When the start parameter is missing, the behabiour is arbitary

It seems that the behaviour of the start parameter being missing is
server (or perhaps a specific client) dependent. The specification
clearely says that it should be inserted at the column of the original
request, but the servers clearly expect either for that column to be the
beginning of an identifier or for the client to ignore the spec and
request from that position anyway.

Reading the VSCode code, we see that the 'word' before the cursor is
guessed, and if only if BOTH 'start' AND 'length' are supplied, then
they are used to determine where insertion starts, otherwise the current
'word' is used. Unclear what 'word' means in the specific contexts, but
we're relying on iskeyword.

autoload/vimspector.vim

@@ -19,6 +19,13 @@ let s:save_cpo = &cpoptions
 set cpoptions&vim
 " }}}
 
+function! s:Debug( ... )
+  py3 <<EOF
+if _vimspector_session is not None:
+  _vimspector_session._logger.debug( *vim.eval( 'a:000' ) )
+EOF
+endfunction
+
 
 let s:enabled = vimspector#internal#state#Reset()
 
@@ -271,28 +278,37 @@ function! vimspector#CompleteFuncSync( prompt, find_start, query ) abort
 
     let line = getline( line( '.' ) )[ len( a:prompt ) : ]
     let col = col( '.' ) - len( a:prompt )
+
+    " It seems that most servers don't implement the 'start' parameter, which is
+    " clearly necessary, as they all seem to assume a specific behaviour, which
+    " is undocumented.
+
     let s:latest_completion_request.items =
           \ py3eval( '_vimspector_session.GetCompletionsSync( '
                    \.'  vim.eval( "line" ), '
                    \.'  int( vim.eval( "col" ) ) )' )
+
     let s:latest_completion_request.line = line
     let s:latest_completion_request.col = col
 
+    let prev_non_keyword_char = match( line[ 0 : col - 1 ], '\k*$' ) + 1
+    let query_len = col - prev_non_keyword_char
+
     let start_pos = col
     for item in s:latest_completion_request.items
-      if !has_key( item, 'start' )
-        let item.start = col
+      if !has_key( item, 'start' ) || !has_key( item, 'length' )
+        " The specification states that if start is not supplied, isertion
+        " should be at the requested column. But about 0 of the servers actually
+        " implement that
+        " (https://github.com/microsoft/debug-adapter-protocol/issues/138)
+        let item.start = prev_non_keyword_char
+        let item.length = query_len
       else
         " For some reason, the returned start value is 0-indexed even though we
         " use columnsStartAt1
-        " TODO need to check this with the other servers that exist
         let item.start += 1
       endif
-      " TODO/FIXME. I don't think it's possible to implement the 'length'
-      " parameter, using vim's completion system
-      " if !has_key( item, 'length' )
-      "   let item.length = 0
-      " endif
+
       if !has_key( item, 'text' )
         let item.text = item.label
       endif
@@ -305,6 +321,14 @@ function! vimspector#CompleteFuncSync( prompt, find_start, query ) abort
     let s:latest_completion_request.start_pos = start_pos
     let s:latest_completion_request.prompt = a:prompt
 
+    call s:Debug( 'FindStart: %s', {
+          \ 'line': line,
+          \ 'col': col,
+          \ 'prompt': len( a:prompt ),
+          \ 'start_pos': start_pos,
+          \ 'returning': ( start_pos + len( a:prompt ) ) - 1,
+          \ } )
+
     " start_pos is 1-based and the return of findstart is 0-based
     return ( start_pos + len( a:prompt ) ) - 1
   else
@@ -316,18 +340,27 @@ function! vimspector#CompleteFuncSync( prompt, find_start, query ) abort
         " that would be erased by the fixed-up earlier start position)
         "
         " both start_pos and item.start are 1-based
-        let item.text = s:latest_completion_request.text[
+        let item.text = s:latest_completion_request.line[
               \ s:latest_completion_request.start_pos + pfxlen - 1 :
               \  item.start + pfxlen - 1 ] . item.text
       endif
 
+      if item.length > len( a:query )
+        call s:Debug( 'Rejecting %s, length is greater than %s',
+              \ item,
+              \ len( a:query ) )
+        continue
+      endif
+
       call add( items, { 'word': item.text,
                        \ 'abbr': item.label,
                        \ 'menu': get( item, 'type', '' ),
                        \ 'icase': 1,
                        \ } )
     endfor
     let s:latest_completion_request = {}
+
+    call s:Debug( 'Items: %s', items )
     return { 'words': items, 'refresh': 'always' }
   endif
 endfunction

support/test/node/simple/simple.js

@@ -1,3 +1,10 @@
 var msg = 'Hello, world!'
 
-console.log( "OK stuff happened" )
+var obj = {
+  test: 'testing',
+  toast: function() {
+    return 'toasty' . this.test;
+  }
+}
+
+console.log( "OK stuff happened " + obj.toast() )