fix(lsp): don't overlay insertion-style inline completions (#36477)

* feat(lua): `Range:is_empty()` to check vim.range emptiness

* fix(lsp): don't overlay insertion-style inline completions

**Problem:** Some servers commonly respond with an empty inline
completion range which acts as a position where text should be inserted.
However, the inline completion module assumes that all responses with a
range are deletions + insertions that thus require an `overlay` display
style. This causes an incorrect preview, because the virtual text should
have the `inline` display style (to reflect that this is purely an
insertion).

**Solution:** Only use `overlay` for non-empty replacement ranges.

runtime/doc/lua.txt

@@ -4136,6 +4136,7 @@ Provides operations to compare, calculate, and convert ranges represented by
     Fields: ~
       • {start}      (`vim.Pos`) Start position.
       • {end_}       (`vim.Pos`) End position, exclusive.
+      • {is_empty}   (`fun(self: vim.Range): boolean`) See |Range:is_empty()|.
       • {has}        (`fun(outer: vim.Range, inner: vim.Range): boolean`) See
                      |Range:has()|.
       • {intersect}  (`fun(r1: vim.Range, r2: vim.Range): vim.Range?`) See
@@ -4167,6 +4168,12 @@ Range:intersect({r1}, {r2})                                *Range:intersect()*
         (`vim.Range?`) range that is present inside both `r1` and `r2`. `nil`
         if such range does not exist. See |vim.Range|.
 
+Range:is_empty()                                            *Range:is_empty()*
+    Checks whether the given range is empty; i.e., start >= end.
+
+    Return: ~
+        (`boolean`) `true` if the given range is empty
+
 Range:lsp({buf}, {range}, {position_encoding})                   *Range:lsp()*
     Creates a new |vim.Range| from `lsp.Range`.
 

runtime/doc/news.txt

@@ -280,6 +280,7 @@ LUA
 • Experimental `vim.pos` and `vim.range` for Position/Range abstraction.
 • |vim.json.encode()| has an `indent` option for pretty-formatting.
 • |vim.json.encode()| has an `sort_keys` option.
+• |Range:is_empty()| to check if a |vim.Range| is empty.
 
 OPTIONS
 

runtime/lua/vim/lsp/inline_completion.lua

@@ -248,7 +248,7 @@ function Completor:show(hint)
   api.nvim_buf_set_extmark(self.bufnr, namespace, row, col, {
     virt_text = virt_text,
     virt_lines = virt_lines,
-    virt_text_pos = current.range and 'overlay' or 'inline',
+    virt_text_pos = (current.range and not current.range:is_empty() and 'overlay') or 'inline',
     hl_mode = 'combine',
   })
 end

runtime/lua/vim/range.lua

@@ -99,6 +99,13 @@ function Range.__eq(r1, r2)
   return r1.start == r2.start and r1.end_ == r2.end_
 end
 
+--- Checks whether the given range is empty; i.e., start >= end.
+---
+---@return boolean `true` if the given range is empty
+function Range:is_empty()
+  return self.start >= self.end_
+end
+
 --- Checks whether {outer} range contains {inner} range.
 ---
 ---@param outer vim.Range

test/functional/plugin/lsp/inline_completion_spec.lua

@@ -83,6 +83,27 @@ describe('vim.lsp.inline_completion', function()
         },
         handlers = {
           ['textDocument/inlineCompletion'] = function(_, _, callback)
+            if _G.empty then
+              callback(nil, {
+                items = {
+                  {
+                    insertText = 'foobar',
+                    range = {
+                      start = {
+                        line = 0,
+                        character = 19,
+                      },
+                      ['end'] = {
+                        line = 0,
+                        character = 19,
+                      },
+                    },
+                  },
+                },
+              })
+              return
+            end
+
             callback(nil, {
               items = {
                 {
@@ -198,6 +219,19 @@ describe('vim.lsp.inline_completion', function()
       screen:expect({ grid = grid_applied_candidates })
     end)
 
+    it('correctly displays with absent/empty range', function()
+      exec_lua(function()
+        _G.empty = true
+      end)
+      feed('I')
+      screen:expect([[
+        function fibonacci({1:foobar})                           |
+        ^                                                     |
+        {1:~                                                    }|*11
+        {3:-- INSERT --}                                         |
+      ]])
+    end)
+
     it('accepts on_accept callback', function()
       feed('i')
       screen:expect({ grid = grid_with_candidates })