fix(lsp): revert text edit application order change (#29877)

Reverts https://github.com/neovim/neovim/pull/29212 and adds a few
additional test cases

From the spec

> All text edits ranges refer to positions in the document they are
> computed on. They therefore move a document from state S1 to S2 without
> describing any intermediate state. Text edits ranges must never overlap,
> that means no part of the original document must be manipulated by more
> than one edit. However, it is possible that multiple edits have the same
> start position: multiple inserts, or any number of inserts followed by a
> single remove or replace edit. If multiple inserts have the same
> position, the order in the array defines the order in which the inserted
> strings appear in the resulting text.

The previous fix seems wrong. The important part:

> If multiple inserts have the same position, the order in the array
> defines the order in which the inserted strings appear in the
> resulting text.

Emphasis on _appear in the resulting text_

Which means that in:

    local edits1 = {
      make_edit(0, 3, 0, 3, { 'World' }),
      make_edit(0, 3, 0, 3, { 'Hello' }),
    }

`World` must appear before `Hello` in the final text. That means the old
logic was correct, and the fix was wrong.

runtime/lua/vim/lsp/util.lua

@@ -396,10 +396,7 @@ function M.apply_text_edits(text_edits, bufnr, offset_encoding)
     if a.range.start.character ~= b.range.start.character then
       return a.range.start.character > b.range.start.character
     end
-    if a._index ~= b._index then
+    return a._index > b._index
-      return a._index < b._index
-    end
-    return false
   end)
 
   -- save and restore local marks since they get deleted by nvim_buf_set_lines

test/functional/plugin/lsp_spec.lua

@@ -1756,6 +1756,58 @@ describe('LSP', function()
     }
   end
 
+  describe('apply vscode text_edits', function()
+    it('single replace', function()
+      insert('012345678901234567890123456789')
+      local edits = {
+        make_edit(0, 3, 0, 6, { 'Hello' }),
+      }
+      exec_lua('vim.lsp.util.apply_text_edits(...)', edits, 1, 'utf-16')
+      eq({ '012Hello678901234567890123456789' }, buf_lines(1))
+    end)
+    it('two replaces', function()
+      insert('012345678901234567890123456789')
+      local edits = {
+        make_edit(0, 3, 0, 6, { 'Hello' }),
+        make_edit(0, 6, 0, 9, { 'World' }),
+      }
+      exec_lua('vim.lsp.util.apply_text_edits(...)', edits, 1, 'utf-16')
+      eq({ '012HelloWorld901234567890123456789' }, buf_lines(1))
+    end)
+    it('same start pos insert are kept in order', function()
+      insert('012345678901234567890123456789')
+      local edits1 = {
+        make_edit(0, 3, 0, 3, { 'World' }),
+        make_edit(0, 3, 0, 3, { 'Hello' }),
+      }
+      exec_lua('vim.lsp.util.apply_text_edits(...)', edits1, 1, 'utf-16')
+      eq({ '012WorldHello345678901234567890123456789' }, buf_lines(1))
+    end)
+    it('same start pos insert and replace are kept in order', function()
+      insert('012345678901234567890123456789')
+      local edits1 = {
+        make_edit(0, 3, 0, 3, { 'World' }),
+        make_edit(0, 3, 0, 3, { 'Hello' }),
+        make_edit(0, 3, 0, 8, { 'No' }),
+      }
+      exec_lua('vim.lsp.util.apply_text_edits(...)', edits1, 1, 'utf-16')
+      eq({ '012WorldHelloNo8901234567890123456789' }, buf_lines(1))
+    end)
+    it('multiline', function()
+      exec_lua([[
+        vim.api.nvim_buf_set_lines(1, 0, 0, true, {'  {', '    "foo": "bar"', '  }'})
+      ]])
+      eq({ '  {', '    "foo": "bar"', '  }', '' }, buf_lines(1))
+      local edits = {
+        make_edit(0, 0, 3, 0, { '' }),
+        make_edit(3, 0, 3, 0, { '{\n' }),
+        make_edit(3, 0, 3, 0, { '  "foo": "bar"\n' }),
+        make_edit(3, 0, 3, 0, { '}\n' }),
+      }
+      exec_lua('vim.lsp.util.apply_text_edits(...)', edits, 1, 'utf-16')
+      eq({ '{', '  "foo": "bar"', '}', '' }, buf_lines(1))
+    end)
+  end)
   describe('apply_text_edits', function()
     before_each(function()
       insert(dedent([[
@@ -1794,9 +1846,9 @@ describe('LSP', function()
       }
       exec_lua('vim.lsp.util.apply_text_edits(...)', edits, 1, 'utf-16')
       eq({
-        '3',
+        '',
-        'foo',
+        '123',
-        '12Fbar',
+        'fooFbar',
         '123irst guy',
         'baz line of text',
         'The next line of text',
@@ -1818,9 +1870,9 @@ describe('LSP', function()
       }
       exec_lua('vim.lsp.util.apply_text_edits(...)', edits, 1, 'utf-16')
       eq({
-        '3',
+        '',
-        'foo',
+        '123',
-        '12Fbar',
+        'fooFbar',
         '123irst guy',
         'baz line of text',
         'The next line of text',