koivunej / derive-into-owned

Procedural macro for deriving helper methods for types with Cow fields

derive-into-owned

Rust procedural macros for deriving methods to help with working with types that contain Cow fields. Please note that this derive somewhat strangely works with duck-typing, at least for now. It was originally created to help me reduce the boilerplate with Cow types.

[derive(IntoOwned)] generates a method similar to:

use std::borrow::Cow;

struct Foo<'a> {
	field: Cow<'a, str>,
}

impl<'a> Foo<'a> {
	/// This method would be derived using #[derive(IntoOwned)]
	pub fn into_owned(self) -> Foo<'static> {
		Foo {
			field: Cow::Owned(self.field.into_owned()),
		}
	}
}

Originally based off of deep-clone-derive example but supports:

• tuple structs
• normal structs
• enums with tuple variants tuple enums
• IntoOwned alike fields (actually assumes all fields with types with lifetimes are IntoOwned alike)
• options of Cow or Cow-like types Option<Cow<'a, str>> and Option<Foo<'a>>
• vectors of Cow or Cow-like types

But wait there is even more! [derive(Borrowed)] generates a currently perhaps a bit limited version of a method like:

impl<'a> Foo<'a> {
	pub fn borrowed<'b>(&'b self) -> Foo<'b> {
		Foo {
			field: Cow::Borrowed(self.field.as_ref()),
		}
	}
}

Types with lifetimes

If your struct has a field with type Bar<'a> then Bar is assumed to have a method fn into_owned(self) -> Bar<'static>.

Note, there's no trait implementation expected because I didn't find one at the time and didn't think to create my own, assumed the Cow::into_owned might be getting an extension in standard library which never happened and so on.

Limitations

Currently deriving will fail miserably for at least but not limited to:

• IntoOwned: borrowed fields like &'a str
• Borrowed: struct/enum has more than one lifetime
• both: arrays not supported
• both: into_owned/borrowed types inside tuples inside vectors

Using with incompatible types results in not so understandable error messages. For example, given a struct:

#[derive(IntoOwned)]
struct Foo<'a> {
	field: &'a str,
}

The compiler error will be:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter `'a` due to conflicting requirements
 --> tests/does_not_compile.rs:4:10
  |
4 | #[derive(IntoOwned)]
  |          ^^^^^^^^^
  |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the impl at 4:10...
 --> tests/does_not_compile.rs:4:10
  |
4 | #[derive(IntoOwned)]
  |          ^^^^^^^^^
note: ...so that reference does not outlive borrowed content
 --> tests/does_not_compile.rs:4:10
  |
4 | #[derive(IntoOwned)]
  |          ^^^^^^^^^
  = note: but, the lifetime must be valid for the static lifetime...
note: ...so that expression is assignable (expected Foo<'static>, found Foo<'_>)
 --> tests/does_not_compile.rs:4:10
  |
4 | #[derive(IntoOwned)]
  |          ^^^^^^^^^
error: aborting due to previous error(s)