message: Add notes for bugs
diff: @@ -42,11 +42,19 @@ impl From<semantics::Error> for Message {
  impl Message {
  pub fn to_diagnostic(&self, interner: &StringInterner) -> Diagnostic<()> {
  fn semantics_diagnostic(error: &semantics::Error) -> Diagnostic<()> {
  - Diagnostic::bug().with_message(match error {
  + Diagnostic::bug()
  + .with_message(match error {
  semantics::Error::InvalidRigidVar => "invalid rigid variable",
  semantics::Error::InvalidFlexibleVar => "invalid flexible variable",
  semantics::Error::InvalidFunctionElimHead => "invalid function elim head",
  })
  + .with_notes(vec![
  + "This is almost certainly a bug!".to_owned(),
  + format!(
  + "If possible, please file a bug report at {}.",
  + env!("CARGO_PKG_REPOSITORY"),
  + ),
  + ])
  }
  match self {
  