Display metadata conditionally across kernel modes. In Meta mode, concrete types display their content. In Anon mode, `()` signals no content via `has_meta() == false`, and callers provide a positional or hash fallback. This enables a single generic `Display` impl per zero kernel type instead of separate Meta/Anon impls.
| 99 | /// fallback. This enables a single generic `Display` impl per zero kernel type |
| 100 | /// instead of separate Meta/Anon impls. |
| 101 | pub trait MetaDisplay { |
| 102 | /// Whether this field carries displayable metadata. |
| 103 | /// `false` for `()` (Anon mode) and anonymous `Name`s. |
| 104 | fn has_meta(&self) -> bool; |
| 105 | |
| 106 | /// Format the metadata value. Callers should check `has_meta()` first |
| 107 | /// and provide a fallback (e.g., positional index) when `false`. |
| 108 | fn meta_fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result; |
| 109 | } |
| 110 | |
| 111 | impl MetaDisplay for Name { |
| 112 | fn has_meta(&self) -> bool { |