lsp: add :GoLSPDebugBrowser

autoload/go/lsp.vim

@@ -315,6 +315,10 @@ function! s:newlsp() abort
   endfunction
 
   function! l:lsp.err_cb(ch, msg) dict abort
+    if a:msg =~ '^\tPort = \d\+$' && !get(self, 'debugport', 0)
+      let self.debugport = substitute(a:msg, '^\tPort = \(\d\+\).*$', '\1', '')
+    endif
+
     call s:debug('stderr', a:msg)
   endfunction
 
@@ -337,9 +341,12 @@ function! s:newlsp() abort
     return
   endif
 
-  " TODO(bc): output a message indicating which directory lsp is going to
-  " start in.
-  let l:lsp.job = go#job#Start([l:bin_path], l:opts)
+  let l:cmd = [l:bin_path]
+  if go#util#HasDebug('lsp')
+    let l:cmd = extend(l:cmd, ['-debug', 'localhost:0'])
+  endif
+
+  let l:lsp.job = go#job#Start(l:cmd, l:opts)
 
   return l:lsp
 endfunction
@@ -759,6 +766,17 @@ function! go#lsp#ResetWorkspaceDirectories() abort
   return 0
 endfunction
 
+function! go#lsp#DebugBrowser() abort
+  let l:lsp = s:lspfactory.get()
+  let l:port = get(l:lsp, 'debugport', 0)
+  if !l:port
+    call go#util#EchoError("gopls was not started with debugging enabled. See :help g:go_debug.")
+    return
+  endif
+
+  call go#util#OpenBrowser(printf('http://localhost:%d', l:port))
+endfunction
+
 function! s:debug(event, data) abort
   if !go#util#HasDebug('lsp')
     return

doc/vim-go.txt

@@ -906,11 +906,16 @@ CTRL-t
     tries to preserve cursor position and avoids replacing the buffer with
     stderr output.
 
-                                                           *:GoAddWorkspace*
+                                                            *:GoAddWorkspace*
 :GoAddWorkspace [dir] ...
 
     Add directories to the `gopls` workspace.
 
+                                                         *:GoLSPDebugBrowser*
+:GoLSPDebugBrowser
+
+    Open a browser to see gopls debugging information.
+
 ==============================================================================
 MAPPINGS                                                        *go-mappings*
 
@@ -1241,9 +1246,9 @@ set to 10 seconds . >
 <
                                                  *'g:go_play_browser_command'*
 
-Browser to use for |:GoPlay| or |:GoDocBrowser|. The url must be added with
-`%URL%`, and it's advisable to include `&` to make sure the shell returns. For
-example:
+Browser to use for |:GoPlay|, |:GoDocBrowser|, and |:GoLSPDebugBrowser|. The
+url must be added with `%URL%`, and it's advisable to include `&` to make sure
+the shell returns. For example:
 >
   let g:go_play_browser_command = 'firefox-developer %URL% &'
 <
@@ -1817,7 +1822,9 @@ Currently accepted values:
   debugger-commands  Echo communication between vim-go and `dlv`; requests and
                      responses are recorded in `g:go_debug_commands`.
   lsp                Echo communication between vim-go and `gopls`. All
-                     communication is shown in a dedicated window.
+                     communication is shown in a dedicated window. When
+                     enabled before gopls is started, `:GoLSPDebugBrowser` can
+                     be used to open a browser window to help debug gopls.
 >
       let g:go_debug = []
 <

ftplugin/go/commands.vim

@@ -118,6 +118,7 @@ command! -nargs=0 GoIfErr call go#iferr#Generate()
 
 " -- lsp
 command! -nargs=+ -complete=dir GoAddWorkspace call go#lsp#AddWorkspaceDirectory(<f-args>)
+command! -nargs=0 GoLSPDebugBrowser call go#lsp#DebugBrowser()
 
 " -- term
 command! GoToggleTermCloseOnExit call go#term#ToggleCloseOnExit()