| Returns `None` if the pointer is null, or else returns a shared reference to |
| the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_ref`] |
| must be used instead. If the value is known to be non-null, [`as_ref_unchecked`] |
| can be used instead. |
| |
| # Safety |
| |
| When calling this method, you have to ensure that *either* the pointer is null *or* |
| the pointer is [convertible to a reference](crate::ptr#pointer-to-reference-conversion). |
| |
| # Panics during const evaluation |
| |
| This method will panic during const evaluation if the pointer cannot be |
| determined to be null or not. See [`is_null`] for more information. |
| |
| # Null-unchecked version |
| |
| If you are sure the pointer can never be null, you can use `as_ref_unchecked` which returns |
| `&mut T` instead of `Option<&mut T>`. |
