fix: map errors from formatting generated files back to their locations in the templ file

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 +223,12 @@ 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)
+		mapped, ok := mapFormatterError(err, sourceMap, fileName, targetFileName)
+		if !ok {
+			return false, false, nil, fmt.Errorf("generated file %s contains source formatting error %w", targetFileName, err)
+		}
+
+		return false, false, nil, mapped
 	}
 
 	// Hash output, and write out the file if the goCodeHash has changed.
@@ -257,6 +264,43 @@ 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 mapFormatterError(err error, sourceMap *parser.SourceMap, fileName string, targetFileName string) (error, bool) {
+	list, ok := err.(scanner.ErrorList)
+	if !ok {
+		return nil, false
+	}
+	list.Sort()
+
+	mapped := scanner.ErrorList{}
+
+	for _, e := range list {
+		// The positions in the source map are off by one line because of the package.
+		srcPos, ok := sourceMap.SourcePositionFromTarget(uint32(e.Pos.Line-1), uint32(e.Pos.Column))
+
+		var mappedPos token.Position
+		if ok {
+			mappedPos = token.Position{
+				Filename: fileName,
+				Offset:   int(srcPos.Index),
+				Line:     int(srcPos.Line) + 1,
+				Column:   int(srcPos.Col),
+			}
+		} else {
+			mappedPos = token.Position{
+				Filename: targetFileName,
+				Offset:   e.Pos.Offset,
+				Line:     e.Pos.Line,
+				Column:   e.Pos.Column,
+			}
+		}
+		mapped.Add(mappedPos, e.Msg)
+	}
+
+	return mapped, true
+}
+
 func generateSourceMapVisualisation(ctx context.Context, templFileName, goFileName string, sourceMap *parser.SourceMap) error {
 	if err := ctx.Err(); err != nil {
 		return err

cmd/templ/generatecmd/eventhandler_test.go

@@ -0,0 +1,61 @@
+package generatecmd
+
+import (
+	"context"
+	"go/scanner"
+	"go/token"
+	"io"
+	"log/slog"
+	"testing"
+
+	"github.com/a-h/templ/generator"
+)
+
+func TestEventHandler(t *testing.T) {
+	tests := []struct {
+		name           string
+		fileName       string
+		errorPositions []token.Position
+	}{
+		{
+			name:     "single error outputs location in srcFile",
+			fileName: "single_error.templ.error",
+			errorPositions: []token.Position{
+				{Filename: "single_error.templ.error", Offset: 41, Line: 3, Column: 20},
+			},
+		},
+		{
+			name:     "multiple errors all output locations in srcFile",
+			fileName: "multiple_errors.templ.error",
+			errorPositions: []token.Position{
+				{Filename: "multiple_errors.templ.error", Offset: 36, Line: 3, Column: 15},
+				{Filename: "multiple_errors.templ.error", Offset: 96, Line: 7, Column: 22},
+			},
+		},
+	}
+
+	slog := slog.New(slog.NewTextHandler(io.Discard, &slog.HandlerOptions{}))
+	fseh := NewFSEventHandler(slog, ".", false, []generator.GenerateOpt{}, false, false, true)
+	for _, test := range tests {
+		_, _, _, err := fseh.generate(context.Background(), test.fileName)
+		if err == nil {
+			t.Errorf("No error was thrown for file %s", test.fileName)
+		}
+
+		list, ok := err.(scanner.ErrorList)
+		if !ok {
+			t.Errorf("Error is not of type scanner.ErrorList")
+		}
+
+		if len(list) != len(test.errorPositions) {
+			t.Errorf("Expected %d errors but got %d", len(test.errorPositions), len(list))
+		}
+
+		for i, err := range list {
+			if err.Pos != test.errorPositions[i] {
+				t.Errorf("Got error %s at pos %v, expected this error at pos %v", err.Msg, err.Pos, test.errorPositions[i])
+			}
+		}
+
+	}
+}

cmd/templ/generatecmd/multiple_errors.templ.error

@@ -0,0 +1,9 @@
+package generatecmd
+
+func invalid(a: string) string {
+    return "foo"
+}
+
+templ multipleError(a: string) {
+    <div/>
+}

cmd/templ/generatecmd/single_error.templ.error

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