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# Description
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# User-Facing Changes
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# Tests + Formatting
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# After Submitting
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# Description
In #13031 I added the derive macros for `FromValue` and `IntoValue`. In
that implementation, in particular for structs with named fields, it was
not possible to omit fields while loading them from a value, when the
field is an `Option`. This PR adds extra handling for this behavior, so
if a field is an `Option` and that field is missing in the `Value`, then
the field becomes `None`. This behavior is also tested in
`nu_protocol::value::test_derive::missing_options`.
# User-Facing Changes
When using structs for options or similar, users can now just emit
fields in the record and the derive `from_value` method will be able to
understand this, if the struct has an `Option` type for that field.
# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`
# After Submitting
A showcase for this feature would be great, I tried to use the current
derive macro in a plugin of mine for a config but without this addition,
they are annoying to use. So, when this is done, I would add an example
for such plugin configs that may be loaded via `FromValue`.
# Description
After discussing with @sholderbach the cumbersome usage of
`nu_protocol::Value` in Rust, I created a derive macro to simplify it.
I’ve added a new crate called `nu-derive-value`, which includes two
macros, `IntoValue` and `FromValue`. These are re-exported in
`nu-protocol` and should be encouraged to be used via that re-export.
The macros ensure that all types can easily convert from and into
`Value`. For example, as a plugin author, you can define your plugin
configuration using a Rust struct and easily convert it using
`FromValue`. This makes plugin configuration less of a hassle.
I introduced the `IntoValue` trait for a standardized approach to
converting values into `Value` (and a fallible variant `TryIntoValue`).
This trait could potentially replace existing `into_value` methods.
Along with this, I've implemented `FromValue` for several standard types
and refined other implementations to use blanket implementations where
applicable.
I made these design choices with input from @devyn.
There are more improvements possible, but this is a solid start and the
PR is already quite substantial.
# User-Facing Changes
For `nu-protocol` users, these changes simplify the handling of
`Value`s. There are no changes for end-users of nushell itself.
# Tests + Formatting
Documenting the macros itself is not really possible, as they cannot
really reference any other types since they are the root of the
dependency graph. The standard library has the same problem
([std::Debug](https://doc.rust-lang.org/stable/std/fmt/derive.Debug.html)).
However I documented the `FromValue` and `IntoValue` traits completely.
For testing, I made of use `proc-macro2` in the derive macro code. This
would allow testing the generated source code. Instead I just tested
that the derived functionality is correct. This is done in
`nu_protocol::value::test_derive`, as a consumer of `nu-derive-value`
needs to do the testing of the macro usage. I think that these tests
should provide a stable baseline so that users can be sure that the impl
works.
# After Submitting
With these macros available, we can probably use them in some examples
for plugins to showcase the use of them.
