output: convert backslashes to forward slashes for GitHub Action annotations printer

pkg/printers/github.go

@@ -3,6 +3,7 @@ package printers
 import (
 	"fmt"
 	"io"
+	"path/filepath"
 
 	"github.com/golangci/golangci-lint/pkg/result"
 )
@@ -26,7 +27,12 @@ func formatIssueAsGithub(issue *result.Issue) string {
 		severity = issue.Severity
 	}
 
-	ret := fmt.Sprintf("::%s file=%s,line=%d", severity, issue.FilePath(), issue.Line())
+	// Convert backslashes to forward slashes.
+	// This is needed when running on windows.
+	// Otherwise, GitHub won't be able to show the annotations pointing to the file path with backslashes.
+	file := filepath.ToSlash(issue.FilePath())
+
+	ret := fmt.Sprintf("::%s file=%s,line=%d", severity, file, issue.Line())
 	if issue.Pos.Column != 0 {
 		ret += fmt.Sprintf(",col=%d", issue.Pos.Column)
 	}

pkg/printers/github_test.go

@@ -4,6 +4,7 @@ package printers
 import (
 	"bytes"
 	"go/token"
+	"runtime"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -72,3 +73,24 @@ func TestFormatGithubIssue(t *testing.T) {
 	sampleIssue.Pos.Column = 0
 	require.Equal(t, "::error file=path/to/file.go,line=10::some issue (sample-linter)", formatIssueAsGithub(&sampleIssue))
 }
+
+func TestFormatGithubIssueWindows(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Skipping test on non Windows")
+	}
+
+	sampleIssue := result.Issue{
+		FromLinter: "sample-linter",
+		Text:       "some issue",
+		Pos: token.Position{
+			Filename: "path\\to\\file.go",
+			Offset:   2,
+			Line:     10,
+			Column:   4,
+		},
+	}
+	require.Equal(t, "::error file=path/to/file.go,line=10,col=4::some issue (sample-linter)", formatIssueAsGithub(&sampleIssue))
+
+	sampleIssue.Pos.Column = 0
+	require.Equal(t, "::error file=path/to/file.go,line=10::some issue (sample-linter)", formatIssueAsGithub(&sampleIssue))
+}