Just a quick one, as I’ve found nvim-lsp’s documentation to be a bit lacking. I hope this will get you up and running with this great tool:

nvim-lsp is Neovim’s fully integrated language server protocol implementation that allows syntax checking, linting and omnifunc code completion within the editor. Currently, it is available as part of Neovim’s master branch, so in many cases (unless you are running Ubuntu, in which case there is a nice snap package available for you, so you can simply install the latest development release with snap install --classic --edge neovim), you will have to compile Neovim from source to start using it. As of Neovim 0.4.4, nvim-lsp has been supported in the release build.

Without any language server configurations, the lsp client is useless. Configurations are distributed separate from the editor itself; using a plugin manager like vim-plug or dein, you can install these configurations just like any other plugin:

" In your .config/nvim/init.vim (Unix):
Plug 'neovim/nvim-lsp'

Each of these configurations targets a different LSP server. You must activate those that you wish to use individually. For example, if I want to have rust completion using rust-analyzer, I must add the following to the bottom of $MYVIMRC:

" Enable rust-analyzer
lua require'nvim_lsp'.rust_analyzer.setup{}

Note: ensure the LSP is installed and available on your PATH environment variable first, otherwise you will get an error message claiming that the LSP is not executable when you attempt to open an associated file type in your editor.

Note: see that lua keyword? nvim-lsp is entirely written in Lua, so make sure you know how to embed lua code in your Neovim configuration.

See also: some LSPs have configuration options which you may want to toggle to work with your environment. Have a look at nvim-lsp’s README for further information concerning your specific LSP configuration.

Now, syntax highlighting and diagnostics facilities now begin immediately when you open the editor again on a new source file associated with the enabled LSP configurations, but you can’t navigate code using the LSP’s features and verbs or use code completion just yet.

You can configure navigation mappings yourself in $MYVIMRC:

nnoremap <silent> gd    <cmd>lua vim.lsp.buf.declaration()<CR>
nnoremap <silent> <c-]> <cmd>lua vim.lsp.buf.definition()<CR>
nnoremap <silent> K     <cmd>lua vim.lsp.buf.hover()<CR>
nnoremap <silent> gD    <cmd>lua vim.lsp.buf.implementation()<CR>
nnoremap <silent> <c-k> <cmd>lua vim.lsp.buf.signature_help()<CR>
nnoremap <silent> 1gD   <cmd>lua vim.lsp.buf.type_definition()<CR>
nnoremap <silent> gr    <cmd>lua vim.lsp.buf.references()<CR>
nnoremap <silent> g0    <cmd>lua vim.lsp.buf.document_symbol()<CR>
nnoremap <silent> gW    <cmd>lua vim.lsp.buf.workspace_symbol()<CR>

And, finally, to enable omnifunc completion in insert mode (i.e. <C-X><C-O>):

" Enable nvim-lsp as the default omnifunc globally:
set omnifunc=v:lua.vim.lsp.omnifunc
" Or only for a specific filetype:
autocmd Filetype rust setlocal omnifunc=v:lua.vim.lsp.omnifunc

Pretty much all of nvim-lsp should now be accessible to you from when you source your .vimrc. Give it a go!

But what if you want insert-mode completion using <Tab>? You’ll need another plugin that can do that for you, such as deoplete.nvim:

" Insert-mode completion
Plug 'Shougo/deoplete.nvim'

let g:deoplete#enable_at_startup = 1

" And a plugin connecting deoplete to nvim-lsp
Plug 'Shougo/deoplete-lsp'

Once installed, you should be able to just tab away to get your automatic code completion. Smashing.