This refactors the code to make it possible to use `get_or_insert_with`
instead of unwrapping in `segwit_cache()`. To achieve it `common_cache`
is refactored into two functions: one taking only the required borrows
and the original calling the new one. `segwit_cache` then calls the new
function so that borrows are OK.
Apart from removing unwrap, this avoids calling `common_cache` multiple
times.
There was a question whether this is equally performant. There are
multiple good reasons why it should be:
1. `get_or_insert_with` is marked `#[inline]`
2. Any good optimizer will inline a function that is used exactly once
3. 1 and 2 conclude that the closure will get inlined
4. Computing self.tx can then be moved to the only branch where it is
required.
5. Even if get_or_insert_with didn't get optimized, which is extremely
unlikely, the `tx` field is at the beginning of the struct and it
probably has pointer alignment (`Deref` suggests it's a pointer).
Alignment larger than pointer is not used, so we can expect the
fields to be ordered as-defined. (This is not guaranteed by Rust but
there's not good reason to change the order in this case.) We can
assume that offset to tx is zero in most cases which means no
computation is actually needed so the expression before closure is
no-op short of passing it into the closure as an argument.
At the time of writing `#[inline]` can be seen at
https://doc.rust-lang.org/src/core/option.rs.html#933
Instead of using magic numbers we can define constants for the address
prefix bytes. This makes it easier for future readers of the code to see
what these values are if they don't know them and/or see that they are
correct if they do know them.
Based on the original work by Justin Moon.
*MSRV unchanged from 1.29.0.*
When `std` is off, `no-std` must be on, and we use the [`alloc`](https://doc.rust-lang.org/alloc/) and core2 crates. The `alloc` crate requires the user define a global allocator.
* Import from `core` and `alloc` instead of `std`
* `alloc` only used if `no-std` is on
* Create `std` feature
* Create `no-std` feature which adds a core2 dependency to polyfill `std::io` features. This is an experimental feature and should be
used with caution.
* CI runs tests `no-std`
* MSRV for `no-std` is 1.51 or so
Dr Maxim Orlovsky
Andrew Poelstra
Riccardo Casatta
Sanket Kanjalkar
Martin Habovstiak
Devrandom
Tobin Harding
Vis Virial