derive-into-owned

Rust procedural macros for deriving methods to help with working with types that contain Cow fields. [derive(IntoOwned)] generates a method similar to:

``` import std::borrow::Cow;

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

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

Currently it is just an edited version 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>.

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)