Crate synstructure

This crate provides helper types for matching against enum variants, and extracting bindings to each of the fields in the deriving Struct or Enum in a generic way.

If you are writing a #[derive] which needs to perform some operation on every field, then you have come to the right place!

Example: WalkFields

/*
 * Trait
 */
pub trait WalkFields: std::any::Any {
    fn walk_fields(&self, walk: &mut FnMut(&WalkFields));
}
impl WalkFields for i32 {
    fn walk_fields(&self, walk: &mut FnMut(&WalkFields)) {}
}

/*
 * Derive Implementation
 */
fn walkfields_derive(s: synstructure::Structure) -> quote::Tokens {
    let body = s.each(|bi| quote!{
        walk(#bi)
    });

    s.bound_impl("::WalkFields", quote!{
        fn walk_fields(&self, walk: &mut FnMut(&::WalkFields)) {
            match *self { #body }
        }
    })
}
decl_derive!([WalkFields] => walkfields_derive);

/*
 * Test Case
 */
fn main() {
    test_derive! {
        walkfields_derive {
            enum A<T> {
                B(i32, T),
                C(i32),
            }
        }
        expands to {
            impl<T> ::WalkFields for A<T> where T: ::WalkFields {
                fn walk_fields(&self, walk: &mut FnMut(&::WalkFields)) {
                    match *self {
                        A::B(ref __binding_0, ref __binding_1,) => {
                            { walk(__binding_0) }
                            { walk(__binding_1) }
                        }
                        A::C(ref __binding_0,) => {
                            { walk(__binding_0) }
                        }
                    }
                }
            }
        }
    }
}

Example: Interest

/*
 * Trait
 */
pub trait Interest {
    fn interesting(&self) -> bool;
}
impl Interest for i32 {
    fn interesting(&self) -> bool { *self > 0 }
}

/*
 * Derive Implementation
 */
fn interest_derive(mut s: synstructure::Structure) -> quote::Tokens {
    let body = s.fold(false, |acc, bi| quote!{
        #acc || ::Interest::interesting(#bi)
    });

    s.bound_impl("::Interest", quote!{
        fn interesting(&self) -> bool {
            match *self {
                #body
            }
        }
    })
}
decl_derive!([Interest] => interest_derive);

/*
 * Test Case
 */
fn main() {
    test_derive!{
        interest_derive {
            enum A<T> {
                B(i32, T),
                C(i32),
            }
        }
        expands to {
            impl<T> ::Interest for A<T> where T: ::Interest {
                fn interesting(&self) -> bool {
                    match *self {
                        A::B(ref __binding_0, ref __binding_1,) => {
                            false || ::Interest::interesting(__binding_0) ||
                                ::Interest::interesting(__binding_1)
                        }
                        A::C(ref __binding_0,) => {
                            false || ::Interest::interesting(__binding_0)
                        }
                    }
                }
            }
        }
    }
}

For more example usage, consider investigating the abomonation_derive crate, which makes use of this crate, and is fairly simple.

Macros

decl_derive	The decl_derive! macro declares a custom derive wrapper. It will parse the incoming TokenStream into a synstructure::Structure object, and pass it into the inner function.
test_derive	Run a test on a custom derive. This macro expands both the original struct and the expansion to ensure that they compile correctly, and confirms that feeding the original struct into the named derive will produce the written output.

Structs

BindingInfo	Information about a specific binding. This contains both an Ident reference to the given field, and the syn &'a Field descriptor for that field.
Structure	A wrapper around a syn DeriveInput which provides utilities for creating custom derive trait implementations.
VariantAst	This type is similar to syn's Variant type, however each of the fields are references rather than owned. When this is used as the AST for a real variant, this struct simply borrows the fields of the syn Variant, however this type may also be used as the sole variant for astruct.
VariantInfo	A wrapper around a syn DeriveInput's variant which provides utilities for destructuring Variants with match expressions.

Enums

BindStyle

The type of binding to use when generating a pattern.