modules: Ignore non-require blocks in go.mod rewrite

The Go 1.24 tool directive uses single-token entries inside a
tool ( ... ) block. The previous splitter treated any tab-indented
line as a require entry, causing an index out of range panic when
running hugo mod tidy on a module with a tool block.

Track the require block state explicitly so other blocks (tool,
replace, exclude, retract) are left untouched.

Fixes #14783

modules/client.go

@@ -1045,18 +1045,32 @@ func (modules goModules) GetMain() *goModule {
 
 func getModlineSplitter(isGoMod bool) func(line string) []string {
 	if isGoMod {
+		var inRequireBlock bool
 		return func(line string) []string {
 			if strings.HasPrefix(line, "require (") {
+				inRequireBlock = true
 				return nil
 			}
-			if !strings.HasPrefix(line, "require") && !strings.HasPrefix(line, "\t") {
+			if inRequireBlock {
+				if strings.HasPrefix(line, ")") {
+					inRequireBlock = false
+					return nil
+				}
+				if !strings.HasPrefix(line, "\t") {
+					return nil
+				}
+			} else if !strings.HasPrefix(line, "require ") {
 				return nil
 			}
 			line = strings.TrimPrefix(line, "require")
 			line = strings.TrimSpace(line)
 			line = strings.TrimSuffix(line, "// indirect")
 
-			return strings.Fields(line)
+			parts := strings.Fields(line)
+			if len(parts) < 2 {
+				return nil
+			}
+			return parts
 		}
 	}
 

modules/client_test.go

@@ -210,14 +210,27 @@ func TestGetModlineSplitter(t *testing.T) {
 
 	gomodSplitter := getModlineSplitter(true)
 
+	c.Assert(gomodSplitter("require ("), qt.IsNil)
 	c.Assert(gomodSplitter("\tgithub.com/BurntSushi/toml v0.3.1"), qt.DeepEquals, []string{"github.com/BurntSushi/toml", "v0.3.1"})
 	c.Assert(gomodSplitter("\tgithub.com/cpuguy83/go-md2man v1.0.8 // indirect"), qt.DeepEquals, []string{"github.com/cpuguy83/go-md2man", "v1.0.8"})
-	c.Assert(gomodSplitter("require ("), qt.IsNil)
 
 	gosumSplitter := getModlineSplitter(false)
 	c.Assert(gosumSplitter("github.com/BurntSushi/toml v0.3.1"), qt.DeepEquals, []string{"github.com/BurntSushi/toml", "v0.3.1"})
 }
 
+// Issue 14783
+func TestGetModlineSplitterIgnoresToolBlockIssue14783(t *testing.T) {
+	c := qt.New(t)
+
+	gomodSplitter := getModlineSplitter(true)
+
+	c.Assert(gomodSplitter("tool ("), qt.IsNil)
+	c.Assert(gomodSplitter("\tgithub.com/gohugoio/hugo"), qt.IsNil)
+	c.Assert(gomodSplitter(")"), qt.IsNil)
+	c.Assert(gomodSplitter("require ("), qt.IsNil)
+	c.Assert(gomodSplitter("\tgithub.com/foo/bar v1.2.3"), qt.DeepEquals, []string{"github.com/foo/bar", "v1.2.3"})
+}
+
 func TestClientConfigToEnv(t *testing.T) {
 	c := qt.New(t)
 

testscripts/commands/mod_tidy.txt

@@ -11,9 +11,17 @@ go 1.19
 
 require github.com/bep/empty-hugo-module v1.0.0
 
+tool (
+    github.com/gohugoio/hugo
+)
+
 module github.com/gohugoio/testmod
 -- golden/go.mod.cleaned --
 go 1.19
 
 
+tool (
+    github.com/gohugoio/hugo
+)
+
 module github.com/gohugoio/testmod