fix(diff): set default diff flags properly (#35450)

src/nvim/diff.c

@@ -87,10 +87,10 @@ static bool diff_need_update = false;  // ex_diffupdate needs to be called
 #define DIFF_FOLLOWWRAP 0x800   // follow the wrap option
 #define DIFF_LINEMATCH  0x1000  // match most similar lines within diff
 #define ALL_WHITE_DIFF (DIFF_IWHITE | DIFF_IWHITEALL | DIFF_IWHITEEOL)
-static int diff_flags = DIFF_INTERNAL | DIFF_FILLER | DIFF_CLOSE_OFF;
+static int diff_flags = DIFF_INTERNAL | DIFF_FILLER | DIFF_CLOSE_OFF | DIFF_LINEMATCH;
 
 static int diff_algorithm = 0;
-static int linematch_lines = 0;
+static int linematch_lines = 40;
 
 #define LBUFLEN 50               // length of line in diff file
 

test/functional/core/startup_spec.lua

@@ -814,6 +814,42 @@ describe('startup', function()
     ]])
   end)
 
+  it("default 'diffopt' is applied with -d", function()
+    clear({
+      args = {
+        '-d',
+        'test/functional/fixtures/diff/startup_old.txt',
+        'test/functional/fixtures/diff/startup_new.txt',
+        '--cmd',
+        'set laststatus=0',
+      },
+    })
+    local screen = Screen.new(80, 24)
+    screen:expect([[
+      {7:+ }{13:^+-- 15 lines: package main············}│{7:+ }{13:+-- 15 lines: package main···········}|
+      {7:  }                                      │{7:  }                                     |
+      {7:  }func printCharacters(str string) strin│{7:  }func printCharacters(str string) stri|
+      {7:  }        return str                    │{7:  }        return str                   |
+      {7:  }}                                     │{7:  }}                                    |
+      {7:  }                                      │{7:  }                                     |
+      {7:  }func main() {                         │{7:  }func main() {                        |
+      {7:  }{23:--------------------------------------}│{7:  }{22:        hello := "Hello, World!"     }|
+      {7:  }{23:--------------------------------------}│{7:  }{22:                                     }|
+      {7:  }{4:        fmt.Print}{27:Ln}{4:(compressString("aa}│{7:  }{4:        fmt.Print(compressString("aaa}|
+      {7:  }{4:        fmt.Print}{27:Ln}{4:(compressString("go}│{7:  }{4:        fmt.Print(compressString("gol}|
+      {7:  }{4:        fmt.Print}{27:Ln}{4:(removeDuplicate("a}│{7:  }{4:        fmt.Print(removeDuplicate("aa}|
+      {7:  }{4:        fmt.Print}{27:Ln}{4:(reverseAndDouble("}│{7:  }{4:        fmt.Print(reverseAndDouble("a}|
+      {7:  }{4:        fmt.Print}{27:Ln}{4:(printCharacters("g}│{7:  }{4:        fmt.Print(printCharacters("go}|
+      {7:  }{23:--------------------------------------}│{7:  }{22:                                     }|
+      {7:  }{23:--------------------------------------}│{7:  }{22:        fmt.Println(hello)           }|
+      {7:  }}                                     │{7:  }}                                    |
+      {1:~                                       }│{1:~                                      }|*6
+                                                                                      |
+    ]])
+    command('let &diffopt = &diffopt')
+    screen:expect_unchanged()
+  end)
+
   it('does not crash if --embed is given twice', function()
     clear { args = { '--embed' } }
     assert_alive()

test/functional/fixtures/diff/startup_new.txt

@@ -0,0 +1,31 @@
+package main
+
+import "fmt"
+
+func compressString(str string) string {
+	return str
+}
+
+func removeDuplicate(str string) string {
+	return str
+}
+
+func reverseAndDouble(str string) string {
+	return str
+}
+
+func printCharacters(str string) string {
+	return str
+}
+
+func main() {
+	hello := "Hello, World!"
+
+	fmt.Print(compressString("aaaabbccgh"))
+	fmt.Print(compressString("golang"))
+	fmt.Print(removeDuplicate("aabccchbbccaaa"))
+	fmt.Print(reverseAndDouble("abcdfgh"))
+	fmt.Print(printCharacters("golang"))
+
+	fmt.Println(hello)
+}

test/functional/fixtures/diff/startup_old.txt

@@ -0,0 +1,27 @@
+package main
+
+import "fmt"
+
+func compressString(str string) string {
+	return str
+}
+
+func removeDuplicate(str string) string {
+	return str
+}
+
+func reverseAndDouble(str string) string {
+	return str
+}
+
+func printCharacters(str string) string {
+	return str
+}
+
+func main() {
+	fmt.PrintLn(compressString("aaaabbccgh"))
+	fmt.PrintLn(compressString("golang"))
+	fmt.PrintLn(removeDuplicate("aabccchbbccaaa"))
+	fmt.PrintLn(reverseAndDouble("abcdfgh"))
+	fmt.PrintLn(printCharacters("golang"))
+}

test/functional/ui/diff_spec.lua

@@ -1249,6 +1249,7 @@ end)
 it('diff updates line numbers below filler lines', function()
   local screen = Screen.new(40, 14)
   exec([[
+    set diffopt=internal,filler,closeoff
     call setline(1, ['a', 'a', 'a', 'y', 'b', 'b', 'b', 'b', 'b'])
     vnew
     call setline(1, ['a', 'a', 'a', 'x', 'x', 'x', 'b', 'b', 'b', 'b', 'b'])
@@ -1384,6 +1385,7 @@ end)
 
 it("'relativenumber' doesn't draw beyond end of window in diff mode #29403", function()
   local screen = Screen.new(60, 12)
+  command('set diffopt=internal,filler,closeoff')
   command('set relativenumber')
   feed('10aa<CR><Esc>gg')
   command('vnew')