vim: Better snippets (and split completion file)

hm/vim/completion.nix

@@ -0,0 +1,54 @@
+{ pkgs, lib, config, ... }:
+{
+  config = {
+    # Completion
+    programs.nixvim = {
+      extraConfigLuaPre = builtins.readFile ./completion_pre.lua;
+      plugins = {
+        # Snippets
+        # UltiSnips should be the better snippet engine, but:
+        # - Not built-in into Nixvim yet (UPST?)
+        # - Couldn't manage to make it work with friendly-snippets, which seems to give better snippets for Python at least
+        # - Tab switching between fields seems hard to configure
+        luasnip = {
+          enable = true;
+          fromVscode = [
+            { paths = "${pkgs.vimPlugins.friendly-snippets}"; }
+            # { paths = "${pkgs.vimPlugins.vim-snippets}"; } # UPST Needs snipmate support
+          ];
+        };
+
+        # Completion
+        nvim-cmp = {
+          enable = true;
+          mapping = {
+            # Proposed example, since there's no default
+            "<C-Space>" = "cmp.mapping.complete()";
+            "<C-d>" = "cmp.mapping.scroll_docs(-4)";
+            "<C-e>" = "cmp.mapping.close()";
+            "<C-f>" = "cmp.mapping.scroll_docs(4)";
+            "<CR>" = "cmp.mapping.confirm({ select = true })";
+            "<S-Tab>" = {
+              action = "nvim_cmp_stab";
+              modes = [ "i" "s" ];
+            };
+            "<Tab>" = {
+              action = "nvim_cmp_tab";
+              modes = [ "i" "s" ];
+            };
+          };
+          sources = [
+            # Respective plugins will get installed automatically
+            { name = "buffer"; }
+            { name = "calc"; }
+            { name = "nvim_lsp"; }
+            { name = "path"; }
+            { name = "luasnip"; }
+          ];
+          snippet.expand = "luasnip";
+        };
+        lspkind.enable = true; # Add icons to LSP completions
+      };
+    };
+  };
+}