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
IntoOwnedalike fields (actually assumes all fields with types with lifetimes areIntoOwnedalike)- options of Cow or Cow-like types
Option<Cow<'a, str>>andOption<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()),
}
}
}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.
Currently deriving will fail miserably for at least but not limited to:
IntoOwned: borrowed fields like&'a strBorrowed: 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)
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