#owned #derive #no-std #into-owned

no-std ownable

Derive macro for structs/enums with Cow, which can convert Type<'a> to Type<'static> and more

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0.6.2 Feb 4, 2024
0.6.1 Sep 20, 2023
0.6.0 May 6, 2023
0.5.0 Apr 12, 2023

#332 in Rust patterns

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crate ownable

Derive macro for structs/enums with Cow

To automatically convert Type<'a> to Type<'static> and more.

Example

This can be very helpful for types which use borrow with serde.

#[derive(IntoOwned, ToBorrowed, ToOwned)]
// #[derive(serde::Serialize, serde::Deserialize)]
pub struct Type<'a> {
  // #[serde(borrow)]
  cow: Cow<'a, str>,
  #[ownable(clone)] owned: String, // always clone this field
}

Will derive something functionally similar to:

impl Type<'_> {
  /// Copy the structure and clone the original values if it's not owned.
  /// This is always a deep copy of the structure.
  pub fn into_owned(self) -> Type<'static> {
    Type {
      cow: Cow::Owned(Cow::into_owned(self.cow)),
      owned: self.owned.clone(), // always cloned, as requested
    }
  }
  /// Copy the structure and clone the original values.
  /// This is always a deep copy.
  pub fn to_owned(&self) -> Type<'static> {
    Type {
      cow: Cow::Owned(str::to_owned(self.cow.borrow())),
      owned: self.owned.clone(), // always cloned, as requested
    }
  }
  /// Copy the structure and reference the original values.
  /// This is always a deep copy of the structure.
  pub fn to_borrowed(&self) -> Type {
    Type {
      cow: Cow::Borrowed(self.cow.borrow()),
      owned: self.owned.clone(), // always cloned, as requested
    }
  }
}

But actually each function only calls a function of traits, which are derived.

If the derive does not work it can be implemented by hand and still derived for types which use it.

Generics

The derive macro supports all kinds of generics: lifetimes, types, consts. And the first two with bounds and all as many times as you want.

References

References are not supported out of the box, because they can't be changed into an owned type.

But it's possible to specify which lifetime(s) are used solely for references and then those will be always copied (the reference) and thus the lifetime is not changed.

Please note that not only the type containing a reference but also types containing such a type are required to be marked.

Example

#[derive(IntoOwned, ToBorrowed, ToOwned)]
#[ownable(reference = "'b")]
pub struct Inner<'a, 'b> {
  cow: Cow<'a, str>,
  referenced: &'b str,
}

// Also types, containing types with references, must be marked.
#[derive(IntoOwned, ToBorrowed, ToOwned)]
#[ownable(reference = "'b")]
pub struct Outer<'a, 'b> {
  inner: Inner<'a, 'b>,
}

Will derive functions with these signatures:

impl<'b> Inner<'_, 'b> {
    pub fn into_owned(self) -> Inner<'static, 'b> {
        // Call the trait, which is also derived
    }
    pub fn to_owned(&self) -> Inner<'static, 'b> {
        // Call the trait, which is also derived
    }
    pub fn to_borrowed(&self) -> Inner<'_, 'b> {
        // Call the trait, which is also derived
    }
}

// The `Outer` will look similar.

Possible Errors

If the following error occurs then one of the fields has a missing trait.

error[E0277]: the trait bound `String: IntoOwned` is not satisfied

This can sometimes be fixed with #[ownable(clone)] as seen in the example above, otherwise AsCopy/AsClone can help.

And as the last resort the impl for the surrounding structure can be hand written.

Attributes

clone

With #[ownable(clone)] and #[ownable(clone = false|true)] it's possible to denote that this enum|struct/variant/field should always be cloned. It can be overwritten (i.e. set to true at top level and then false at the fields to not be cloned).

For an example see the at the top.

function

With #[ownable(function = false)] at top level (enum/struct) the functions mentioned above are not implemented, only the traits.

reference

With #[ownable(reference = "..")] one or more comma separated lifetimes can be supplied to be used for references only, see References above.

AsCopy/AsClone

If the impls for the copy types are not enough or #[ownable(clone)] does not work in that position then AsCopy and AsClone can be used to wrap a value which then works in this environment as expected. Both are transparent and do use only exact the same space as the original type and all impls (Eq, Display, Hash, ...) only pass the calls though to the inner type

Example

Example of an more complex type:

#[derive(IntoOwned, ToBorrowed, ToOwned)]
pub struct Type<'a> {
  cow: Cow<'a, str>,
  nested: Option<Box<Type<'a>>>,
  map: HashMap<AsClone<String>, Cow<'a, str>>,
  #[ownable(clone)] owned: String, // always clone this field
  number: usize, // many copy types have a trait impl, and thus can be used without the `#[ownable(clone)]`
}

Features

  • std - Traits are also implemented for types which are not in core or alloc (currently HashMap and HashSet).

std is enabled by default.

Usage

With defaults (includes std):

[dependencies]
ownable = "0.5"

With no_std (but still requires alloc):

[dependencies]
ownable = { version = "0.5", default-features = false }

License

This project is licensed under either of

at your option.

Contributing

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Dependencies

~0.4–0.8MB
~19K SLoC