A reimplementation of my Meson language server in Swift.

Current feature set

  • Hovering
  • Symbol resolving
  • Jump-To-Definition
  • Jump-To-Subdir
  • A limited set of diagnostics
  • Formatting
  • Document symbols
  • Rudimentary autocompletion


  • High memory usage (100-400MB)
  • Not implemented inefficiently: On each textDocument/didChange notification, the entire tree is rebuilt.
  • Only very partial support for anything regarding set_variable/get_variable
  • No wrap/subproject support
  • Non constant subdir-calls are not supported
  • Type deduction is not 100% correct yet
  • Autocompletion is very flaky
  • Type definitions may have minor errors regarding:
    • Is this argument optional?
    • What is the type of the argument?

some_var = 'foo'
set_variable('foo_' + some_var, 1)
x = foo_foo # Unknown identifier 'foo_foo'
foreach backend : backends
  # The file backend-$backend/meson.build
  # won't be parsed
  subdir('backend-' + backend)

A majority of these limitations come from the fact that Swift-MesonLSP won’t interpret the code, but instead will just traverse the ast and check what subdirs are included.

Why a reimplementation?

The first version, written in Vala, had some code maintenance problems because basically everything was done in one file. I had the choice between untangling that mess or rewriting it as cleanly as possible. I have chosen the latter because I wanted to learn Swift.


I made a matrix channel: #mesonlsp:matrix.org Feel free to join in case you want to ask questions, get support or contribute.




  • Language Server: sudo dnf install swift-lang libstdc++-static
  • GNOME-Builder plugin: sudo dnf install git vala meson gcc libgee-devel json-glib-devel gtk4-devel gtksourceview5-devel libadwaita-devel libpeas-devel template-glib-devel g++ libsoup3-devel


TBD: For the moment, just follow what the docker files do:

Install the language server

git clone https://github.com/JCWasmx86/Swift-MesonLSP
cd Swift-MesonLSP
swift build -c release --static-swift-stdlib
sudo cp .build/release/Swift-MesonLSP /usr/local/bin

Or you can use podman (Maybe even docker, but only podman is tested):

DOCKER_BUILDKIT=1 podman build --file Dockerfile --output out --no-cache .
# If you want to use Ubuntu as docker image
DOCKER_BUILDKIT=1 podman build --file Dockerfile.ubuntu --output out --no-cache .

This will place a file “Fedora37.zip” (Or Ubuntu22.04.zip) in the directory out. It contains two statically linked binaries. Copy Swift-MesonLSP to /usr/local/bin.

A debug build is provided, too. Just rename it from Swift-MesonLSP.debug to Swift-MesonLSP and copy it to the right destination.

Install the GNOME Builder plugin (Requires GNOME Builder Nightly)

git clone https://github.com/JCWasmx86/GNOME-Builder-Plugins
cd GNOME-Builder-Plugins
# The other plugins in the repo
# get disabled
meson -Dplugin_cabal=disabled \
      -Dplugin_clangd=disabled \
      -Dplugin_gitgui=disabled \
      -Dplugin_hadolint=disabled \
      -Dplugin_hls=disabled \
      -Dplugin_icon_installer=disabled \
      -Dplugin_markdown=disabled \
      -Dplugin_muon=disabled \
      -Dplugin_pylint=disabled \
      -Dplugin_shfmt=disabled \
      -Dplugin_sqls=disabled \
      -Dplugin_stack=disabled \
      -Dplugin_swift=disabled \
      -Dplugin_swift_format=disabled \
      -Dplugin_swift_lint=disabled \
      -Dplugin_swift_templates=disabled \
      -Dplugin_sourcekit=disabled \
      -Dplugin_texlab=disabled \
      -Dplugin_xmlfmt=disabled \
      -Dplugin_xmlfmt=disabled \
      -Duse_swift_meson_lsp=true \
cd _build
# Don't do "sudo ninja install"
ninja -j $(nproc) install


On Fedora, you can use the install.sh script. It will automatically install all dependencies. (It will ask you for your root password using sudo, but don’t execute the script using sudo)

Want to contribute?

Take an item from the TODO list in PROGRESS.md and work on it. Feel free to join the matrix channel #mesonlsp:matrix.org

Debugging performance problems

The language server is measuring the duration needed by certain operations and exposes it using a HTTP-Server on http://localhost:65000 (Or any bigger port, if 65000 is already used)

Timing This picture shows an example of the rendered timing information. The project used is mesa and it got fully parsed 35 times.

  • analyzingTypes is the section in which all nodes of the AST are annotated with their possible type(s)
  • sendingDiagnostics is the section, where all diagnostics are sent back to the editor/client
  • rebuildTree is responsible for clearing all diagnostics, parsing all meson files, annotating the types, sending the newdiagnostics
  • parsingEntireTree is the section for parsing the entire project, patching the ASTs, etc.
  • SelectionStatementTypeMerge is part of the type analyzer. It is responsible for merging the possible types after different branches.
  • patchingAST is the section in which all subdir('foo') calls are replaced by nodes referencing the corresponding source files.
  • buildingAST is the section in which the tree sitter nodes are converted to easier to use objects
  • parsing is the section for just parsing a file using tree-sitter
  • checkCall is for checking the arguments of the call (Except type checking at the moment)
  • clearingDiagnostics is the section in that all diagnostics are cleared before rebuilding the tree.
  • checkIdentifier checks, if an identifier follows snake_case
  • guessingMethod is a method that attempts to deduce the possible methods that are called on an any object.
  • evalStack is responsible for getting types of variables that were overwritten in previous branches of selection statements.

In this example, it took around half-second to parse the entire mesa meson files, deduce the types and emit diagnostics.



  • SwiftTreeSitter – Swift API for the tree-sitter incremental parsing system
  • tree-sitter-meson – A tree-sitter grammar for meson.build files. Forked and enhanced by me.
  • swift-argument-parser – Straightforward, type-safe argument parsing for Swift
  • PathKit – Effortless path operations in Swift
  • sourcekit-lsp – Language Server Protocol implementation for Swift and C-based languages. (I used the JSONRPC definitions from them)
  • Swifter – Tiny http server engine written in Swift programming language.
  • swift-log – A Logging API for Swift
  • ConsoleKit – APIs for creating interactive CLI tools. (Used the loghandler from there to have nice logs)

Projects I tested the language server with


View Github