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Contributing to Metals

Whenever you are stuck or unsure, please open an issue or ask on Gitter. This project follows Scalameta's contribution guidelines.

Requirements

You will need the following applications installed:

  • Java 11 or 8 - Make sure JAVA_HOME points to a Java 11 or 8 installation. Metals will need to build and run on both.
  • git
  • sbt (for building a local version of the server)

Project structure

  • metals the main project with sources of the Metals language server.
  • mtags source file indexer for Java and Scala, it's a dependency of the metals project but kept in a separate module so it can be used by other projects like Metabrowse.
  • mtags-interfaces - java interfaces for the presentation compiler.
  • tests/cross - tests targeting cross builds for common features such as hover, completions, signatures etc.
  • tests/input example Scala code that is used as testing data for unit tests.
  • tests/unit moderately fast-running unit tests.
  • tests/slow slow integration tests.
  • test-workspace demo project for manually testing Metals through an editor.
  • docs documentation markdown for the Metals website.
  • metals-docs methods used for generating documentation across multiple pages in docs.
  • website holds the static site configuraton, style and blogs posts for the Metals website.

Git hooks

This git repository has a pre-push hook to run Scalafmt.

The CI also uses Scalafix to assert that there a no unused imports. To automatically remove unused imports run sbt scalafixAll. We don't run Scalafix as a pre-push git hook since starting sbt takes a long time.

Related projects

The improvement you are looking to contribute may belong in a separate repository:

Unit tests

To run the unit tests open an sbt shell and run unit/test

sbt
# (recommended) run a specific test suite, great for edit/test/debug workflows.
> metals/testOnly -- tests.DefinitionSuite
# run unit tests, moderately fast but still a bit too slow for edit/test/debug workflows.
> unit/test
# run slow integration tests, takes several minutes.
> slow/test
# (not recommended) run all tests, slow. It's better to target individual projects.
> test

Manually testing a LspSuite

Every test suite that extends LspSuite generates a workspace directory under tests/unit/target/e2e/$suitename/$testname. To debug why a LspSuite might be failing, run the test once and then open it directly in your editor. For example, for the test case "deprecated-scala" in WarningsLspSuite run the following command:

code tests/unit/target/e2e/warnings/deprecated-scala

If you are using VS Code, make sure to update the "Server Version" setting to use your locally published version of Metals.

Manual tests

Some functionality is best to manually test through an editor. A common workflow while iterating on a new feature is to run publishLocal and then open an editor in a small demo build.

Visual Studio Code

Install the Metals extension from the Marketplace by searching for "Metals".

Click here to install the Metals VS Code plugin

Next, update the "Server version" setting under preferences to point to the version you published locally via sbt publishLocal.

Metals server version setting

When you make changes in the Metals Scala codebase

  • run sbt publishLocal
  • execute the "Metals: Restart server" command in Visual Studio Code (via command palette)

Vim

First, follow the vim installation instruction.

Next, write a new-metals-vim script that builds a new metals-vim bootstrap script using the locally published version.

coursier bootstrap \
  --java-opt -Dmetals.client=coc.nvim \
  org.scalameta:metals_2.12:0.8.4-SNAPSHOT \ # double-check version here
  -r bintray:scalacenter/releases \
  -o /usr/local/bin/metals-vim -f

Finally, start vim with the local Metals version

cd test-workspace # any directory you want to manually test Metals
new-metals-vim && vim build.sbt # remember to have the script in your $PATH

When you make changes in the Metals Scala codebase, run sbt publishLocal, quit vim and re-run new-metals-vim && vim build.sbt.

Workspace logs

Metals logs workspace-specific information to the $WORKSPACE/.metals/metals.log file.

tail -f .metals/metals.log

These logs contain information that may be relevant for regular users.

JSON-RPC trace

To see trace of incoming/outgoing JSON communication with the text editor or build server, create empty files in your machine cache directory.

# macOS
touch -f ~/Library/Caches/org.scalameta.metals/lsp.trace.json # text editor
touch -f ~/Library/Caches/org.scalameta.metals/bsp.trace.json # build server
# Linux
touch ~/.cache/metals/lsp.trace.json # text editor
touch ~/.cache/metals/bsp.trace.json # build server

Next when you start Metals, watch the logs with tail -f.

# macOS
tail -f ~/Library/Caches/org.scalameta.metals/lsp.trace.json
# Linux
tail -f ~/.cache/metals/lsp.trace.json

The traces are very verbose so it's recommended to delete the files if you are not interested in debugging the JSON communication.

JVM Debugging

To debug the JVM with the Metals server, add a property to your Server Properties with the usual Java debugging flags, making sure you have the quiet option on. It's important to remember about the flag, as the server uses standard input/output to communicate with the client, and the default output of the debuggee interferes with that.

This property will make your server run in debug mode on port 5005 without waiting for the debugger to connect:

-agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=5005,quiet=y

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