adding "rust-analyzer.cargo.target": "aarch64-linux-android". Note that some LSP features might not work in this configuration.

Zed

If you are using Zed, you must do something very similar to what is described for Visual Studio Code, but the Zed configuration file expects a slightly different syntax. In your ./zed/settings.json file you need something like this:

{
  "lsp": {
    "rust-analyzer": {
      "initialization_options": {
        "checkOnSave": true,
        "check": {
          "overrideCommand": [
            "./mach",
            "clippy",
            "--message-format=json",
            "--target-dir",
            "target/lsp",
            "--feature",
            "tracing,tracing-perfetto"
          ]
        },
        "cargo": {
          "allTargets": false,
          "buildScripts": {
            "overrideCommand": [
              "./mach",
              "clippy",
              "--message-format=json",
              "--target-dir",
              "target/lsp",
              "--feature",
              "tracing,tracing-perfetto"
            ]
          }
        },
        "rustfmt": {
          "extraArgs": [
            "--config",
            "unstable_features=true",
            "--config",
            "binop_separator=Back",
            "--config",
            "imports_granularity=Module",
            "--config",
            "group_imports=StdExternalCrate"
          ]
        }
      }
    }
  }
}

NixOS

If you are on NixOS, you might get errors about pkg-config or crown:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "Could not run `PKG_CONFIG_ALLOW_SYSTEM_CFLAGS=\"1\" PKG_CONFIG_ALLOW_SYSTEM_LIBS=\"1\" \"pkg-config\" \"--libs\" \"--cflags\" \"fontconfig\"`

[ERROR rust_analyzer::main_loop] FetchWorkspaceError: rust-analyzer failed to load workspace: Failed to load the project at /path/to/servo/Cargo.toml: Failed to read Cargo metadata from Cargo.toml file /path/to/servo/Cargo.toml, Some(Version { major: 1, minor: 74, patch: 1 }): Failed to run `cd "/path/to/servo" && "cargo" "metadata" "--format-version" "1" "--manifest-path" "/path/to/servo/Cargo.toml" "--filter-platform" "x86_64-unknown-linux-gnu"`: `cargo metadata` exited with an error: error: could not execute process `crown -vV` (never executed)

mach passes different RUSTFLAGS to the Rust compiler than plain cargo, so if you try to build Servo with cargo, it will undo all the work done by mach (and vice versa). Because of this, and because Servo can currently only be built with mach, you need to configure the rust-analyzer extension to use mach in .vscode/settings.json:

Using crown

If you are using --use-crown, you should also set CARGO_BUILD_RUSTC in .vscode/settings.json as follows, where /nix/store/.../crown is the output of nix-shell --run 'command -v crown'.

{
    "rust-analyzer.server.extraEnv": {
        "CARGO_BUILD_RUSTC": "/nix/store/.../crown",
    },
}

These settings should be enough to not need to run code . from within a nix-shell, but it wouldn’t hurt to try that if you still have problems.

Problems with proc macros

When enabling rust-analyzer’s proc macro support, you may start to see errors like

proc macro `MallocSizeOf` not expanded: Cannot create expander for /path/to/servo/target/debug/deps/libfoo-0781e5a02b945749.so: unsupported ABI `rustc 1.69.0-nightly (dc1d9d50f 2023-01-31)` rust-analyzer(unresolved-proc-macro)

This means rust-analyzer is using the wrong proc macro server, and you will need to configure the correct one manually. Use mach to query the current sysroot path, and copy the last line of output:

$ ./mach rustc --print sysroot
NOTE: Entering nix-shell /path/to/servo/shell.nix
info: component 'llvm-tools' for target 'x86_64-unknown-linux-gnu' is up to date
/home/me/.rustup/toolchains/nightly-2023-02-01-x86_64-unknown-linux-gnu

Then configure either your sysroot path or proc macro server path in .vscode/settings.json:

{
    "rust-analyzer.procMacro.enable": true,
    "rust-analyzer.cargo.sysroot": "[paste what you copied]",
    "rust-analyzer.procMacro.server": "[paste what you copied]/libexec/rust-analyzer-proc-macro-srv",
}