message: Only allow internal backends
diff: @@ -564,8 +564,14 @@ impl TomlProfile {
  features.require(Feature::strip())?;
  }
  - if self.codegen_backend.is_some() {
  + if let Some(codegen_backend) = &self.codegen_backend {
  features.require(Feature::codegen_backend())?;
  + if codegen_backend.contains('.') {
  + bail!(
  + "`profile.{}.codegen-backend` is an external backend, but only builtin codegen \
  + backends are allowed."
  + );
  + }
  }
  Ok(())
  