feat(ex_checkhealth): provide function for command line completion

Move away from providing completion with ExpandRTDir to ExpandGeneric
providing the function get_healthcheck_name which caches the results for
the current command line prompt.

It does the almost the same thing the Vim function 'get_healthcheck'
implemented in 'runtime/autoload/health.vim' does.

src/nvim/ex_getln.c

@@ -230,7 +230,22 @@ static int compl_match_arraysize;
 static int compl_startcol;
 static int compl_selected;
 
+/// |:checkhealth| completion items
+///
+/// Regenerates on every new command line prompt, to accomodate changes on the
+/// runtime files.
+typedef struct {
+  garray_T names;  // healthcheck names
+  unsigned last_gen;  // last_prompt_id where names were generated
+} CheckhealthComp;
 
+/// Cookie used when converting filepath to name
+struct healthchecks_cookie {
+  garray_T *names;  // global healthchecks
+  bool is_lua;  // true if the current entry is a Lua healthcheck
+};
+
+static CheckhealthComp healthchecks = { GA_INIT(sizeof(char_u *), 10), 0 };
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "ex_getln.c.generated.h"
@@ -273,6 +288,68 @@ static void init_incsearch_state(incsearch_state_T *s)
   save_viewstate(&s->old_viewstate);
 }
 
+/// Completion for |:checkhealth| command.
+///
+/// Given to ExpandGeneric() to obtain all available heathcheck names.
+/// @param[in] idx  Index of the healthcheck item.
+/// @param[in] xp  Not used.
+static char_u *get_healthcheck_names(expand_T *xp, int idx)
+{
+  // Generate the first time or on new prompt.
+  if (healthchecks.last_gen == 0 || healthchecks.last_gen != last_prompt_id) {
+    ga_clear_strings(&healthchecks.names);
+    char *patterns[3] = { "autoload/health/**.vim", "lua/**/**/health/init.lua",  // NOLINT
+                          "lua/**/**/health.lua" };  // NOLINT
+    for (int i = 0; i < 3; i++) {
+      struct healthchecks_cookie hcookie = { .names = &healthchecks.names, .is_lua = i != 0 };
+      do_in_runtimepath((char_u *)patterns[i], DIP_ALL, get_healthcheck_cb, &hcookie);
+
+      if (healthchecks.names.ga_len > 0) {
+        ga_remove_duplicate_strings(&healthchecks.names);
+      }
+    }
+    // Tracked to regenerate items on next prompt.
+    healthchecks.last_gen = last_prompt_id;
+  }
+  return idx <
+         (int)healthchecks.names.ga_len ? ((char_u **)(healthchecks.names.ga_data))[idx] : NULL;
+}
+
+/// Transform healthcheck file path into it's name.
+///
+/// Used as a callback for do_in_runtimepath
+/// @param[in] path  Expanded path to a possible healthcheck.
+/// @param[out] cookie  Array where names will be inserted.
+static void get_healthcheck_cb(char_u *path, void *cookie)
+{
+  if (path != NULL) {
+    struct healthchecks_cookie *hcookie = (struct healthchecks_cookie *)cookie;
+    char *pattern;
+    char *sub = "\\1";
+    char_u *res;
+
+    if (hcookie->is_lua) {
+      // Lua: transform "../lua/vim/lsp/health.lua" into "vim.lsp"
+      pattern = ".*lua[\\/]\\(.\\{-}\\)[\\/]health\\([\\/]init\\)\\?\\.lua$";
+    } else {
+      // Vim: transform "../autoload/health/provider.vim" into "provider"
+      pattern = ".*[\\/]\\([^\\/]*\\)\\.vim$";
+    }
+
+    res = do_string_sub(path, (char_u *)pattern, (char_u *)sub, NULL, (char_u *)"g");
+    if (hcookie->is_lua && res != NULL) {
+      // Replace slashes with dots as represented by the healthcheck plugin.
+      char_u *ares = do_string_sub(res, (char_u *)"[\\/]", (char_u *)".", NULL, (char_u *)"g");
+      xfree(res);
+      res = ares;
+    }
+
+    if (res != NULL) {
+      GA_APPEND(char_u *, hcookie->names, res);
+    }
+  }
+}
+
 // Return true when 'incsearch' highlighting is to be done.
 // Sets search_first_line and search_last_line to the address range.
 static bool do_incsearch_highlighting(int firstc, int *search_delim, incsearch_state_T *s,
@@ -4902,10 +4979,6 @@ static int ExpandFromContext(expand_T *xp, char_u *pat, int *num_file, char_u **
     char *directories[] = { "syntax", "indent", "ftplugin", NULL };
     return ExpandRTDir(pat, DIP_LUA, num_file, file, directories);
   }
-  if (xp->xp_context == EXPAND_CHECKHEALTH) {
-    char *directories[] = { "autoload/health", NULL };
-    return ExpandRTDir(pat, 0, num_file, file, directories);
-  }
   if (xp->xp_context == EXPAND_USER_LIST) {
     return ExpandUserList(xp, num_file, file);
   }
@@ -4982,6 +5055,7 @@ static int ExpandFromContext(expand_T *xp, char_u *pat, int *num_file, char_u **
       { EXPAND_ENV_VARS, get_env_name, true, true },
       { EXPAND_USER, get_users, true, false },
       { EXPAND_ARGLIST, get_arglist_name, true, false },
+      { EXPAND_CHECKHEALTH, get_healthcheck_names, true, false },
     };
     int i;
 

test/functional/plugin/health_spec.lua

@@ -36,6 +36,7 @@ describe(':checkhealth', function()
     clear()
     eq('nvim', getcompletion('nvim', 'checkhealth')[1])
     eq('provider', getcompletion('prov', 'checkhealth')[1])
+    eq('vim.lsp', getcompletion('vim.ls', 'checkhealth')[1])
   end)
 end)