fix: map go formatting errors to their locations in templ files (#737)

Co-authored-by: Adrian Hesketh <adrianhesketh@hushmail.com>

cmd/templ/generatecmd/eventhandler.go

@@ -7,6 +7,8 @@ import (
 	"crypto/sha256"
 	"fmt"
 	"go/format"
+	"go/scanner"
+	"go/token"
 	"log/slog"
 	"os"
 	"path"
@@ -221,7 +223,8 @@ func (h *FSEventHandler) generate(ctx context.Context, fileName string) (goUpdat
 
 	formattedGoCode, err := format.Source(b.Bytes())
 	if err != nil {
-		return false, false, nil, fmt.Errorf("%s source formatting error: %w", fileName, err)
+		err = remapErrorList(err, sourceMap, fileName, targetFileName)
+		return false, false, nil, fmt.Errorf("%s source formatting error %w", fileName, err)
 	}
 
 	// Hash output, and write out the file if the goCodeHash has changed.
@@ -257,6 +260,29 @@ func (h *FSEventHandler) generate(ctx context.Context, fileName string) (goUpdat
 	return goUpdated, textUpdated, parsedDiagnostics, err
 }
 
+// Takes an error from the formatter and attempts to convert the positions reported in the target file to their positions
+// in the source file.
+func remapErrorList(err error, sourceMap *parser.SourceMap, fileName string, targetFileName string) error {
+	list, ok := err.(scanner.ErrorList)
+	if !ok || len(list) == 0 {
+		return err
+	}
+	for i, e := range list {
+		// The positions in the source map are off by one line because of the package definition.
+		srcPos, ok := sourceMap.SourcePositionFromTarget(uint32(e.Pos.Line-1), uint32(e.Pos.Column))
+		if !ok {
+			continue
+		}
+		list[i].Pos = token.Position{
+			Filename: fileName,
+			Offset:   int(srcPos.Index),
+			Line:     int(srcPos.Line) + 1,
+			Column:   int(srcPos.Col),
+		}
+	}
+	return list
+}
+
 func generateSourceMapVisualisation(ctx context.Context, templFileName, goFileName string, sourceMap *parser.SourceMap) error {
 	if err := ctx.Err(); err != nil {
 		return err

cmd/templ/generatecmd/test-eventhandler/eventhandler_test.go

@@ -0,0 +1,100 @@
+package testeventhandler
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"go/scanner"
+	"go/token"
+	"io"
+	"log/slog"
+	"os"
+	"testing"
+
+	"github.com/a-h/templ/cmd/templ/generatecmd"
+	"github.com/a-h/templ/generator"
+	"github.com/fsnotify/fsnotify"
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestErrorLocationMapping(t *testing.T) {
+	tests := []struct {
+		name           string
+		rawFileName    string
+		errorPositions []token.Position
+	}{
+		{
+			name:        "single error outputs location in srcFile",
+			rawFileName: "single_error.templ.error",
+			errorPositions: []token.Position{
+				{Offset: 46, Line: 3, Column: 20},
+			},
+		},
+		{
+			name:        "multiple errors all output locations in srcFile",
+			rawFileName: "multiple_errors.templ.error",
+			errorPositions: []token.Position{
+				{Offset: 41, Line: 3, Column: 15},
+				{Offset: 101, Line: 7, Column: 22},
+				{Offset: 126, Line: 10, Column: 1},
+			},
+		},
+	}
+
+	slog := slog.New(slog.NewTextHandler(io.Discard, &slog.HandlerOptions{}))
+	fseh := generatecmd.NewFSEventHandler(slog, ".", false, []generator.GenerateOpt{}, false, false, true)
+	for _, test := range tests {
+		// The raw files cannot end in .templ because they will cause the generator to fail. Instead,
+		// we create a tmp file that ends in .templ only for the duration of the test.
+		rawFile, err := os.Open(test.rawFileName)
+		if err != nil {
+			t.Errorf("%s: Failed to open file %s: %v", test.name, test.rawFileName, err)
+			break
+		}
+		file, err := os.CreateTemp("", fmt.Sprintf("*%s.templ", test.rawFileName))
+		if err != nil {
+			t.Errorf("%s: Failed to create a tmp file at %s: %v", test.name, file.Name(), err)
+			break
+		}
+		defer os.Remove(file.Name())
+		if _, err = io.Copy(file, rawFile); err != nil {
+			t.Errorf("%s: Failed to copy contents from raw file %s to tmp %s: %v", test.name, test.rawFileName, file.Name(), err)
+		}
+
+		event := fsnotify.Event{Name: file.Name(), Op: fsnotify.Write}
+		_, _, err = fseh.HandleEvent(context.Background(), event)
+		if err == nil {
+			t.Errorf("%s: no error was thrown", test.name)
+			break
+		}
+		list, ok := err.(scanner.ErrorList)
+		for !ok {
+			err = errors.Unwrap(err)
+			if err == nil {
+				t.Errorf("%s: reached end of error wrapping before finding an ErrorList", test.name)
+				break
+			} else {
+				list, ok = err.(scanner.ErrorList)
+			}
+		}
+		if !ok {
+			break
+		}
+
+		if len(list) != len(test.errorPositions) {
+			t.Errorf("%s: expected %d errors but got %d", test.name, len(test.errorPositions), len(list))
+			break
+		}
+		for i, err := range list {
+			test.errorPositions[i].Filename = file.Name()
+			diff := cmp.Diff(test.errorPositions[i], err.Pos)
+			if diff != "" {
+				t.Error(diff)
+				t.Error("expected:")
+				t.Error(test.errorPositions[i])
+				t.Error("actual:")
+				t.Error(err.Pos)
+			}
+		}
+	}
+}

cmd/templ/generatecmd/test-eventhandler/multiple_errors.templ.error

@@ -0,0 +1,10 @@
+package testeventhandler
+
+func invalid(a: string) string {
+    return "foo"
+}
+
+templ multipleError(a: string) {
+    <div/>
+}
+l

cmd/templ/generatecmd/test-eventhandler/single_error.templ.error

@@ -0,0 +1,5 @@
+package testeventhandler
+
+templ singleError(a: string) {
+    <div/>
+}