startup: load files from &packpath . '/start/{pluginname}'

Quoting the existing docs:

    Packages are loaded.  These are plugins, as above [&runtimepath], but
    found in the "start" directory of each entry in 'packpath'.  Every
    plugin directory found is added in 'runtimepath' and then the plugins
    are sourced.

Also tj didn't think I could do it.

src/nvim/ex_cmds2.c

@@ -2551,6 +2551,17 @@ int do_in_path_and_pp(char_u *path, char_u *name, int flags,
     done = do_in_path(p_pp, s, flags, callback, cookie);
 
     xfree(s);
+
+    if (done == FAIL|| (flags & DIP_ALL)) {
+      start_dir = "start/*/%s";  // NOLINT
+      len = STRLEN(start_dir) + STRLEN(name);
+      s = xmallocz(len);
+
+      vim_snprintf((char *)s, len, start_dir, name);
+      done = do_in_path(p_pp, s, flags, callback, cookie);
+
+      xfree(s);
+    }
   }
 
   if ((done == FAIL || (flags & DIP_ALL)) && (flags & DIP_OPT)) {
@@ -2562,6 +2573,17 @@ int do_in_path_and_pp(char_u *path, char_u *name, int flags,
     done = do_in_path(p_pp, s, flags, callback, cookie);
 
     xfree(s);
+
+    if (done == FAIL || (flags & DIP_ALL)) {
+      opt_dir = "opt/*/%s";  // NOLINT
+      len = STRLEN(opt_dir) + STRLEN(name);
+      s = xmallocz(len);
+
+      vim_snprintf((char *)s, len, opt_dir, name);
+      done = do_in_path(p_pp, s, flags, callback, cookie);
+
+      xfree(s);
+    }
   }
 
   return done;
@@ -2822,6 +2844,8 @@ void add_pack_start_dirs(void)
 {
   do_in_path(p_pp, (char_u *)"pack/*/start/*", DIP_ALL + DIP_DIR,  // NOLINT
              add_pack_plugin, &APP_ADD_DIR);
+  do_in_path(p_pp, (char_u *)"start/*", DIP_ALL + DIP_DIR,  // NOLINT
+             add_pack_plugin, &APP_ADD_DIR);
 }
 
 /// Load plugins from all packages in the "start" directory.
@@ -2830,6 +2854,8 @@ void load_start_packages(void)
   did_source_packages = true;
   do_in_path(p_pp, (char_u *)"pack/*/start/*", DIP_ALL + DIP_DIR,  // NOLINT
              add_pack_plugin, &APP_LOAD);
+  do_in_path(p_pp, (char_u *)"start/*", DIP_ALL + DIP_DIR,  // NOLINT
+             add_pack_plugin, &APP_LOAD);
 }
 
 // ":packloadall"

src/nvim/ex_getln.c

@@ -5533,6 +5533,7 @@ static int ExpandRTDir(char_u *pat, int flags, int *num_file, char_u ***file,
   garray_T ga;
   ga_init(&ga, (int)sizeof(char *), 10);
 
+  // TODO(bfredl): this is bullshit, exandpath should not reinvent path logic.
   for (int i = 0; dirnames[i] != NULL; i++) {
     size_t size = STRLEN(dirnames[i]) + pat_len + 7;
     char_u *s = xmalloc(size);
@@ -5549,6 +5550,14 @@ static int ExpandRTDir(char_u *pat, int flags, int *num_file, char_u ***file,
       globpath(p_pp, s, &ga, 0);
       xfree(s);
     }
+
+    for (int i = 0; dirnames[i] != NULL; i++) {
+      size_t size = STRLEN(dirnames[i]) + pat_len + 22;
+      char_u *s = xmalloc(size);
+      snprintf((char *)s, size, "start/*/%s/%s*.vim", dirnames[i], pat);  // NOLINT
+      globpath(p_pp, s, &ga, 0);
+      xfree(s);
+    }
   }
 
   if (flags & DIP_OPT) {
@@ -5559,6 +5568,14 @@ static int ExpandRTDir(char_u *pat, int flags, int *num_file, char_u ***file,
       globpath(p_pp, s, &ga, 0);
       xfree(s);
     }
+
+    for (int i = 0; dirnames[i] != NULL; i++) {
+      size_t size = STRLEN(dirnames[i]) + pat_len + 20;
+      char_u *s = xmalloc(size);
+      snprintf((char *)s, size, "opt/*/%s/%s*.vim", dirnames[i], pat);  // NOLINT
+      globpath(p_pp, s, &ga, 0);
+      xfree(s);
+    }
   }
 
   for (int i = 0; i < ga.ga_len; i++) {
@@ -5606,6 +5623,8 @@ static int ExpandPackAddDir(char_u *pat, int *num_file, char_u ***file)
   char_u *s = xmalloc(buflen);
   snprintf((char *)s, buflen, "pack/*/opt/%s*", pat);  // NOLINT
   globpath(p_pp, s, &ga, 0);
+  snprintf((char *)s, buflen, "opt/%s*", pat);  // NOLINT
+  globpath(p_pp, s, &ga, 0);
   xfree(s);
 
   for (int i = 0; i < ga.ga_len; i++) {

test/functional/core/startup_spec.lua

@@ -313,14 +313,33 @@ describe('startup', function()
 
 
   it("handles &packpath during startup", function()
-    pack_clear [[ let g:x = bar#test() ]]
+    pack_clear [[
+      let g:x = bar#test()
+      let g:y = leftpad#pad("heyya")
+    ]]
     eq(-3, eval 'g:x')
+    eq("  heyya", eval 'g:y')
 
-    pack_clear [[ lua _G.y = require'bar'.doit() ]]
+    pack_clear [[ lua _G.y = require'bar'.doit() _G.z = require'leftpad''howdy' ]]
-    eq(9003, exec_lua [[ return _G.y ]])
+    eq({9003, '\thowdy'}, exec_lua [[ return { _G.y, _G.z } ]])
   end)
 
   it("handles :packadd during startup", function()
+    -- control group: opt/bonus is not availabe by default
+    pack_clear [[
+      try
+        let g:x = bonus#secret()
+      catch
+        let g:err = v:exception
+      endtry
+    ]]
+    eq('Vim(let):E117: Unknown function: bonus#secret', eval 'g:err')
+
+    pack_clear [[ lua _G.test = {pcall(function() require'bonus'.launch() end)} ]]
+    eq({false, [[[string ":lua"]:1: module 'bonus' not found:]]},
+       exec_lua [[ _G.test[2] = string.gsub(_G.test[2], '[\r\n].*', '') return _G.test ]])
+
+    -- ok, time to launch the nukes:
     pack_clear [[ packadd! bonus | let g:x = bonus#secret() ]]
     eq('halloj', eval 'g:x')
 

test/functional/fixtures/start/nvim-leftpad/autoload/leftpad.vim

@@ -0,0 +1,3 @@
+function! leftpad#pad(str)
+  return '  '.a:str
+endfunction

test/functional/fixtures/start/nvim-leftpad/lua/leftpad.lua

@@ -0,0 +1 @@
+return function (str) return '\t' .. str end