fix(lsp): prevent desync due to empty buffer (#29904)

Problem:
Some language servers (e.g., rust-analyzer, texlab) are desynced when
the user deletes the entire contents of the buffer. This is due to the
discrepancy between how nvim computes diff and how nvim treats empty
buffer.
* diff: If the buffer became empty, then the diff includes the last
  line's eol.
* empty buffer: Even if the buffer is empty, nvim regards it as having
  a single empty line with eol.

Solution:
Add special case for diff computation when the buffer becomes empty so
that it does not include the eol of the last line.

runtime/lua/vim/lsp/sync.lua

@@ -212,7 +212,8 @@ end
 ---@param lastline integer
 ---@param new_lastline integer
 ---@param offset_encoding string
----@return vim.lsp.sync.Range, vim.lsp.sync.Range
+---@return vim.lsp.sync.Range prev_end_range
+---@return vim.lsp.sync.Range curr_end_range
 local function compute_end_range(
   prev_lines,
   curr_lines,
@@ -222,6 +223,16 @@ local function compute_end_range(
   new_lastline,
   offset_encoding
 )
+  -- A special case for the following `firstline == new_lastline` case where lines are deleted.
+  -- Even if the buffer has become empty, nvim behaves as if it has an empty line with eol.
+  if #curr_lines == 1 and curr_lines[1] == '' then
+    local prev_line = prev_lines[lastline - 1]
+    return {
+      line_idx = lastline - 1,
+      byte_idx = #prev_line + 1,
+      char_idx = compute_line_length(prev_line, offset_encoding) + 1,
+    }, { line_idx = 1, byte_idx = 1, char_idx = 1 }
+  end
   -- If firstline == new_lastline, the first change occurred on a line that was deleted.
   -- In this case, the last_byte...
   if firstline == new_lastline then

test/functional/plugin/lsp/incremental_sync_spec.lua

@@ -170,7 +170,7 @@ describe('incremental synchronization', function()
       }
       test_edit({ 'a' }, { 'rb' }, expected_text_changes, 'utf-16', '\n')
     end)
-    it('deleting a line', function()
+    it('deleting the first line', function()
       local expected_text_changes = {
         {
           range = {
@@ -183,11 +183,49 @@ describe('incremental synchronization', function()
               line = 1,
             },
           },
-          rangeLength = 12,
+          rangeLength = 6,
           text = '',
         },
       }
-      test_edit({ 'hello world' }, { 'dd' }, expected_text_changes, 'utf-16', '\n')
+      test_edit({ 'hello', 'world' }, { 'ggdd' }, expected_text_changes, 'utf-16', '\n')
+    end)
+    it('deleting the last line', function()
+      local expected_text_changes = {
+        {
+          range = {
+            ['start'] = {
+              character = 0,
+              line = 1,
+            },
+            ['end'] = {
+              character = 0,
+              line = 2,
+            },
+          },
+          rangeLength = 6,
+          text = '',
+        },
+      }
+      test_edit({ 'hello', 'world' }, { '2ggdd' }, expected_text_changes, 'utf-16', '\n')
+    end)
+    it('deleting all lines', function()
+      local expected_text_changes = {
+        {
+          range = {
+            ['start'] = {
+              character = 0,
+              line = 0,
+            },
+            ['end'] = {
+              character = 5,
+              line = 1,
+            },
+          },
+          rangeLength = 11,
+          text = '',
+        },
+      }
+      test_edit({ 'hello', 'world' }, { 'ggdG' }, expected_text_changes, 'utf-16', '\n')
     end)
     it('deleting an empty line', function()
       local expected_text_changes = {