Highlight active parameter in signatureHelp (#497)

* Highlight active parameter in signatureHelp

* Support `[number, number]` form of parameter label.

* Support parameter's documentation.

* Fix extracting parameter label

Author: hrsh7th

autoload/lsp/ui/vim/signature_help.vim

@@ -38,11 +38,43 @@ function! s:handle_signature_help(server, data) abort
     endif
 
     if !empty(a:data['response']['result']) && !empty(a:data['response']['result']['signatures'])
-        let l:signature = a:data['response']['result']['signatures'][0]
-        let l:contents = [l:signature['label']]
+        " Get current signature.
+        let l:signatures = get(a:data['response']['result'], 'signatures', [])
+        let l:signature_index = get(a:data['response']['result'], 'activeSignature', 0)
+        let l:signature = get(l:signatures, l:signature_index, {})
+        if empty(l:signature)
+            return
+        endif
+
+        " Signature label.
+        let l:label = l:signature['label']
+
+        " Mark current parameter.
+        if has_key(a:data['response']['result'], 'activeParameter')
+            let l:parameters = get(l:signature, 'parameters', [])
+            let l:parameter_index = a:data['response']['result']['activeParameter']
+            let l:parameter = get(l:parameters, l:parameter_index, {})
+            let l:parameter_label = s:get_parameter_label(l:signature, l:parameter)
+            if !empty(l:parameter_label)
+                let l:label = substitute(l:label, '\V\(' . escape(l:parameter_label, '\/?') . '\)', '`\1`', 'g')
+            endif
+        endif
+
+        let l:contents = [l:label]
+
+        if exists('l:parameter')
+            let l:parameter_doc = s:get_parameter_doc(l:parameter)
+            if !empty(l:parameter_doc)
+                call add(l:contents, '')
+                call add(l:contents, l:parameter_doc)
+                call add(l:contents, '')
+            endif
+        endif
+
         if has_key(l:signature, 'documentation')
             call add(l:contents, l:signature['documentation'])
         endif
+
         call lsp#ui#vim#output#preview(a:server, l:contents, {'statusline': ' LSP SignatureHelp'})
         return
     else
@@ -51,6 +83,33 @@ function! s:handle_signature_help(server, data) abort
     endif
 endfunction
 
+function! s:get_parameter_label(signature, parameter) abort
+    if has_key(a:parameter, 'label')
+        if type(a:parameter['label']) == v:t_list
+            let l:string_range = a:parameter['label']
+            return strcharpart(
+                        \ a:signature['label'],
+                        \ l:string_range[0],
+                        \ l:string_range[1] - l:string_range[0])
+        endif
+        return a:parameter['label']
+    endif
+    return ''
+endfunction
+
+function! s:get_parameter_doc(parameter) abort
+    if !has_key(a:parameter, 'documentation')
+        return ''
+    endif
+
+    if type(a:parameter['documentation']) == v:t_dict
+        let l:doc = copy(a:parameter['documentation'])
+        let l:doc['value'] = printf('***%s*** - %s', a:parameter['label'], l:doc['value'])
+        return l:doc
+    endif
+    return printf('***%s*** - %s', a:parameter['label'], l:doc)
+endfunction
+
 function! s:insert_char_pre() abort
     let l:buf = bufnr('%')
     for l:server_name in lsp#get_whitelisted_servers(l:buf)