message: Make SourceLoc transparent
diff: @@ -12,6 +12,7 @@ use serde::{Deserialize, Serialize};
  /// The default source location uses the all-ones bit pattern `!0`. It is used for instructions
  /// that can't be given a real source location.
  #[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
  +#[serde(transparent)]
  pub struct SourceLoc(u32);
  impl SourceLoc {
  