diff --git a/runtime/doc/lua.txt b/runtime/doc/lua.txt
index cbe283065d..e793a97fe1 100644
--- a/runtime/doc/lua.txt
+++ b/runtime/doc/lua.txt
@@ -715,7 +715,7 @@ vim.diff({a}, {b}, {opts})                                        *vim.diff()*
       • {b}     (`string`) Second string to compare
       • {opts}  (`table?`) Optional parameters:
                 • {on_hunk}?
-                  (`fun(start_a: integer, count_a: integer, start_b: integer, count_b: integer): integer`)
+                  (`fun(start_a: integer, count_a: integer, start_b: integer, count_b: integer): integer?`)
                   Invoked for each hunk in the diff. Return a negative number
                   to cancel the callback for any remaining hunks. Arguments:
                   • `start_a` (`integer`): Start line of hunk in {a}.
diff --git a/runtime/lua/vim/_meta/diff.lua b/runtime/lua/vim/_meta/diff.lua
index 5a0de68119..4803ed4775 100644
--- a/runtime/lua/vim/_meta/diff.lua
+++ b/runtime/lua/vim/_meta/diff.lua
@@ -11,7 +11,7 @@
 ---   - `count_a` (`integer`): Hunk size in {a}.
 ---   - `start_b` (`integer`): Start line of hunk in {b}.
 ---   - `count_b` (`integer`): Hunk size in {b}.
---- @field on_hunk? fun(start_a: integer, count_a: integer, start_b: integer, count_b: integer): integer
+--- @field on_hunk? fun(start_a: integer, count_a: integer, start_b: integer, count_b: integer): integer?
 ---
 --- Form of the returned diff:
 ---   - `unified`: String in unified format.
diff --git a/test/functional/lua/xdiff_spec.lua b/test/functional/lua/xdiff_spec.lua
index ed65193244..269dbde10a 100644
--- a/test/functional/lua/xdiff_spec.lua
+++ b/test/functional/lua/xdiff_spec.lua
@@ -12,15 +12,11 @@ describe('xdiff bindings', function()
   end)
 
   describe('can diff text', function()
-    before_each(function()
-      exec_lua [[
-        a1 = 'Hello\n'
-        b1 = 'Helli\n'
+    local a1 = 'Hello\n'
+    local b1 = 'Helli\n'
 
-        a2 = 'Hello\nbye\nfoo\n'
-        b2 = 'Helli\nbye\nbar\nbaz\n'
-      ]]
-    end)
+    local a2 = 'Hello\nbye\nfoo\n'
+    local b2 = 'Helli\nbye\nbar\nbaz\n'
 
     it('with no callback', function()
       eq(
@@ -30,7 +26,9 @@ describe('xdiff bindings', function()
           '+Helli',
           '',
         }, '\n'),
-        exec_lua('return vim.diff(a1, b1)')
+        exec_lua(function()
+          return vim.diff(a1, b1)
+        end)
       )
 
       eq(
@@ -44,63 +42,81 @@ describe('xdiff bindings', function()
           '+baz',
           '',
         }, '\n'),
-        exec_lua('return vim.diff(a2, b2)')
+        exec_lua(function()
+          return vim.diff(a2, b2)
+        end)
       )
     end)
 
     it('with callback', function()
-      exec_lua([[on_hunk = function(sa, ca, sb, cb)
-          exp[#exp+1] = {sa, ca, sb, cb}
-        end]])
-
       eq(
         { { 1, 1, 1, 1 } },
-        exec_lua [[
-          exp = {}
-          assert(vim.diff(a1, b1, {on_hunk = on_hunk}) == nil)
+        exec_lua(function()
+          local exp = {} --- @type table[]
+          assert(vim.diff(a1, b1, {
+            on_hunk = function(...)
+              exp[#exp + 1] = { ... }
+            end,
+          }) == nil)
           return exp
-        ]]
+        end)
       )
 
       eq(
         { { 1, 1, 1, 1 }, { 3, 1, 3, 2 } },
-        exec_lua [[
-          exp = {}
-          assert(vim.diff(a2, b2, {on_hunk = on_hunk}) == nil)
+        exec_lua(function()
+          local exp = {} --- @type table[]
+          assert(vim.diff(a2, b2, {
+            on_hunk = function(...)
+              exp[#exp + 1] = { ... }
+            end,
+          }) == nil)
           return exp
-        ]]
+        end)
       )
 
       -- gives higher precedence to on_hunk over result_type
       eq(
         { { 1, 1, 1, 1 }, { 3, 1, 3, 2 } },
-        exec_lua [[
-          exp = {}
-          assert(vim.diff(a2, b2, {on_hunk = on_hunk, result_type='indices'}) == nil)
+        exec_lua(function()
+          local exp = {} --- @type table[]
+          assert(vim.diff(a2, b2, {
+            on_hunk = function(...)
+              exp[#exp + 1] = { ... }
+            end,
+            result_type = 'indices',
+          }) == nil)
           return exp
-        ]]
+        end)
       )
     end)
 
     it('with error callback', function()
-      exec_lua [[
-        on_hunk = function(sa, ca, sb, cb)
-          error('ERROR1')
-        end
-      ]]
-
       eq(
-        [[error running function on_hunk: [string "<nvim>"]:0: ERROR1]],
-        pcall_err(exec_lua, [[vim.diff(a1, b1, {on_hunk = on_hunk})]])
+        [[.../xdiff_spec.lua:0: error running function on_hunk: .../xdiff_spec.lua:0: ERROR1]],
+        pcall_err(exec_lua, function()
+          vim.diff(a1, b1, {
+            on_hunk = function()
+              error('ERROR1')
+            end,
+          })
+        end)
       )
     end)
 
     it('with hunk_lines', function()
-      eq({ { 1, 1, 1, 1 } }, exec_lua([[return vim.diff(a1, b1, {result_type = 'indices'})]]))
+      eq(
+        { { 1, 1, 1, 1 } },
+        exec_lua(function()
+          return vim.diff(a1, b1, { result_type = 'indices' })
+        end)
+      )
 
       eq(
         { { 1, 1, 1, 1 }, { 3, 1, 3, 2 } },
-        exec_lua([[return vim.diff(a2, b2, {result_type = 'indices'})]])
+        exec_lua(function()
+          return vim.diff(a2, b2, { result_type = 'indices' })
+        end)
       )
     end)
 
@@ -143,16 +159,11 @@ describe('xdiff bindings', function()
           '+}',
           '',
         }, '\n'),
-        exec_lua(
-          [[
-          local args = {...}
-          return vim.diff(args[1], args[2], {
-            algorithm = 'patience'
+        exec_lua(function()
+          return vim.diff(a, b, {
+            algorithm = 'patience',
           })
-        ]],
-          a,
-          b
-        )
+        end)
       )
     end)
   end)