doc: get package from LSP

Get the package name of the identifier under the cursor using the doc
link provided by lsp to improve performance vs trying to rely on go list
via go#tool#Imports() to get the package name.

Fixes fatih#3546

autoload/go/doc.vim

@@ -11,42 +11,56 @@ scriptencoding utf-8
 let s:buf_nr = -1
 
 function! go#doc#OpenBrowser(...) abort
+  let l:url = call('s:docURL', a:000)
+  if l:url is ''
+    call go#util#EchoWarning("could not find path for doc URL")
+    return
+  endif
+  call go#util#OpenBrowser(l:url)
+endfunction
+
+function! s:docURL() abort
   if len(a:000) == 0
+    " call go#lsp#DocLink directly instead of s:docURLFor, because s:docURLFor
+    " will strip any version information from the URL.
     let [l:out, l:err] = go#lsp#DocLink()
-    if l:err
-      call go#util#EchoError(l:out)
-      return
-    endif
-
-    if len(l:out) == 0
-      call go#util#EchoWarning("could not find path for doc URL")
-      return
+    if !(l:err || len(l:out) is 0)
+      let l:url = printf('%s/%s', go#config#DocUrl(), l:out)
+    else
+      let l:url = ''
     endif
+  else
+    let l:url = call('s:docURLFor', a:000)
+  endif
 
-    let l:godoc_url = printf('%s/%s', go#config#DocUrl(), l:out)
+  return l:url
+endfunction
 
-    call go#util#OpenBrowser(l:godoc_url)
-    return
+function! s:docURLFor(...) abort
+  let l:identifier = call('s:godocIdentifier', a:000)
+  if empty(l:identifier)
+    return ''
   endif
 
-  let pkgs = s:godocWord(a:000)
-  if empty(pkgs)
-    return
+  let l:pkg = l:identifier[0]
+  let l:exported_name = ''
+  if len(l:identifier) > 1
+    let l:exported_name = l:identifier[1]
   endif
 
-  let pkg = pkgs[0]
-  let exported_name = pkgs[1]
-
   " example url: https://godoc.org/github.com/fatih/set#Set
-  let godoc_url = printf('%s/%s#%s', go#config#DocUrl(), pkg, exported_name)
-  call go#util#OpenBrowser(godoc_url)
+  return printf('%s/%s#%s', go#config#DocUrl(), l:pkg, l:exported_name)
 endfunction
 
 function! go#doc#Open(newmode, mode, ...) abort
   let l:words = a:000
   let l:package = ''
   if a:0 is 0
-    let l:words = s:godocWord()
+    let l:words = s:godocIdentifier()
+    if len(l:words) is 0
+      call go#util#EchoWarning("could not find doc identifier")
+      return
+    endif
     let l:package = l:words[0]
   endif
 
@@ -200,19 +214,9 @@ endfunction
 " returns the package and exported name. exported name might be empty.
 " ie: fmt and Println
 " ie: github.com/fatih/set and New
-function! s:godocWord(...) abort
-  let words = a:000
-  if a:0 is 0
-    let oldiskeyword = &iskeyword
-    " TODO(bc): include / in iskeyword when filetype is godoc?
-    setlocal iskeyword+=.
-    let word = expand('<cword>')
-    let &iskeyword = oldiskeyword
-    let word = substitute(word, '[^a-zA-Z0-9\\/._~-]', '', 'g')
-    let words = split(word, '\.\ze[^./]\+$')
-  endif
-
-  if !len(words)
+function s:godocIdentifier(...) abort
+  let l:words = call('s:godocWord', a:000)
+  if empty(l:words)
     return []
   endif
 
@@ -229,15 +233,37 @@ function! s:godocWord(...) abort
     let exported_name = words[1]
   endif
 
-  if &filetype isnot 'godoc'
-    let packages = go#tool#Imports()
-    if has_key(packages, pkg)
-      let pkg = packages[pkg]
+  return [pkg, exported_name]
+endfunction
+
+function! s:godocWord(...) abort
+  let l:words = a:000
+  if a:0 is 0
+    if &filetype isnot 'godoc'
+      let [l:out, l:err] = go#lsp#DocLink()
+      if !(l:err || len(l:out) is 0)
+        " strip out any version string in the doc link path.
+        let l:out = substitute(l:out, '@v[^/]\+', '', '')
+        let words = split(l:out, '#')
+      else
+        let l:words = s:godocCursorWord()
+      endif
+    else
+      let l:words = s:godocCursorWord()
     endif
   endif
 
+  return l:words
+endfunction
 
-  return [pkg, exported_name]
+function! s:godocCursorWord() abort
+  let l:oldiskeyword = &iskeyword
+  " TODO(bc): include / in iskeyword when filetype is godoc?
+  setlocal iskeyword+=.
+  let l:word = expand('<cword>')
+  let &iskeyword = l:oldiskeyword
+  let l:word = substitute(l:word, '[^a-zA-Z0-9\\/._~-]', '', 'g')
+  return split(l:word, '\.\ze[^./]\+$')
 endfunction
 
 " restore Vi compatibility settings