This is a no-op in terms of function, but provides a nicer way to derive the
ContentHash trait for structs using the `#[derive(ContentHash)]` syntax used
for other traits such as `Debug`.
This commit only adds the macro. A subsequent commit will replace uses of
`content_hash!{}` with `#[derive(ContentHash)]`.
The new macro generates nice error messages, just like the old macro:
```
error[E0277]: the trait bound `NotImplemented: content_hash::ContentHash` is not satisfied
--> lib/src/content_hash.rs:265:16
|
265 | z: NotImplemented,
| ^^^^^^^^^^^^^^ the trait `content_hash::ContentHash` is not implemented for `NotImplemented`
|
= help: the following other types implement trait `content_hash::ContentHash`:
bool
i32
i64
u8
u32
u64
std::collections::HashMap<K, V>
BTreeMap<K, V>
and 38 others
```
This commit does two things to make proc macros re-exported by jj_lib useable
by deps:
1. jj_lib needs to be able refer to itself as `jj_lib` which it does
by adding an `extern crate self as jj_lib` declaration.
2. jj_lib::content_hash needs to re-export the `digest::Update` type so that
users of jj_lib can use the `#[derive(ContentHash)]` proc macro without
directly depending on the digest crate. This is done by re-exporting it
as `DigestUpdate`.
#3054
