I think I finally have a working setup using the built-in LSP integration.
Things seem to have changed quite a lot since this guide was written, most notably that nvim-lspconfig and nvim-cmp no longer seem to be required. I still had to install nvim-treesitter, though. Here are a few notes from my setup, in case it helps anyone else.
- Install
nvim-treesitter. Also don’t forget to install the prerequisite CLI via for example brew install tree-sitter-cli. At the time of writing, they are in the process of a complete rewrite on the main branch, and the master one will become unsupported. Also, the docs state that we should run :TSUpdate when updating the plugin, so I set that up too. Using vimplug:Plug 'nvim-treesitter/nvim-treesitter', { 'branch': 'main', 'build': ':TSUpdate' }
require'nvim-treesitter'.install { 'elixir', 'eex', 'heex' }
- Set up ElixirLS. We need to supply the
cmd to tell it where it’s installed, and root_markers so neovim can find the project root. vim.lsp.config('elixirls', {
cmd = { "/path/to/language_server.sh" },
-- required to make language server aware of other files
root_markers = { 'mix.exs', '.git' },
})
vim.lsp.enable('elixirls')
- Set up an autocmd to start tree-sitter when an elixir file is opened. I also set up a sub-autocmd to configure some LSP-related buffer settings when the LSP attaches to the buffer: enabling completion and auto-format on save. I was stuck here for a while, because while in most places the
pattern for an autocmd is something like *.ex, for the FileType autocmd it’s the detected filetype name, e.g. elixir 
vim.api.nvim_create_autocmd('FileType', {
pattern = { 'elixir', 'eex', 'heex' },
callback = function(ft_args)
vim.treesitter.start()
-- https://github.com/nvim-treesitter/nvim-treesitter/tree/main?tab=readme-ov-file#indentation
vim.bo.indentexpr = "v:lua.require'nvim-treesitter'.indentexpr()"
vim.api.nvim_create_autocmd('LspAttach', {
buffer = ft_args.buf,
callback = function(args)
local client = assert(vim.lsp.get_client_by_id(args.data.client_id))
-- Enable auto-completion
vim.lsp.completion.enable(true, client.id, args.buf, {autotrigger = true})
-- autoformat on save
-- Apparently not needed if server supports "textDocument/willSaveWaitUntil".
vim.api.nvim_create_autocmd('BufWritePre', {
buffer = args.buf,
callback = function()
vim.lsp.buf.format({ bufnr = args.buf, id = client.id, timeout_ms = 1000 })
end,
})
end,
})
end,
})
- Turn on “virtual lines” for diagnostics, so warnings and error messages show up below the line (otherwise it’s necessary to press CTRL+W d to view the warning - which you can still do if the warning scrolls off the edge of the screen and you want to view it in a popup).
vim.diagnostic.config({ virtual_lines = true })
- Set up any custom keymappings in addition to the defaults. For me I set
gd to go to definition, grr to find references, and Ctrl+m to show the diagnostic in a popup.vim.keymap.set('n', 'gd', function() vim.lsp.buf.definition() end)
vim.keymap.set('n', 'grr', function() vim.lsp.buf.references() end)
vim.keymap.set('n', '<C-m>', function() vim.diagnostic.open_float() end)
- Finally I added some vimscript to customise the autocompletion popup. The key bindings shouldn’t be necessary because neovim apparently already has these tab/shift+tab bindings, but for some reason it wasn’t working for me (also they are in vimscript, not lua, because I edited some existing code rather than figuring out how to do a conditional binding in lua…)
set completeopt+=menuone,noselect,popup
imap <expr> <enter> pumvisible() ? "<C-y>" : "<enter>"
imap <expr> <tab> pumvisible() ? "<C-n>" : "<tab>"
imap <expr> <S-tab> pumvisible() ? "<C-p>" : "<S-tab>"
Assembled config:
" Install tree-sitter with plugin manager
Plug 'nvim-treesitter/nvim-treesitter', { 'branch': 'main', 'build': ':TSUpdate' }
lua << LUA
vim.lsp.config('elixirls', {
cmd = { "/path/to/language_server.sh" },
-- required to make language server aware of other files
root_markers = { 'mix.exs', '.git' },
})
vim.lsp.enable('elixirls')
require'nvim-treesitter'.install { 'elixir', 'eex', 'heex' }
-- start tree-sitter and lsp for elixir files
vim.api.nvim_create_autocmd('FileType', {
pattern = { 'elixir', 'eex', 'heex' },
callback = function(ft_args)
vim.treesitter.start()
-- https://github.com/nvim-treesitter/nvim-treesitter/tree/main?tab=readme-ov-file#indentation
vim.bo.indentexpr = "v:lua.require'nvim-treesitter'.indentexpr()"
vim.api.nvim_create_autocmd('LspAttach', {
buffer = ft_args.buf,
callback = function(args)
local client = assert(vim.lsp.get_client_by_id(args.data.client_id))
-- Enable auto-completion
vim.lsp.completion.enable(true, client.id, args.buf, {autotrigger = true})
-- autoformat on save
-- Apparently not needed if server supports "textDocument/willSaveWaitUntil".
vim.api.nvim_create_autocmd('BufWritePre', {
buffer = args.buf,
callback = function()
vim.lsp.buf.format({ bufnr = args.buf, id = client.id, timeout_ms = 1000 })
end,
})
end,
})
end,
})
-- Enable warning/error messsages in virtual lines
vim.diagnostic.config({ virtual_lines = true })
vim.keymap.set('n', 'gd', function() vim.lsp.buf.definition() end)
vim.keymap.set('n', 'grr', function() vim.lsp.buf.references() end)
vim.keymap.set('n', '<C-m>', function() vim.diagnostic.open_float() end)
LUA
set completeopt+=menuone,noselect,popup
" Set up tab/enter navigation for autocomplete popup
" apparently this is already the default for snippets, no idea why it's not working without this
" https://github.com/neovim/neovim/pull/27339/files#diff-e682fc63c5b105e3eab956c380e8abf951a1b0d14fea03ae0f079bd490686e77
" have no idea what <expr> does but think it allows the ? operator
imap <expr> <enter> pumvisible() ? "<C-y>" : "<enter>"
imap <expr> <tab> pumvisible() ? "<C-n>" : "<tab>"
imap <expr> <S-tab> pumvisible() ? "<C-p>" : "<S-tab>"
