go/analysis/passes/shadow: add filename if shadowed variable is in other file

Author: bkreitch

go/analysis/passes/shadow/shadow.go

@@ -6,9 +6,11 @@ package shadow
 
 import (
 	_ "embed"
+	"fmt"
 	"go/ast"
 	"go/token"
 	"go/types"
+	"path/filepath"
 
 	"golang.org/x/tools/go/analysis"
 	"golang.org/x/tools/go/analysis/passes/inspect"
@@ -262,7 +264,25 @@ func checkShadowing(pass *analysis.Pass, spans map[types.Object]span, ident *ast
 	}
 	// Don't complain if the types differ: that implies the programmer really wants two different things.
 	if types.Identical(obj.Type(), shadowed.Type()) {
-		line := pass.Fset.Position(shadowed.Pos()).Line
-		pass.ReportRangef(ident, "declaration of %q shadows declaration at line %d", obj.Name(), line)
+		shadowedPos := pass.Fset.Position(shadowed.Pos())
+
+		// Build the message, adding filename only if in a different file
+		message := fmt.Sprintf("declaration of %q shadows declaration at line %d", obj.Name(), shadowedPos.Line)
+		currentFile := pass.Fset.Position(ident.Pos()).Filename
+		if shadowedPos.Filename != currentFile {
+			message += fmt.Sprintf(" in %s", filepath.Base(shadowedPos.Filename))
+		}
+
+		endPos := shadowed.Pos() + token.Pos(len(shadowed.Name()))
+		pass.Report(analysis.Diagnostic{
+			Pos:     ident.Pos(),
+			End:     ident.End(),
+			Message: message,
+			Related: []analysis.RelatedInformation{{
+				Pos:     shadowed.Pos(),
+				End:     endPos,
+				Message: fmt.Sprintf("shadowed symbol %q declared here", obj.Name()),
+			}},
+		})
 	}
 }

go/analysis/passes/shadow/shadow_test.go

@@ -15,3 +15,8 @@ func Test(t *testing.T) {
 	testdata := analysistest.TestData()
 	analysistest.Run(t, testdata, shadow.Analyzer, "a")
 }
+
+func TestCrossFile(t *testing.T) {
+	testdata := analysistest.TestData()
+	analysistest.Run(t, testdata, shadow.Analyzer, "crossfile")
+}

go/analysis/passes/shadow/testdata/src/crossfile/crossfile.go

@@ -0,0 +1,15 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file contains a test for the shadowed variable checker with cross-file reference.
+
+package crossfile
+
+func ShadowGlobal() {
+	{
+		global := 1 // want "declaration of .global. shadows declaration at line 7 in other.go"
+		_ = global
+	}
+	_ = global
+}

go/analysis/passes/shadow/testdata/src/crossfile/other.go

@@ -0,0 +1,7 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package crossfile
+
+var global int