Pin first and last path in &rtp (#34)

plug.vim

@@ -175,6 +175,7 @@ function! plug#end()
             \ key, string(key), string(reverse(names)))
     augroup END
   endfor
+  call s:reorg_rtp()
   filetype plugin indent on
   syntax on
 endfunction
@@ -218,6 +219,17 @@ function! s:add_rtp(rtp)
   endif
 endfunction
 
+function! s:reorg_rtp()
+  if !empty(s:first_rtp)
+    execute 'set rtp-='.s:first_rtp
+    execute 'set rtp^='.s:first_rtp
+  endif
+  if s:last_rtp !=# s:first_rtp
+    execute 'set rtp-='.s:last_rtp
+    execute 'set rtp+='.s:last_rtp
+  endif
+endfunction
+
 function! s:lod(plug, types)
   let rtp = s:rtp(a:plug)
   call s:add_rtp(rtp)
@@ -232,6 +244,7 @@ function! s:lod_ft(pat, names)
   for name in a:names
     call s:lod(g:plugs[name], ['plugin', 'after'])
   endfor
+  call s:reorg_rtp()
   execute 'autocmd! PlugLOD FileType ' . a:pat
   silent! doautocmd filetypeplugin FileType
 endfunction
@@ -239,6 +252,7 @@ endfunction
 function! s:lod_cmd(cmd, bang, l1, l2, args, name)
   execute 'delc '.a:cmd
   call s:lod(g:plugs[a:name], ['plugin', 'ftdetect', 'after'])
+  call s:reorg_rtp()
   execute printf("%s%s%s %s", (a:l1 == a:l2 ? '' : (a:l1.','.a:l2)), a:cmd, a:bang, a:args)
 endfunction
 
@@ -246,6 +260,7 @@ function! s:lod_map(map, name, prefix)
   execute 'unmap '.a:map
   execute 'iunmap '.a:map
   call s:lod(g:plugs[a:name], ['plugin', 'ftdetect', 'after'])
+  call s:reorg_rtp()
   let extra = ''
   while 1
     let c = getchar(0)
@@ -1029,6 +1044,9 @@ function! s:diff()
   normal! gg
 endfunction
 
+let s:first_rtp = s:esc(get(split(&rtp, ','), 0, ''))
+let s:last_rtp = s:esc(get(split(&rtp, ','), -1, ''))
+
 let &cpo = s:cpo_save
 unlet s:cpo_save
 

test/workflow.vader

@@ -1,6 +1,9 @@
 Execute (Initialize test environment):
   Save &rtp, g:plugs, g:plug_home, $MYVIMRC
 
+  let first_rtp = split(&rtp, ',')[0]
+  let last_rtp  = split(&rtp, ',')[-1]
+
   let vader = fnamemodify(globpath(&rtp, 'autoload/vader.vim'), ':h:h')
   let plug  = fnamemodify(globpath(&rtp, 'autoload/plug.vim'), ':h:h')
   set rtp=$HOME/.vim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,$HOME/.vim/after
@@ -120,6 +123,8 @@ Execute (PlugClean before installation):
 Execute (plug#end() updates &rtp):
   call plug#end()
   Assert len(&rtp) > len(basertp)
+  AssertEqual first_rtp, split(&rtp, ',')[0]
+  AssertEqual last_rtp, split(&rtp, ',')[-1]
 
 Execute (Yet, plugins are not available):
   Assert empty(globpath(&rtp, 'autoload/emoji.vim'))
@@ -367,6 +372,8 @@ Execute (Partial PlugInstall):
 
 Execute (Check dependent plugin):
   Assert &rtp =~ 'pseudocl', &rtp
+  AssertEqual first_rtp, split(&rtp, ',')[0]
+  AssertEqual last_rtp, split(&rtp, ',')[-1]
 
 Given (Unaligned code):
   a=1
@@ -475,6 +482,8 @@ Execute (Check &rtp after SomeCommand):
   Assert &rtp !~ 'fzf'
   silent! SomeCommand
   Assert &rtp =~ 'fzf'
+  AssertEqual first_rtp, split(&rtp, ',')[0]
+  AssertEqual last_rtp, split(&rtp, ',')[-1]
 
 Execute (Common parent):
   call plug#begin()