neovim: Add autocompletion support with nvim-cmp

Did not expect this to work on the first try but it did. Taken directly
from the nvim-lspconfig wiki.

https://github.com/neovim/nvim-lspconfig/wiki/Autocompletion

home/neovim.nix

@@ -271,6 +271,73 @@ in
           })
         '';
       }
+      {
+        plugin = nvim-cmp;
+        type = "lua";
+        config = /* lua */ ''
+          -- Add additional capabilities supported by nvim-cmp
+          local capabilities = require("cmp_nvim_lsp").default_capabilities()
+
+          local lspconfig = require('lspconfig')
+
+          -- Enable some language servers with the additional completion capabilities offered by nvim-cmp
+          local servers = { 'clangd', 'rust_analyzer', 'pyright', 'tsserver' }
+          for _, lsp in ipairs(servers) do
+            lspconfig[lsp].setup {
+              -- on_attach = my_custom_on_attach,
+              capabilities = capabilities,
+            }
+          end
+
+          -- luasnip setup
+          local luasnip = require 'luasnip'
+
+          -- nvim-cmp setup
+          local cmp = require 'cmp'
+          cmp.setup {
+            snippet = {
+              expand = function(args)
+                luasnip.lsp_expand(args.body)
+              end,
+            },
+            mapping = cmp.mapping.preset.insert({
+              ['<C-u>'] = cmp.mapping.scroll_docs(-4), -- Up
+              ['<C-d>'] = cmp.mapping.scroll_docs(4), -- Down
+              -- C-b (back) C-f (forward) for snippet placeholder navigation.
+              ['<C-Space>'] = cmp.mapping.complete(),
+              ['<CR>'] = cmp.mapping.confirm {
+                behavior = cmp.ConfirmBehavior.Replace,
+                select = true,
+              },
+              ['<Tab>'] = cmp.mapping(function(fallback)
+                if cmp.visible() then
+                  cmp.select_next_item()
+                elseif luasnip.expand_or_jumpable() then
+                  luasnip.expand_or_jump()
+                else
+                  fallback()
+                end
+              end, { 'i', 's' }),
+              ['<S-Tab>'] = cmp.mapping(function(fallback)
+                if cmp.visible() then
+                  cmp.select_prev_item()
+                elseif luasnip.jumpable(-1) then
+                  luasnip.jump(-1)
+                else
+                  fallback()
+                end
+              end, { 'i', 's' }),
+            }),
+            sources = {
+              { name = 'nvim_lsp' },
+              { name = 'luasnip' },
+            },
+          }
+        '';
+      }
+      cmp-nvim-lsp
+      cmp_luasnip
+      luasnip
       {
         plugin = nvim-base16;
         type = "lua";