use gopls for balloons

autoload/go/lsp.vim

@@ -437,6 +437,23 @@ function! s:completionErrorHandler(next, error) abort dict
   call call(a:next, [[]])
 endfunction
 
+function! go#lsp#Hover(fname, line, col, handler)
+  call go#lsp#DidChange(a:fname)
+
+  let l:lsp = s:lspfactory.get()
+  let l:msg = go#lsp#message#Hover(a:fname, a:line, a:col)
+  let l:state = s:newHandlerState('hover')
+  let l:state.handleResult = funcref('s:hoverHandler', [function(a:handler, [], l:state)], l:state)
+  call l:lsp.sendMessage(l:msg, l:state)
+endfunction
+
+function! s:hoverHandler(next, msg) abort dict
+  " gopls returns a MarkupContent.
+  let l:content = substitute(a:msg.contents.value, '```go\n\(.*\)\n```', '\1', '')
+  let l:args = [l:content]
+  call call(a:next, l:args)
+endfunction
+
 " restore Vi compatibility settings
 let &cpo = s:cpo_save
 unlet s:cpo_save

autoload/go/lsp/message.vim

@@ -100,6 +100,19 @@ function! go#lsp#message#Completion(file, line, col)
        \ }
 endfunction
 
+function! go#lsp#message#Hover(file, line, col)
+  return {
+          \ 'notification': 0,
+          \ 'method': 'textDocument/hover',
+          \ 'params': {
+          \   'textDocument': {
+          \       'uri': go#path#ToURI(a:file)
+          \   },
+          \   'position': s:position(a:line, a:col),
+          \ }
+       \ }
+endfunction
+
 function! s:position(line, col)
   return {'line': a:line - 1, 'character': a:col-1}
 endfunction

autoload/go/tool.vim

@@ -112,7 +112,18 @@ function! go#tool#Exists(importpath) abort
 endfunction
 
 function! go#tool#DescribeBalloon()
-  return go#guru#DescribeBalloon()
+  let l:fname = fnamemodify(bufname(v:beval_bufnr), ':p')
+  call go#lsp#Hover(l:fname, v:beval_lnum, v:beval_col, funcref('s:balloon', []))
+  return ''
+endfunction
+
+function! s:balloon(msg)
+  if has('balloon_eval')
+    call balloon_show(a:msg)
+    return
+  endif
+
+  call balloon_show(balloon_split(a:msg))
 endfunction
 
 " restore Vi compatibility settings