Handle length/start in completions response in repl.omnifunc

Doesn't handle mixed cases yet but should fix the cpptools case
mentioned in #1037

Author: mfussenegger

lua/dap/protocol.lua

@@ -248,4 +248,38 @@
 ---@field request 'launch'|'attach'
 
 
+---@alias dap.CompletionItemType
+---|'method'
+---|'function'
+---|'constructor'
+---|'field'
+---|'variable'
+---|'class'
+---|'interface'
+---|'module'
+---|'property'
+---|'unit'
+---|'value'
+---|'enum'
+---|'keyword'
+---|'snippet'
+---|'text'
+---|'color'
+---|'file'
+---|'reference'
+---|'customcolor'
+
+
+---@class dap.CompletionItem
+---@field label string By default this is also the text that is inserted when selecting this completion
+---@field text? string If present and not empty this is inserted instead of the label
+---@field sortText? string Used to sort completion items if present and not empty. Otherwise label is used
+---@field detail? string human-readable string with additional information about this item. Like type or symbol information
+---@field type? dap.CompletionItemType
+---@field start? number Start position in UTF-16 code units. (within the `text` attribute of the `completions` request) 0- or 1-based depending on `columnsStartAt1` capability. If omitted, the text is added at the location of the `column` attribute of the `completions` request.
+---@field length? number How many characters are overwritten by the completion text. Measured in UTF-16 code units. If missing the value 0 is assumed which results in the completion text being inserted.
+---@field selectionStart? number
+---@field selectionLength? number
+
+
 ---@alias dap.SteppingGranularity 'statement'|'line'|'instruction'

lua/dap/repl.lua

@@ -409,6 +409,24 @@ do
     return items
   end
 
+  ---@param items dap.CompletionItem[]
+  ---@return boolean mixed, integer? start
+  local function get_start(items)
+    local start = nil
+    local mixed = false
+    for _, item in ipairs(items) do
+      if item.start and (item.length or 0) > 0 then
+        if start and start ~= item.start then
+          mixed = true
+          start = math.min(start, item.start)
+        else
+          start = item.start
+        end
+      end
+    end
+    return mixed, start
+  end
+
   function M.omnifunc(findstart, base)
     local session = get_session()
     local col = api.nvim_win_get_cursor(0)[2]
@@ -456,8 +474,13 @@ do
         require('dap.utils').notify('completions request failed: ' .. err.message, vim.log.levels.WARN)
         return
       end
-      local items = completions_to_items(response.targets)
-      vim.fn.complete(offset + text_match + 1, items)
+      local items = response.targets --[[@as dap.CompletionItem[]|]]
+      local mixed, start = get_start(items)
+      if start and not mixed then
+        vim.fn.complete(offset + start + 1, completions_to_items(items))
+      else
+        vim.fn.complete(offset + text_match + 1, completions_to_items(items))
+      end
     end)
 
     -- cancel but stay in completion mode for completion via `completions` callback