Preserve source labels for added patterns

README.md

@@ -55,6 +55,13 @@ m.AddFromFile("/path/to/repo/src/.gitignore", "src")
 m.AddPatterns([]byte("*.log\nbuild/\n"), "")
 ```
 
+When pattern bytes came from another filesystem, use `AddPatternsWithSource`
+to preserve the source label returned by `MatchDetail` and `Errors`:
+
+```go
+m.AddPatternsWithSource(data, "src", "src/.gitignore")
+```
+
 To limit the bytes read from each ignore file, pass `MaxIgnoreFileSize` to any constructor or walk function:
 
 ```go

gitignore.go

@@ -350,6 +350,12 @@ func (m *Matcher) AddPatterns(data []byte, dir string) {
 	m.addPatterns(data, dir, "")
 }
 
+// AddPatternsWithSource parses gitignore pattern lines from data, scopes them
+// to the given relative directory, and retains source as match provenance.
+func (m *Matcher) AddPatternsWithSource(data []byte, dir, source string) {
+	m.addPatterns(data, dir, source)
+}
+
 // AddFromFile reads a .gitignore file at the given absolute path and scopes
 // its patterns to the given relative directory. It uses the matcher's file-size
 // limit, if set, and records oversized files in Errors without applying any rules.

gitignore_test.go

@@ -2548,6 +2548,21 @@ func TestAddPatterns(t *testing.T) {
 	}
 }
 
+func TestAddPatternsWithSource(t *testing.T) {
+	m := gitignore.New("")
+	m.AddPatternsWithSource([]byte("*.env\n[[:bogus:]]\n"), "services/api", "services/api/.gitignore")
+
+	result := m.MatchDetail("services/api/local.env")
+	if !result.Ignored || result.Pattern != "*.env" || result.Line != 1 ||
+		result.Source != "services/api/.gitignore" {
+		t.Fatalf("MatchDetail = %#v", result)
+	}
+	errors := m.Errors()
+	if len(errors) != 1 || errors[0].Source != "services/api/.gitignore" || errors[0].Line != 2 {
+		t.Fatalf("Errors = %#v", errors)
+	}
+}
+
 func TestNewEmptyRootSkipsFilesystem(t *testing.T) {
 	// Create a temporary directory with a .gitignore that excludes *.exe
 	dir := t.TempDir()