Update build#dir() to open a popup_menu()

Use `popup_menu()` in `build#dir()` when the `:BuildDir` command is uses
without an argument to specify a build directory. If there are no build
directories, print an error message. If there is a single build
directory, use it. If there are multiple build directories use
`popup_menu()` to prompt the user to select the desired build directory.

Update `build#targets()` and `build#run()` with comments and error
messages.

autoload/build.vim

@@ -1,27 +1,75 @@
-function! build#dir(dir) abort
-  let l:cwd = getcwd()
-  let $BUILD_DIR = l:cwd.'/'.a:dir
+function! build#dir(...) abort
+  if a:0 == 0
+    " No arguments, find build directories.
+    let s:dirs = split(substitute(globpath('.', 'build*'), '\.\/', '', 'g'), '\n')
+    let l:len = len(s:dirs)
+    if l:len == 0
+      echoerr 'no build directories found'
+    elseif l:len == 1
+      " One build directory found, use it.
+      let l:dir = s:dirs[0]
+      unlet s:dirs
+    else
+      " Multiple build directories found, create popup menu to select one.
+      " Set the callback to this function on completion, handled below.
+      call popup_menu(s:dirs, #{
+      \   filter: 'popup_filter_menu',
+      \   callback: 'build#dir',
+      \ })
+    endif
+  else
+    if a:0 == 1
+      " Single argument, invoked by :BuildDir.
+      let l:dir = a:1
+    elseif a:0 == 2
+      " Two arguments, called back by popup_menu().
+      let l:dirs = s:dirs
+      unlet s:dirs
+      if a:2 == -1
+        " Selection in popup_menu() was cancelled.
+        return
+      endif
+      let l:dir = l:dirs[a:2 - 1]
+    else
+      echoerr 'build#dir called with too many arguments'
+    endif
+  endif
+  if exists('l:dir')
+    " Set build directory and restart YouCompleteMe.
+    let $BUILD_DIR = getcwd().'/'.substitute(l:dir, '\/$', '', '')
     let g:ycm_clangd_args = ['--compile-commands-dir='.$BUILD_DIR]
     YcmRestartServer
+  endif
 endfunction
 
 function! build#targets(ArgLead, CmdLine, CursorPos) abort
+  if !exists('$BUILD_DIR')
+    echoerr 'build directory not set'
+    return ''
+  endif
   let l:targets = []
   if filereadable($BUILD_DIR.'/build.ninja')
+    " Ask ninja for the list of targets and prepare for returning.
     for l:target in split(system('ninja -C '.$BUILD_DIR.' -t targets'), '\n')
       call add(l:targets, substitute(l:target, ':.*$', '', ''))
     endfor
   elseif filereadable($BUILD_DIR.'/Makefile')
-    " TODO: support make
+    " TODO: Support make, it's much less straight forwards than ninja.
   endif
   return join(l:targets, "\n")
 endfunction
 
 function! build#run(...) abort
-  let l:build_dir = substitute($BUILD_DIR, '\/$', '', '')
+  if !exists('$BUILD_DIR')
+    call echo#error('build directory not set')
+    return
+  endif
+  let l:build_dir = $BUILD_DIR
   if filereadable($BUILD_DIR.'/build.ninja')
+    " Execute ninja in a terminal window.
     execute 'terminal ninja -C '.l:build_dir.' '.join(a:000, ' ')
   elseif filereadable($BUILD_DIR.'/Makefile')
+    " Execute make in a terminal window.
     execute 'terminal make -C '.l:build_dir.' '.join(a:000, ' ')
   endif
 endfunction

plugin/commands.vim

@@ -31,5 +31,5 @@ command! -nargs=+ -complete=file Debug call do#debug(<f-args>)
 command! TodoFile lvimgrep /todo/ %
 
 " Change build directory
-command! -nargs=1 -complete=dir BuildDir call build#dir(<f-args>)
+command! -nargs=? -complete=dir BuildDir call build#dir(<f-args>)
 command! -nargs=* -complete=custom,build#targets Build call build#run(<f-args>)