fix(html): improve layout variant diagnostics

Co-Authored-By: Virgil <virgil@lethean.io>

layout.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"maps"
 	"slices"
+	"strconv"
 	"strings"
 )
 
@@ -57,18 +58,17 @@ func NewLayout(variant string) *Layout {
 // recognised slot characters.
 // Example: ValidateLayoutVariant("HCF").
 func ValidateLayoutVariant(variant string) error {
-	var invalid bool
+	var invalidSlots []byte
 	for i := range len(variant) {
 		if _, ok := slotRegistry[variant[i]]; ok {
 			continue
 		}
-		invalid = true
-		break
+		invalidSlots = append(invalidSlots, variant[i])
 	}
-	if !invalid {
+	if len(invalidSlots) == 0 {
 		return nil
 	}
-	return &layoutVariantError{variant: variant}
+	return &layoutVariantError{variant: variant, invalidSlots: invalidSlots}
 }
 
 // H appends nodes to the Header slot.
@@ -182,13 +182,46 @@ func (l *Layout) Render(ctx *Context) string {
 }
 
 type layoutVariantError struct {
-	variant string
+	variant      string
+	invalidSlots []byte
 }
 
 func (e *layoutVariantError) Error() string {
-	return "html: invalid layout variant " + e.variant
+	if e == nil {
+		return ErrInvalidLayoutVariant.Error()
+	}
+
+	var b strings.Builder
+	b.WriteString("html: invalid layout variant ")
+	b.WriteString(e.variant)
+	if len(e.invalidSlots) == 0 {
+		return b.String()
+	}
+
+	b.WriteString(" (invalid slot")
+	if len(e.invalidSlots) > 1 {
+		b.WriteString("s")
+	}
+	b.WriteString(": ")
+	for i, slot := range e.invalidSlots {
+		if i > 0 {
+			b.WriteString(", ")
+		}
+		b.WriteString(strconv.QuoteRuneToASCII(rune(slot)))
+	}
+	b.WriteByte(')')
+	return b.String()
 }
 
 func (e *layoutVariantError) Unwrap() error {
 	return ErrInvalidLayoutVariant
 }
+
+// InvalidSlots returns a copy of the invalid slot characters that were present
+// in the original variant string.
+func (e *layoutVariantError) InvalidSlots() []byte {
+	if e == nil || len(e.invalidSlots) == 0 {
+		return nil
+	}
+	return append([]byte(nil), e.invalidSlots...)
+}

layout_test.go

@@ -179,7 +179,7 @@ func TestLayout_VariantError(t *testing.T) {
 			name:          "mixed invalid variant",
 			variant:       "HXC",
 			wantErr:       true,
-			wantErrString: "html: invalid layout variant HXC",
+			wantErrString: "html: invalid layout variant HXC (invalid slot: 'X')",
 			wantRender: `<header role="banner" data-block="H-0">header</header>` +
 				`<main role="main" data-block="C-0">main</main>`,
 		},
@@ -200,6 +200,13 @@ func TestLayout_VariantError(t *testing.T) {
 				if got := layout.VariantError().Error(); got != tt.wantErrString {
 					t.Fatalf("VariantError().Error() = %q, want %q", got, tt.wantErrString)
 				}
+				if err, ok := layout.VariantError().(*layoutVariantError); ok {
+					if got := string(err.InvalidSlots()); got != "X" {
+						t.Fatalf("InvalidSlots() = %q, want %q", got, "X")
+					}
+				} else {
+					t.Fatalf("VariantError() has unexpected concrete type %T", layout.VariantError())
+				}
 			} else if layout.VariantError() != nil {
 				t.Fatalf("VariantError() = %v, want nil", layout.VariantError())
 			}