Impressum lib.rs

Sprache: Rust

//! 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 Implementation
//! ```
//! 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)) {}
//! }
//! ```
//!
//! ### Custom Derive
//! ```
//! # use quote::quote;
//! fn walkfields_derive(s: synstructure::Structure) -> proc_macro2::TokenStream {
//! let body = s.each(|bi| quote!{
//! walk(#bi)
//! });
//!
//! s.gen_impl(quote! {
//! extern crate synstructure_test_traits;
//!
//! gen impl synstructure_test_traits::WalkFields for @Self {
//! fn walk_fields(&self, walk: &mut FnMut(&synstructure_test_traits::WalkFields)) {
//! match *self { #body }
//! }
//! }
//! })
//! }
//! # const _IGNORE: &'static str = stringify!(
//! synstructure::decl_derive!([WalkFields] => walkfields_derive);
//! # );
//!
//! /*
//! * Test Case
//! */
//! fn main() {
//! synstructure::test_derive! {
//! walkfields_derive {
//! enum A<T> {
//! B(i32, T),
//! C(i32),
//! }
//! }
//! expands to {
//! const _: () = {
//! extern crate synstructure_test_traits;
//! impl<T> synstructure_test_traits::WalkFields for A<T>
//! where T: synstructure_test_traits::WalkFields
//! {
//! fn walk_fields(&self, walk: &mut FnMut(&synstructure_test_traits::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 Implementation
//! ```
//! pub trait Interest {
//! fn interesting(&self) -> bool;
//! }
//! impl Interest for i32 {
//! fn interesting(&self) -> bool { *self > 0 }
//! }
//! ```
//!
//! ### Custom Derive
//! ```
//! # use quote::quote;
//! fn interest_derive(mut s: synstructure::Structure) -> proc_macro2::TokenStream {
//! let body = s.fold(false, |acc, bi| quote!{
//! #acc || synstructure_test_traits::Interest::interesting(#bi)
//! });
//!
//! s.gen_impl(quote! {
//! extern crate synstructure_test_traits;
//! gen impl synstructure_test_traits::Interest for @Self {
//! fn interesting(&self) -> bool {
//! match *self {
//! #body
//! }
//! }
//! }
//! })
//! }
//! # const _IGNORE: &'static str = stringify!(
//! synstructure::decl_derive!([Interest] => interest_derive);
//! # );
//!
//! /*
//! * Test Case
//! */
//! fn main() {
//! synstructure::test_derive!{
//! interest_derive {
//! enum A<T> {
//! B(i32, T),
//! C(i32),
//! }
//! }
//! expands to {
//! const _: () = {
//! extern crate synstructure_test_traits;
//! impl<T> synstructure_test_traits::Interest for A<T>
//! where T: synstructure_test_traits::Interest
//! {
//! fn interesting(&self) -> bool {
//! match *self {
//! A::B(ref __binding_0, ref __binding_1,) => {
//! false ||
//! synstructure_test_traits::Interest::interesting(__binding_0) ||
//! synstructure_test_traits::Interest::interesting(__binding_1)
//! }
//! A::C(ref __binding_0,) => {
//! false ||
//! synstructure_test_traits::Interest::interesting(__binding_0)
//! }
//! }
//! }
//! }
//! };
//! }
//! }
//! }
//! ```
//!
//! For more example usage, consider investigating the `abomonation_derive` crate,
//! which makes use of this crate, and is fairly simple.

#![allow(
 clippy::default_trait_access,
 clippy::missing_errors_doc,
 clippy::missing_panics_doc,
 clippy::must_use_candidate,
 clippy::needless_pass_by_value
)]

#[cfg(all(
 not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
 feature = "proc-macro"
))]
extern crate proc_macro;

use std::collections::HashSet;

use syn::parse::{ParseStream, Parser};
use syn::visit::{self, Visit};
use syn::{
 braced, punctuated, token, Attribute, Data, DeriveInput, Error, Expr, Field, Fields,
 FieldsNamed, FieldsUnnamed, GenericParam, Generics, Ident, PredicateType, Result, Token,
 TraitBound, Type, TypeMacro, TypeParamBound, TypePath, WhereClause, WherePredicate,
};

use quote::{format_ident, quote_spanned, ToTokens};
// re-export the quote! macro so we can depend on it being around in our macro's
// implementations.
#[doc(hidden)]
pub use quote::quote;

use proc_macro2::{Span, TokenStream, TokenTree};

// NOTE: This module has documentation hidden, as it only exports macros (which
// always appear in the root of the crate) and helper methods / re-exports used
// in the implementation of those macros.
#[doc(hidden)]
pub mod macros;

/// Changes how bounds are added
#[allow(clippy::manual_non_exhaustive)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum AddBounds {
 /// Add for fields and generics
 Both,
 /// Fields only
 Fields,
 /// Generics only
 Generics,
 /// None
 None,
 #[doc(hidden)]
 __Nonexhaustive,
}

/// The type of binding to use when generating a pattern.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum BindStyle {
 /// `x`
 Move,
 /// `mut x`
 MoveMut,
 /// `ref x`
 Ref,
 /// `ref mut x`
 RefMut,
}

impl ToTokens for BindStyle {
 fn to_tokens(&self, tokens: &mut TokenStream) {
 match self {
 BindStyle::Move => {}
 BindStyle::MoveMut => quote_spanned!(Span::call_site() => mut).to_tokens(tokens),
 BindStyle::Ref => quote_spanned!(Span::call_site() => ref).to_tokens(tokens),
 BindStyle::RefMut => quote_spanned!(Span::call_site() => ref mut).to_tokens(tokens),
 }
 }
}

// Internal method for merging seen_generics arrays together.
fn generics_fuse(res: &mut Vec<bool>, new: &[bool]) {
 for (i, &flag) in new.iter().enumerate() {
 if i == res.len() {
 res.push(false);
 }
 if flag {
 res[i] = true;
 }
 }
}

// Internal method for extracting the set of generics which have been matched.
fn fetch_generics<'a>(set: &[bool], generics: &'a Generics) -> Vec<&>'a Ident> {
 let mut tys = vec![];
 for (&seen, param) in set.iter().zip(generics.params.iter()) {
 if seen {
 if let GenericParam::Type(tparam) = param {
 tys.push(&tparam.ident);
 }
 }
 }
 tys
}

// Internal method to merge two Generics objects together intelligently.
fn merge_generics(into: &mut Generics, from: &Generics) -> Result<()> {
 // Try to add the param into `into`, and merge parmas with identical names.
 for p in &from.params {
 for op in &into.params {
 match (op, p) {
 (GenericParam::Type(otp), GenericParam::Type(tp)) => {
 // NOTE: This is only OK because syn ignores the span for equality purposes.
 if otp.ident == tp.ident {
 return Err(Error::new_spanned(
 p,
 format!(
 "Attempted to merge conflicting generic parameters: {} and {}",
 quote!(#op),
 quote!(#p)
 ),
 ));
 }
 }
 (GenericParam::Lifetime(olp), GenericParam::Lifetime(lp)) => {
 // NOTE: This is only OK because syn ignores the span for equality purposes.
 if olp.lifetime == lp.lifetime {
 return Err(Error::new_spanned(
 p,
 format!(
 "Attempted to merge conflicting generic parameters: {} and {}",
 quote!(#op),
 quote!(#p)
 ),
 ));
 }
 }
 // We don't support merging Const parameters, because that wouldn't make much sense.
 _ => (),
 }
 }
 into.params.push(p.clone());
 }

 // Add any where clauses from the input generics object.
 if let Some(from_clause) = &from.where_clause {
 into.make_where_clause()
 .predicates
 .extend(from_clause.predicates.iter().cloned());
 }

 Ok(())
}

/// Helper method which does the same thing as rustc 1.20's
/// `Option::get_or_insert_with`. This method is used to keep backwards
/// compatibility with rustc 1.15.
fn get_or_insert_with<T, F>(opt: &mut Option<T>, f: F) -> &mut T
where
 F: FnOnce() -> T,
{
 if opt.is_none() {
 *opt = Some(f());
 }

 match opt {
 Some(v) => v,
 None => unreachable!(),
 }
}

/// Information about a specific binding. This contains both an `Ident`
/// reference to the given field, and the syn `&'a Field` descriptor for that
/// field.
///
/// This type supports `quote::ToTokens`, so can be directly used within the
/// `quote!` macro. It expands to a reference to the matched field.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct BindingInfo<'a> {
 /// The name which this BindingInfo will bind to.
 pub binding: Ident,

 /// The type of binding which this BindingInfo will create.
 pub style: BindStyle,

 field: &'a Field,

 // These are used to determine which type parameters are avaliable.
 generics: &'a Generics,
 seen_generics: Vec<bool>,
 // The original index of the binding
 // this will not change when .filter() is called
 index: usize,
}

impl<'a> ToTokens for BindingInfo<'a> {
 fn to_tokens(&self, tokens: &mut TokenStream) {
 self.binding.to_tokens(tokens);
 }
}

impl<'a> BindingInfo<'a> {
 /// Returns a reference to the underlying `syn` AST node which this
 /// `BindingInfo` references
 pub fn ast(&self) -> &'a Field {
 self.field
 }

 /// Generates the pattern fragment for this field binding.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].bindings()[0].pat().to_string(),
 /// quote! {
 /// ref __binding_0
 /// }.to_string()
 /// );
 /// ```
 pub fn pat(&self) -> TokenStream {
 let BindingInfo { binding, style, .. } = self;
 quote!(#style #binding)
 }

 /// Returns a list of the type parameters which are referenced in this
 /// field's type.
 ///
 /// # Caveat
 ///
 /// If the field contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// struct A<T, U> {
 /// a: Option<T>,
 /// b: U,
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].bindings()[0].referenced_ty_params(),
 /// &["e::format_ident!("T")]
 /// );
 /// ```
 pub fn referenced_ty_params(&self) -> Vec<&'a Ident> {
 fetch_generics(&self.seen_generics, self.generics)
 }
}

/// 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 a struct.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub struct VariantAst<'a> {
 pub attrs: &'a [Attribute],
 pub ident: &'a Ident,
 pub fields: &'a Fields,
 pub discriminant: &'a Option<(token::Eq, Expr)>,
}

/// A wrapper around a `syn::DeriveInput`'s variant which provides utilities
/// for destructuring `Variant`s with `match` expressions.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct VariantInfo<'a> {
 pub prefix: Option<&'a Ident>,
 bindings: Vec<BindingInfo<'a>>,
 ast: VariantAst<'a>,
 generics: &'a Generics,
 // The original length of `bindings` before any `.filter()` calls
 original_length: usize,
}

/// Helper function used by the `VariantInfo` constructor. Walks all of the types
/// in `field` and returns a list of the type parameters from `ty_params` which
/// are referenced in the field.
fn get_ty_params(field: &Field, generics: &Generics) -> Vec<bool> {
 // Helper type. Discovers all identifiers inside of the visited type,
 // and calls a callback with them.
 struct BoundTypeLocator<'a> {
 result: Vec<bool>,
 generics: &'a Generics,
 }

 impl<'a> Visit<'a> for BoundTypeLocator<'a> {
 // XXX: This also (intentionally) captures paths like T::SomeType. Is
 // this desirable?
 fn visit_ident(&mut self, id: &Ident) {
 for (idx, i) in self.generics.params.iter().enumerate() {
 if let GenericParam::Type(tparam) = i {
 if tparam.ident == *id {
 self.result[idx] = true;
 }
 }
 }
 }

 fn visit_type_macro(&mut self, x: &'a TypeMacro) {
 // If we see a type_mac declaration, then we can't know what type parameters
 // it might be binding, so we presume it binds all of them.
 for r in &mut self.result {
 *r = true;
 }
 visit::visit_type_macro(self, x);
 }
 }

 let mut btl = BoundTypeLocator {
 result: vec![false; generics.params.len()],
 generics,
 };

 btl.visit_type(&field.ty);

 btl.result
}

impl<'a> VariantInfo<'a> {
 fn new(ast: VariantAst<'a>, prefix: Option<&'a Ident>, generics: &'a Generics) -> Self {
 let bindings = match ast.fields {
 Fields::Unit => vec![],
 Fields::Unnamed(FieldsUnnamed {
 unnamed: fields, ..
 })
 | Fields::Named(FieldsNamed { named: fields, .. }) => {
 fields
 .into_iter()
 .enumerate()
 .map(|(i, field)| {
 BindingInfo {
 // XXX: This has to be call_site to avoid privacy
 // when deriving on private fields.
 binding: format_ident!("__binding_{}", i),
 style: BindStyle::Ref,
 field,
 generics,
 seen_generics: get_ty_params(field, generics),
 index: i,
 }
 })
 .collect::<Vec<_>>()
 }
 };

 let original_length = bindings.len();
 VariantInfo {
 prefix,
 bindings,
 ast,
 generics,
 original_length,
 }
 }

 /// Returns a slice of the bindings in this Variant.
 pub fn bindings(&self) -> &[BindingInfo<'a>] {
 &self.bindings
 }

 /// Returns a mut slice of the bindings in this Variant.
 pub fn bindings_mut(&mut self) -> &mut [BindingInfo<'a>] {
 &mut self.bindings
 }

 /// Returns a `VariantAst` object which contains references to the
 /// underlying `syn` AST node which this `Variant` was created from.
 pub fn ast(&self) -> VariantAst<'a> {
 self.ast
 }

 /// True if any bindings were omitted due to a `filter` call.
 pub fn omitted_bindings(&self) -> bool {
 self.original_length != self.bindings.len()
 }

 /// Generates the match-arm pattern which could be used to match against this Variant.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].pat().to_string(),
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,)
 /// }.to_string()
 /// );
 /// ```
 pub fn pat(&self) -> TokenStream {
 let mut t = TokenStream::new();
 if let Some(prefix) = self.prefix {
 prefix.to_tokens(&mut t);
 quote!(::).to_tokens(&mut t);
 }
 self.ast.ident.to_tokens(&mut t);
 match self.ast.fields {
 Fields::Unit => {
 assert!(self.bindings.is_empty());
 }
 Fields::Unnamed(..) => token::Paren(Span::call_site()).surround(&mut t, |t| {
 let mut expected_index = 0;
 for binding in &self.bindings {
 while expected_index < binding.index {
 quote!(_,).to_tokens(t);
 expected_index += 1;
 }
 binding.pat().to_tokens(t);
 quote!(,).to_tokens(t);
 expected_index += 1;
 }
 if expected_index != self.original_length {
 quote!(..).to_tokens(t);
 }
 }),
 Fields::Named(..) => token::Brace(Span::call_site()).surround(&mut t, |t| {
 for binding in &self.bindings {
 binding.field.ident.to_tokens(t);
 quote!(:).to_tokens(t);
 binding.pat().to_tokens(t);
 quote!(,).to_tokens(t);
 }
 if self.omitted_bindings() {
 quote!(..).to_tokens(t);
 }
 }),
 }
 t
 }

 /// Generates the token stream required to construct the current variant.
 ///
 /// The init array initializes each of the fields in the order they are
 /// written in `variant.ast().fields`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(usize, usize),
 /// C{ v: usize },
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].construct(|_, i| quote!(#i)).to_string(),
 ///
 /// quote!{
 /// A::B(0usize, 1usize,)
 /// }.to_string()
 /// );
 ///
 /// assert_eq!(
 /// s.variants()[1].construct(|_, i| quote!(#i)).to_string(),
 ///
 /// quote!{
 /// A::C{ v: 0usize, }
 /// }.to_string()
 /// );
 /// ```
 pub fn construct<F, T>(&self, mut func: F) -> TokenStream
 where
 F: FnMut(&Field, usize) -> T,
 T: ToTokens,
 {
 let mut t = TokenStream::new();
 if let Some(prefix) = self.prefix {
 quote!(#prefix ::).to_tokens(&mut t);
 }
 self.ast.ident.to_tokens(&mut t);

 match &self.ast.fields {
 Fields::Unit => (),
 Fields::Unnamed(FieldsUnnamed { unnamed, .. }) => {
 token::Paren::default().surround(&mut t, |t| {
 for (i, field) in unnamed.into_iter().enumerate() {
 func(field, i).to_tokens(t);
 quote!(,).to_tokens(t);
 }
 });
 }
 Fields::Named(FieldsNamed { named, .. }) => {
 token::Brace::default().surround(&mut t, |t| {
 for (i, field) in named.into_iter().enumerate() {
 field.ident.to_tokens(t);
 quote!(:).to_tokens(t);
 func(field, i).to_tokens(t);
 quote!(,).to_tokens(t);
 }
 });
 }
 }
 t
 }

 /// Runs the passed-in function once for each bound field, passing in a `BindingInfo`.
 /// and generating a `match` arm which evaluates the returned tokens.
 ///
 /// This method will ignore fields which are ignored through the `filter`
 /// method.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,) => {
 /// { println!("{:?}", __binding_0) }
 /// { println!("{:?}", __binding_1) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn each<F, R>(&self, mut f: F) -> TokenStream
 where
 F: FnMut(&BindingInfo<'_>) -> R,
 R: ToTokens,
 {
 let pat = self.pat();
 let mut body = TokenStream::new();
 for binding in &self.bindings {
 token::Brace::default().surround(&mut body, |body| {
 f(binding).to_tokens(body);
 });
 }
 quote!(#pat => { #body })
 }

 /// Runs the passed-in function once for each bound field, passing in the
 /// result of the previous call, and a `BindingInfo`. generating a `match`
 /// arm which evaluates to the resulting tokens.
 ///
 /// This method will ignore fields which are ignored through the `filter`
 /// method.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].fold(quote!(0), |acc, bi| quote!(#acc + #bi)).to_string(),
 ///
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,) => {
 /// 0 + __binding_0 + __binding_1
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn fold<F, I, R>(&self, init: I, mut f: F) -> TokenStream
 where
 F: FnMut(TokenStream, &BindingInfo<'_>) -> R,
 I: ToTokens,
 R: ToTokens,
 {
 let pat = self.pat();
 let body = self.bindings.iter().fold(quote!(#init), |i, bi| {
 let r = f(i, bi);
 quote!(#r)
 });
 quote!(#pat => { #body })
 }

 /// Filter the bindings created by this `Variant` object. This has 2 effects:
 ///
 /// * The bindings will no longer appear in match arms generated by methods
 /// on this `Variant` or its subobjects.
 ///
 /// * Impl blocks created with the `bound_impl` or `unsafe_bound_impl`
 /// method only consider type parameters referenced in the types of
 /// non-filtered fields.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C{ a: u32 },
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.variants_mut()[0].filter(|bi| {
 /// bi.ast().ident == Some(quote::format_ident!("b"))
 /// });
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ b: ref __binding_1, .. } => {
 /// { println!("{:?}", __binding_1) }
 /// }
 /// A::C{ a: ref __binding_0, } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn filter<F>(&mut self, f: F) -> &mut Self
 where
 F: FnMut(&BindingInfo<'_>) -> bool,
 {
 self.bindings.retain(f);
 self
 }

 /// Iterates all the bindings of this `Variant` object and uses a closure to determine if a
 /// binding should be removed. If the closure returns `true` the binding is removed from the
 /// variant. If the closure returns `false`, the binding remains in the variant.
 ///
 /// All the removed bindings are moved to a new `Variant` object which is otherwise identical
 /// to the current one. To understand the effects of removing a binding from a variant check
 /// the [`VariantInfo::filter`] documentation.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C{ a: u32 },
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// let mut with_b = &mut s.variants_mut()[0];
 ///
 /// let with_a = with_b.drain_filter(|bi| {
 /// bi.ast().ident == Some(quote::format_ident!("a"))
 /// });
 ///
 /// assert_eq!(
 /// with_a.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ a: ref __binding_0, .. } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 ///
 /// assert_eq!(
 /// with_b.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ b: ref __binding_1, .. } => {
 /// { println!("{:?}", __binding_1) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 #[allow(clippy::return_self_not_must_use)]
 pub fn drain_filter<F>(&mut self, mut f: F) -> Self
 where
 F: FnMut(&BindingInfo<'_>) -> bool,
 {
 let mut other = VariantInfo {
 prefix: self.prefix,
 bindings: vec![],
 ast: self.ast,
 generics: self.generics,
 original_length: self.original_length,
 };

 let (other_bindings, self_bindings) = self.bindings.drain(..).partition(&mut f);
 other.bindings = other_bindings;
 self.bindings = self_bindings;

 other
 }

 /// Remove the binding at the given index.
 ///
 /// # Panics
 ///
 /// Panics if the index is out of range.
 pub fn remove_binding(&mut self, idx: usize) -> &mut Self {
 self.bindings.remove(idx);
 self
 }

 /// Updates the `BindStyle` for each of the passed-in fields by calling the
 /// passed-in function for each `BindingInfo`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.variants_mut()[0].bind_with(|bi| BindStyle::RefMut);
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B(ref mut __binding_0, ref mut __binding_1,) => {
 /// { println!("{:?}", __binding_0) }
 /// { println!("{:?}", __binding_1) }
 /// }
 /// A::C(ref __binding_0,) => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn bind_with<F>(&mut self, mut f: F) -> &mut Self
 where
 F: FnMut(&BindingInfo<'_>) -> BindStyle,
 {
 for binding in &mut self.bindings {
 binding.style = f(binding);
 }
 self
 }

 /// Updates the binding name for each fo the passed-in fields by calling the
 /// passed-in function for each `BindingInfo`.
 ///
 /// The function will be called with the `BindingInfo` and its index in the
 /// enclosing variant.
 ///
 /// The default name is `__binding_{}` where `{}` is replaced with an
 /// increasing number.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C{ a: u32 },
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.variants_mut()[0].binding_name(|bi, i| bi.ident.clone().unwrap());
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ a: ref a, b: ref b, } => {
 /// { println!("{:?}", a) }
 /// { println!("{:?}", b) }
 /// }
 /// A::C{ a: ref __binding_0, } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn binding_name<F>(&mut self, mut f: F) -> &>mut Self
 where
 F: FnMut(&Field, usize) -> Ident,
 {
 for (it, binding) in self.bindings.iter_mut().enumerate() {
 binding.binding = f(binding.field, it);
 }
 self
 }

 /// Returns a list of the type parameters which are referenced in this
 /// field's type.
 ///
 /// # Caveat
 ///
 /// If the field contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// struct A<T, U> {
 /// a: Option<T>,
 /// b: U,
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.variants()[0].bindings()[0].referenced_ty_params(),
 /// &["e::format_ident!("T")]
 /// );
 /// ```
 pub fn referenced_ty_params(&self) -> Vec<&'a Ident> {
 let mut flags = Vec::new();
 for binding in &self.bindings {
 generics_fuse(&mut flags, &binding.seen_generics);
 }
 fetch_generics(&flags, self.generics)
 }
}

/// A wrapper around a `syn::DeriveInput` which provides utilities for creating
/// custom derive trait implementations.
#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct Structure<'a> {
 variants: Vec<VariantInfo<'a>>,
 omitted_variants: bool,
 ast: &'a DeriveInput,
 extra_impl: Vec<GenericParam>,
 extra_predicates: Vec<WherePredicate>,
 add_bounds: AddBounds,
}

impl<'a> Structure<'a> {
 /// Create a new `Structure` with the variants and fields from the passed-in
 /// `DeriveInput`.
 ///
 /// # Panics
 ///
 /// This method will panic if the provided AST node represents an untagged
 /// union.
 pub fn new(ast: &'a DeriveInput) -> Self {
 Self::try_new(ast).expect("Unable to create synstructure::Structure")
 }

 /// Create a new `Structure` with the variants and fields from the passed-in
 /// `DeriveInput`.
 ///
 /// Unlike `Structure::new`, this method does not panic if the provided AST
 /// node represents an untagged union.
 pub fn try_new(ast: &'a DeriveInput) -> Result<Self> {
 let variants = match &ast.data {
 Data::Enum(data) => (&data.variants)
 .into_iter()
 .map(|v| {
 VariantInfo::new(
 VariantAst {
 attrs: &v.attrs,
 ident: &v.ident,
 fields: &v.fields,
 discriminant: &v.discriminant,
 },
 Some(&ast.ident),
 &ast.generics,
 )
 })
 .collect::<Vec<_>>(),
 Data::Struct(data) => {
 vec![VariantInfo::new(
 VariantAst {
 attrs: &ast.attrs,
 ident: &ast.ident,
 fields: &data.fields,
 discriminant: &None,
 },
 None,
 &ast.generics,
 )]
 }
 Data::Union(_) => {
 return Err(Error::new_spanned(
 ast,
 "unexpected unsupported untagged union",
 ));
 }
 };

 Ok(Structure {
 variants,
 omitted_variants: false,
 ast,
 extra_impl: vec![],
 extra_predicates: vec![],
 add_bounds: AddBounds::Both,
 })
 }

 /// Returns a slice of the variants in this Structure.
 pub fn variants(&self) -> &[VariantInfo<'a>] {
 &self.variants
 }

 /// Returns a mut slice of the variants in this Structure.
 pub fn variants_mut(&mut self) -> &mut [VariantInfo<'a>] {
 &mut self.variants
 }

 /// Returns a reference to the underlying `syn` AST node which this
 /// `Structure` was created from.
 pub fn ast(&self) -> &'a DeriveInput {
 self.ast
 }

 /// True if any variants were omitted due to a `filter_variants` call.
 pub fn omitted_variants(&self) -> bool {
 self.omitted_variants
 }

 /// Runs the passed-in function once for each bound field, passing in a `BindingInfo`.
 /// and generating `match` arms which evaluate the returned tokens.
 ///
 /// This method will ignore variants or fields which are ignored through the
 /// `filter` and `filter_variant` methods.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,) => {
 /// { println!("{:?}", __binding_0) }
 /// { println!("{:?}", __binding_1) }
 /// }
 /// A::C(ref __binding_0,) => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn each<F, R>(&self, mut f: F) -> TokenStream
 where
 F: FnMut(&BindingInfo<'_>) -> R,
 R: ToTokens,
 {
 let mut t = TokenStream::new();
 for variant in &self.variants {
 variant.each(&mut f).to_tokens(&mut t);
 }
 if self.omitted_variants {
 quote!(_ => {}).to_tokens(&mut t);
 }
 t
 }

 /// Runs the passed-in function once for each bound field, passing in the
 /// result of the previous call, and a `BindingInfo`. generating `match`
 /// arms which evaluate to the resulting tokens.
 ///
 /// This method will ignore variants or fields which are ignored through the
 /// `filter` and `filter_variant` methods.
 ///
 /// If a variant has been ignored, it will return the `init` value.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.fold(quote!(0), |acc, bi| quote!(#acc + #bi)).to_string(),
 ///
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,) => {
 /// 0 + __binding_0 + __binding_1
 /// }
 /// A::C(ref __binding_0,) => {
 /// 0 + __binding_0
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn fold<F, I, R>(&self, init: I, mut f: F) -> TokenStream
 where
 F: FnMut(TokenStream, &BindingInfo<'_>) -> R,
 I: ToTokens,
 R: ToTokens,
 {
 let mut t = TokenStream::new();
 for variant in &self.variants {
 variant.fold(&init, &mut f).to_tokens(&mut t);
 }
 if self.omitted_variants {
 quote!(_ => { #init }).to_tokens(&mut t);
 }
 t
 }

 /// Runs the passed-in function once for each variant, passing in a
 /// `VariantInfo`. and generating `match` arms which evaluate the returned
 /// tokens.
 ///
 /// This method will ignore variants and not bind fields which are ignored
 /// through the `filter` and `filter_variant` methods.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let s = Structure::new(&di);
 ///
 /// assert_eq!(
 /// s.each_variant(|v| {
 /// let name = &v.ast().ident;
 /// quote!(println!(stringify!(#name)))
 /// }).to_string(),
 ///
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,) => {
 /// println!(stringify!(B))
 /// }
 /// A::C(ref __binding_0,) => {
 /// println!(stringify!(C))
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn each_variant<F, R>(&self, mut f: F) -> TokenStream
 where
 F: FnMut(&VariantInfo<'_>) -> R,
 R: ToTokens,
 {
 let mut t = TokenStream::new();
 for variant in &self.variants {
 let pat = variant.pat();
 let body = f(variant);
 quote!(#pat => { #body }).to_tokens(&mut t);
 }
 if self.omitted_variants {
 quote!(_ => {}).to_tokens(&mut t);
 }
 t
 }

 /// Filter the bindings created by this `Structure` object. This has 2 effects:
 ///
 /// * The bindings will no longer appear in match arms generated by methods
 /// on this `Structure` or its subobjects.
 ///
 /// * Impl blocks created with the `bound_impl` or `unsafe_bound_impl`
 /// method only consider type parameters referenced in the types of
 /// non-filtered fields.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C{ a: u32 },
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter(|bi| {
 /// bi.ast().ident == Some(quote::format_ident!("a"))
 /// });
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ a: ref __binding_0, .. } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// A::C{ a: ref __binding_0, } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn filter<F>(&mut self, mut f: F) -> &mut Self
 where
 F: FnMut(&BindingInfo<'_>) -> bool,
 {
 for variant in &mut self.variants {
 variant.filter(&mut f);
 }
 self
 }

 /// Iterates all the bindings of this `Structure` object and uses a closure to determine if a
 /// binding should be removed. If the closure returns `true` the binding is removed from the
 /// structure. If the closure returns `false`, the binding remains in the structure.
 ///
 /// All the removed bindings are moved to a new `Structure` object which is otherwise identical
 /// to the current one. To understand the effects of removing a binding from a structure check
 /// the [`Structure::filter`] documentation.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C{ a: u32 },
 /// }
 /// };
 /// let mut with_b = Structure::new(&di);
 ///
 /// let with_a = with_b.drain_filter(|bi| {
 /// bi.ast().ident == Some(quote::format_ident!("a"))
 /// });
 ///
 /// assert_eq!(
 /// with_a.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ a: ref __binding_0, .. } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// A::C{ a: ref __binding_0, } => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 ///
 /// assert_eq!(
 /// with_b.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ b: ref __binding_1, .. } => {
 /// { println!("{:?}", __binding_1) }
 /// }
 /// A::C{ .. } => {
 ///
 /// }
 /// }.to_string()
 /// );
 /// ```
 #[allow(clippy::return_self_not_must_use)]
 pub fn drain_filter<F>(&mut self, mut f: F) -> Self
 where
 F: FnMut(&BindingInfo<'_>) -> bool,
 {
 Self {
 variants: self
 .variants
 .iter_mut()
 .map(|variant| variant.drain_filter(&mut f))
 .collect(),
 omitted_variants: self.omitted_variants,
 ast: self.ast,
 extra_impl: self.extra_impl.clone(),
 extra_predicates: self.extra_predicates.clone(),
 add_bounds: self.add_bounds,
 }
 }

 /// Specify additional where predicate bounds which should be generated by
 /// impl-generating functions such as `gen_impl`, `bound_impl`, and
 /// `unsafe_bound_impl`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// // Add an additional where predicate.
 /// s.add_where_predicate(syn::parse_quote!(T: std::fmt::Display));
 ///
 /// assert_eq!(
 /// s.bound_impl(quote!(krate::Trait), quote!{
 /// fn a() {}
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U> krate::Trait for A<T, U>
 /// where T: std::fmt::Display,
 /// T: krate::Trait,
 /// Option: krate::Trait,
 /// U: krate::Trait
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 pub fn add_where_predicate(&mut self, pred: WherePredicate) -> &n style='color:red'>mut Self {
 self.extra_predicates.push(pred);
 self
 }

 /// Specify which bounds should be generated by impl-generating functions
 /// such as `gen_impl`, `bound_impl`, and `unsafe_bound_impl`.
 ///
 /// The default behaviour is to generate both field and generic bounds from
 /// type parameters.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// // Limit bounds to only generics.
 /// s.add_bounds(AddBounds::Generics);
 ///
 /// assert_eq!(
 /// s.bound_impl(quote!(krate::Trait), quote!{
 /// fn a() {}
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U> krate::Trait for A<T, U>
 /// where T: krate::Trait,
 /// U: krate::Trait
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 pub fn add_bounds(&mut self, mode: AddBounds) -> &mut Self {
 self.add_bounds = mode;
 self
 }

 /// Filter the variants matched by this `Structure` object. This has 2 effects:
 ///
 /// * Match arms destructuring these variants will no longer be generated by
 /// methods on this `Structure`
 ///
 /// * Impl blocks created with the `bound_impl` or `unsafe_bound_impl`
 /// method only consider type parameters referenced in the types of
 /// fields in non-fitered variants.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 ///
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "B");
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::C(ref __binding_0,) => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// _ => {}
 /// }.to_string()
 /// );
 /// ```
 pub fn filter_variants<F>(&mut self, f: F) -> &>mut Self
 where
 F: FnMut(&VariantInfo<'_>) -> bool,
 {
 let before_len = self.variants.len();
 self.variants.retain(f);
 if self.variants.len() != before_len {
 self.omitted_variants = true;
 }
 self
 }
 /// Iterates all the variants of this `Structure` object and uses a closure to determine if a
 /// variant should be removed. If the closure returns `true` the variant is removed from the
 /// structure. If the closure returns `false`, the variant remains in the structure.
 ///
 /// All the removed variants are moved to a new `Structure` object which is otherwise identical
 /// to the current one. To understand the effects of removing a variant from a structure check
 /// the [`Structure::filter_variants`] documentation.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 ///
 /// let mut with_c = Structure::new(&di);
 ///
 /// let with_b = with_c.drain_filter_variants(|v| v.ast().ident == "B");
 ///
 /// assert_eq!(
 /// with_c.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::C(ref __binding_0,) => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 ///
 /// assert_eq!(
 /// with_b.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B(ref __binding_0, ref __binding_1,) => {
 /// { println!("{:?}", __binding_0) }
 /// { println!("{:?}", __binding_1) }
 /// }
 /// }.to_string()
 /// );
 #[allow(clippy::return_self_not_must_use)]
 pub fn drain_filter_variants<F>(&mut self, mut f: F) -> Self
 where
 F: FnMut(&VariantInfo<'_>) -> bool,
 {
 let mut other = Self {
 variants: vec![],
 omitted_variants: self.omitted_variants,
 ast: self.ast,
 extra_impl: self.extra_impl.clone(),
 extra_predicates: self.extra_predicates.clone(),
 add_bounds: self.add_bounds,
 };

 let (other_variants, self_variants) = self.variants.drain(..).partition(&mut f);
 other.variants = other_variants;
 self.variants = self_variants;

 other
 }

 /// Remove the variant at the given index.
 ///
 /// # Panics
 ///
 /// Panics if the index is out of range.
 pub fn remove_variant(&mut self, idx: usize) -> &mut Self {
 self.variants.remove(idx);
 self.omitted_variants = true;
 self
 }

 /// Updates the `BindStyle` for each of the passed-in fields by calling the
 /// passed-in function for each `BindingInfo`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B(i32, i32),
 /// C(u32),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.bind_with(|bi| BindStyle::RefMut);
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B(ref mut __binding_0, ref mut __binding_1,) => {
 /// { println!("{:?}", __binding_0) }
 /// { println!("{:?}", __binding_1) }
 /// }
 /// A::C(ref mut __binding_0,) => {
 /// { println!("{:?}", __binding_0) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn bind_with<F>(&mut self, mut f: F) -> &mut Self
 where
 F: FnMut(&BindingInfo<'_>) -> BindStyle,
 {
 for variant in &mut self.variants {
 variant.bind_with(&mut f);
 }
 self
 }

 /// Updates the binding name for each fo the passed-in fields by calling the
 /// passed-in function for each `BindingInfo`.
 ///
 /// The function will be called with the `BindingInfo` and its index in the
 /// enclosing variant.
 ///
 /// The default name is `__binding_{}` where `{}` is replaced with an
 /// increasing number.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A {
 /// B{ a: i32, b: i32 },
 /// C{ a: u32 },
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.binding_name(|bi, i| bi.ident.clone().unwrap());
 ///
 /// assert_eq!(
 /// s.each(|bi| quote!(println!("{:?}", #bi))).to_string(),
 ///
 /// quote!{
 /// A::B{ a: ref a, b: ref b, } => {
 /// { println!("{:?}", a) }
 /// { println!("{:?}", b) }
 /// }
 /// A::C{ a: ref a, } => {
 /// { println!("{:?}", a) }
 /// }
 /// }.to_string()
 /// );
 /// ```
 pub fn binding_name<F>(&mut self, mut f: F) -> &>mut Self
 where
 F: FnMut(&Field, usize) -> Ident,
 {
 for variant in &mut self.variants {
 variant.binding_name(&mut f);
 }
 self
 }

 /// Returns a list of the type parameters which are refrenced in the types
 /// of non-filtered fields / variants.
 ///
 /// # Caveat
 ///
 /// If the struct contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T, i32),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "C");
 ///
 /// assert_eq!(
 /// s.referenced_ty_params(),
 /// &["e::format_ident!("T")]
 /// );
 /// ```
 pub fn referenced_ty_params(&self) -> Vec<&'a Ident> {
 let mut flags = Vec::new();
 for variant in &self.variants {
 for binding in &variant.bindings {
 generics_fuse(&mut flags, &binding.seen_generics);
 }
 }
 fetch_generics(&flags, &self.ast.generics)
 }

 /// Adds an `impl<>` generic parameter.
 /// This can be used when the trait to be derived needs some extra generic parameters.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 /// let generic: syn::GenericParam = syn::parse_quote!(X: krate::AnotherTrait);
 ///
 /// assert_eq!(
 /// s.add_impl_generic(generic)
 /// .bound_impl(quote!(krate::Trait<X>),
 /// quote!{
 /// fn a() {}
 /// }
 /// ).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U, X: krate::AnotherTrait> krate::Trait<X> for A<T, U>
 /// where T : krate :: Trait < X >,
 /// Option: krate::Trait<X>,
 /// U: krate::Trait<X>
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 pub fn add_impl_generic(&mut self, param: GenericParam) -> &yle='color:red'>mut Self {
 self.extra_impl.push(param);
 self
 }

 /// Add trait bounds for a trait with the given path for each type parmaeter
 /// referenced in the types of non-filtered fields.
 ///
 /// # Caveat
 ///
 /// If the method contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 pub fn add_trait_bounds(
 &self,
 bound: &TraitBound,
 where_clause: &mut Option<WhereClause>,
 mode: AddBounds,
 ) {
 // If we have any explicit where predicates, make sure to add them first.
 if !self.extra_predicates.is_empty() {
 let clause = get_or_insert_with(&mut *where_clause, || WhereClause {
 where_token: Default::default(),
 predicates: punctuated::Punctuated::new(),
 });
 clause
 .predicates
 .extend(self.extra_predicates.iter().cloned());
 }

 let mut seen = HashSet::new();
 let mut pred = |ty: Type| {
 if !seen.contains(&ty) {
 seen.insert(ty.clone());

 // Add a predicate.
 let clause = get_or_insert_with(&mut *where_clause, || WhereClause {
 where_token: Default::default(),
 predicates: punctuated::Punctuated::new(),
 });
 clause.predicates.push(WherePredicate::Type(PredicateType {
 lifetimes: None,
 bounded_ty: ty,
 colon_token: Default::default(),
 bounds: Some(punctuated::Pair::End(TypeParamBound::Trait(bound.clone())))
 .into_iter()
 .collect(),
 }));
 }
 };

 for variant in &self.variants {
 for binding in &variant.bindings {
 match mode {
 AddBounds::Both | AddBounds::Fields => {
 for &seen in &binding.seen_generics {
 if seen {
 pred(binding.ast().ty.clone());
 break;
 }
 }
 }
 _ => {}
 }

 match mode {
 AddBounds::Both | AddBounds::Generics => {
 for param in binding.referenced_ty_params() {
 pred(Type::Path(TypePath {
 qself: None,
 path: (*param).clone().into(),
 }));
 }
 }
 _ => {}
 }
 }
 }
 }

 /// This method is a no-op, underscore consts are used by default now.
 pub fn underscore_const(&mut self, _enabled: bool) -> &mut Self {
 self
 }

 /// > NOTE: This methods' features are superceded by `Structure::gen_impl`.
 ///
 /// Creates an `impl` block with the required generic type fields filled in
 /// to implement the trait `path`.
 ///
 /// This method also adds where clauses to the impl requiring that all
 /// referenced type parmaeters implement the trait `path`.
 ///
 /// # Hygiene and Paths
 ///
 /// This method wraps the impl block inside of a `const` (see the example
 /// below). In this scope, the first segment of the passed-in path is
 /// `extern crate`-ed in. If you don't want to generate that `extern crate`
 /// item, use a global path.
 ///
 /// This means that if you are implementing `my_crate::Trait`, you simply
 /// write `s.bound_impl(quote!(my_crate::Trait), quote!(...))`, and for the
 /// entirety of the definition, you can refer to your crate as `my_crate`.
 ///
 /// # Caveat
 ///
 /// If the method contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 ///
 /// # Panics
 ///
 /// Panics if the path string parameter is not a valid `TraitBound`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "B");
 ///
 /// assert_eq!(
 /// s.bound_impl(quote!(krate::Trait), quote!{
 /// fn a() {}
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U> krate::Trait for A<T, U>
 /// where Option: krate::Trait,
 /// U: krate::Trait
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 pub fn bound_impl<P: ToTokens, B: ToTokens>(&self, path: P, body: B) -> TokenStream {
 self.impl_internal(
 path.into_token_stream(),
 body.into_token_stream(),
 quote!(),
 None,
 )
 }

 /// > NOTE: This methods' features are superceded by `Structure::gen_impl`.
 ///
 /// Creates an `impl` block with the required generic type fields filled in
 /// to implement the unsafe trait `path`.
 ///
 /// This method also adds where clauses to the impl requiring that all
 /// referenced type parmaeters implement the trait `path`.
 ///
 /// # Hygiene and Paths
 ///
 /// This method wraps the impl block inside of a `const` (see the example
 /// below). In this scope, the first segment of the passed-in path is
 /// `extern crate`-ed in. If you don't want to generate that `extern crate`
 /// item, use a global path.
 ///
 /// This means that if you are implementing `my_crate::Trait`, you simply
 /// write `s.bound_impl(quote!(my_crate::Trait), quote!(...))`, and for the
 /// entirety of the definition, you can refer to your crate as `my_crate`.
 ///
 /// # Caveat
 ///
 /// If the method contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 ///
 /// # Panics
 ///
 /// Panics if the path string parameter is not a valid `TraitBound`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "B");
 ///
 /// assert_eq!(
 /// s.unsafe_bound_impl(quote!(krate::Trait), quote!{
 /// fn a() {}
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// unsafe impl<T, U> krate::Trait for A<T, U>
 /// where Option: krate::Trait,
 /// U: krate::Trait
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 pub fn unsafe_bound_impl<P: ToTokens, B: ToTokens>(&self, path: P, body: B) -> TokenStream {
 self.impl_internal(
 path.into_token_stream(),
 body.into_token_stream(),
 quote!(unsafe),
 None,
 )
 }

 /// > NOTE: This methods' features are superceded by `Structure::gen_impl`.
 ///
 /// Creates an `impl` block with the required generic type fields filled in
 /// to implement the trait `path`.
 ///
 /// This method will not add any where clauses to the impl.
 ///
 /// # Hygiene and Paths
 ///
 /// This method wraps the impl block inside of a `const` (see the example
 /// below). In this scope, the first segment of the passed-in path is
 /// `extern crate`-ed in. If you don't want to generate that `extern crate`
 /// item, use a global path.
 ///
 /// This means that if you are implementing `my_crate::Trait`, you simply
 /// write `s.bound_impl(quote!(my_crate::Trait), quote!(...))`, and for the
 /// entirety of the definition, you can refer to your crate as `my_crate`.
 ///
 /// # Panics
 ///
 /// Panics if the path string parameter is not a valid `TraitBound`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "B");
 ///
 /// assert_eq!(
 /// s.unbound_impl(quote!(krate::Trait), quote!{
 /// fn a() {}
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U> krate::Trait for A<T, U> {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 pub fn unbound_impl<P: ToTokens, B: ToTokens>(&self, path: P, body: B) -> TokenStream {
 self.impl_internal(
 path.into_token_stream(),
 body.into_token_stream(),
 quote!(),
 Some(AddBounds::None),
 )
 }

 /// > NOTE: This methods' features are superceded by `Structure::gen_impl`.
 ///
 /// Creates an `impl` block with the required generic type fields filled in
 /// to implement the unsafe trait `path`.
 ///
 /// This method will not add any where clauses to the impl.
 ///
 /// # Hygiene and Paths
 ///
 /// This method wraps the impl block inside of a `const` (see the example
 /// below). In this scope, the first segment of the passed-in path is
 /// `extern crate`-ed in. If you don't want to generate that `extern crate`
 /// item, use a global path.
 ///
 /// This means that if you are implementing `my_crate::Trait`, you simply
 /// write `s.bound_impl(quote!(my_crate::Trait), quote!(...))`, and for the
 /// entirety of the definition, you can refer to your crate as `my_crate`.
 ///
 /// # Panics
 ///
 /// Panics if the path string parameter is not a valid `TraitBound`.
 ///
 /// # Example
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "B");
 ///
 /// assert_eq!(
 /// s.unsafe_unbound_impl(quote!(krate::Trait), quote!{
 /// fn a() {}
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// unsafe impl<T, U> krate::Trait for A<T, U> {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 #[deprecated]
 pub fn unsafe_unbound_impl<P: ToTokens, B: ToTokens>(&self, path: P, body: B) -> TokenStream {
 self.impl_internal(
 path.into_token_stream(),
 body.into_token_stream(),
 quote!(unsafe),
 Some(AddBounds::None),
 )
 }

 fn impl_internal(
 &self,
 path: TokenStream,
 body: TokenStream,
 safety: TokenStream,
 mode: Option<AddBounds>,
 ) -> TokenStream {
 let mode = mode.unwrap_or(self.add_bounds);
 let name = &self.ast.ident;
 let mut gen_clone = self.ast.generics.clone();
 gen_clone.params.extend(self.extra_impl.iter().cloned());
 let (impl_generics, _, _) = gen_clone.split_for_impl();
 let (_, ty_generics, where_clause) = self.ast.generics.split_for_impl();

 let bound = syn::parse2::<TraitBound>(path)
 .expect("`path` argument must be a valid rust trait bound");

 let mut where_clause = where_clause.cloned();
 self.add_trait_bounds(&bound, &mut where_clause, mode);

 // This function is smart. If a global path is passed, no extern crate
 // statement will be generated, however, a relative path will cause the
 // crate which it is relative to to be imported within the current
 // scope.
 let mut extern_crate = quote!();
 if bound.path.leading_colon.is_none() {
 if let Some(seg) = bound.path.segments.first() {
 let seg = &seg.ident;
 extern_crate = quote! { extern crate #seg; };
 }
 }

 let generated = quote! {
 #extern_crate
 #safety impl #impl_generics #bound for #name #ty_generics #where_clause {
 #body
 }
 };

 quote! {
 const _: () = { #generated };
 }
 }

 /// Generate an impl block for the given struct. This impl block will
 /// automatically use hygiene tricks to avoid polluting the caller's
 /// namespace, and will automatically add trait bounds for generic type
 /// parameters.
 ///
 /// # Syntax
 ///
 /// This function accepts its arguments as a `TokenStream`. The recommended way
 /// to call this function is passing the result of invoking the `quote!`
 /// macro to it.
 ///
 /// ```ignore
 /// s.gen_impl(quote! {
 /// // You can write any items which you want to import into scope here.
 /// // For example, you may want to include an `extern crate` for the
 /// // crate which implements your trait. These items will only be
 /// // visible to the code you generate, and won't be exposed to the
 /// // consuming crate
 /// extern crate krate;
 ///
 /// // You can also add `use` statements here to bring types or traits
 /// // into scope.
 /// //
 /// // WARNING: Try not to use common names here, because the stable
 /// // version of syn does not support hygiene and you could accidentally
 /// // shadow types from the caller crate.
 /// use krate::Trait as MyTrait;
 ///
 /// // The actual impl block is a `gen impl` or `gen unsafe impl` block.
 /// // You can use `@Self` to refer to the structure's type.
 /// gen impl MyTrait for @Self {
 /// fn f(&self) { ... }
 /// }
 /// })
 /// ```
 ///
 /// The most common usage of this trait involves loading the crate the
 /// target trait comes from with `extern crate`, and then invoking a `gen
 /// impl` block.
 ///
 /// # Hygiene
 ///
 /// This method tries to handle hygiene intelligently for both stable and
 /// unstable proc-macro implementations, however there are visible
 /// differences.
 ///
 /// The output of every `gen_impl` function is wrapped in a dummy `const`
 /// value, to ensure that it is given its own scope, and any values brought
 /// into scope are not leaked to the calling crate.
 ///
 /// By default, the above invocation may generate an output like the
 /// following:
 ///
 /// ```ignore
 /// const _: () = {
 /// extern crate krate;
 /// use krate::Trait as MyTrait;
 /// impl<T> MyTrait for Struct<T> where T: MyTrait {
 /// fn f(&self) { ... }
 /// }
 /// };
 /// ```
 ///
 /// ### Using the `std` crate
 ///
 /// If you are using `quote!()` to implement your trait, with the
 /// `proc-macro2/nightly` feature, `std` isn't considered to be in scope for
 /// your macro. This means that if you use types from `std` in your
 /// procedural macro, you'll want to explicitly load it with an `extern
 /// crate std;`.
 ///
 /// ### Absolute paths
 ///
 /// You should generally avoid using absolute paths in your generated code,
 /// as they will resolve very differently when using the stable and nightly
 /// versions of `proc-macro2`. Instead, load the crates you need to use
 /// explictly with `extern crate` and
 ///
 /// # Trait Bounds
 ///
 /// This method will automatically add trait bounds for any type parameters
 /// which are referenced within the types of non-ignored fields.
 ///
 /// Additional type parameters may be added with the generics syntax after
 /// the `impl` keyword.
 ///
 /// ### Type Macro Caveat
 ///
 /// If the method contains any macros in type position, all parameters will
 /// be considered bound. This is because we cannot determine which type
 /// parameters are bound by type macros.
 ///
 /// # Errors
 ///
 /// This function will generate a `compile_error!` if additional type
 /// parameters added by `impl<..>` conflict with generic type parameters on
 /// the original struct.
 ///
 /// # Panics
 ///
 /// This function will panic if the input `TokenStream` is not well-formed.
 ///
 /// # Example Usage
 ///
 /// ```
 /// # use synstructure::*;
 /// let di: syn::DeriveInput = syn::parse_quote! {
 /// enum A<T, U> {
 /// B(T),
 /// C(Option),
 /// }
 /// };
 /// let mut s = Structure::new(&di);
 ///
 /// s.filter_variants(|v| v.ast().ident != "B");
 ///
 /// assert_eq!(
 /// s.gen_impl(quote! {
 /// extern crate krate;
 /// gen impl krate::Trait for @Self {
 /// fn a() {}
 /// }
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U> krate::Trait for A<T, U>
 /// where
 /// Option: krate::Trait,
 /// U: krate::Trait
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 ///
 /// // NOTE: You can also add extra generics after the impl
 /// assert_eq!(
 /// s.gen_impl(quote! {
 /// extern crate krate;
 /// gen impl<X: krate::OtherTrait> krate::Trait<X> for @Self
 /// where
 /// X: Send + Sync,
 /// {
 /// fn a() {}
 /// }
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<X: krate::OtherTrait, T, U> krate::Trait<X> for A<T, U>
 /// where
 /// X: Send + Sync,
 /// Option: krate::Trait<X>,
 /// U: krate::Trait<X>
 /// {
 /// fn a() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 ///
 /// // NOTE: you can generate multiple traits with a single call
 /// assert_eq!(
 /// s.gen_impl(quote! {
 /// extern crate krate;
 ///
 /// gen impl krate::Trait for @Self {
 /// fn a() {}
 /// }
 ///
 /// gen impl krate::OtherTrait for @Self {
 /// fn b() {}
 /// }
 /// }).to_string(),
 /// quote!{
 /// const _: () = {
 /// extern crate krate;
 /// impl<T, U> krate::Trait for A<T, U>
 /// where
 /// Option: krate::Trait,
 /// U: krate::Trait
 /// {
 /// fn a() {}
 /// }
 ///
 /// impl<T, U> krate::OtherTrait for A<T, U>
 /// where
 /// Option: krate::OtherTrait,
 /// U: krate::OtherTrait
 /// {
 /// fn b() {}
 /// }
 /// };
 /// }.to_string()
 /// );
 /// ```
 ///
 /// Use `add_bounds` to change which bounds are generated.
 pub fn gen_impl(&self, cfg: TokenStream) -> TokenStream {
 Parser::parse2(
 |input: ParseStream<'_>| -> Result<TokenStream> { self.gen_impl_parse(input, true) },
 cfg,
 )
 .expect("Failed to parse gen_impl")
 }

 fn gen_impl_parse(&self, input: ParseStream<'_>, wrap: bool) -> Result<TokenStream> {
 fn parse_prefix(input: ParseStream<'_>) -> Result<Option<Token![unsafe]>> {
 if input.parse::<Ident>()? != "gen" {
 return Err(input.error("Expected keyword `gen`"));
 }
 let safety = input.parse::<Option<Token![unsafe]>>()?;
 let _ = input.parse::<Token![impl]>()?;
 Ok(safety)
 }

 let mut before = vec![];
 loop {
 if parse_prefix(&input.fork()).is_ok() {
 break;
 }
 before.push(input.parse::<TokenTree>()?);
 }

 // Parse the prefix "for real"
 let safety = parse_prefix(input)?;

 // optional `<>`
 let mut generics = input.parse::<Generics>()?;

 // @bound
 let bound = input.parse::<TraitBound>()?;

 // `for @Self`
 let _ = input.parse::<Token![for]>()?;
 let _ = input.parse::<Token![@]>()?;
 let _ = input.parse::<Token![Self]>()?;

 // optional `where ...`
 generics.where_clause = input.parse()?;

 // Body of the impl
 let body;
 braced!(body in input);
 let body = body.parse::<TokenStream>()?;

 // Try to parse the next entry in sequence. If this fails, we'll fall
 // back to just parsing the entire rest of the TokenStream.
 let maybe_next_impl = self.gen_impl_parse(&input.fork(), false);

 // Eat tokens to the end. Whether or not our speculative nested parse
 // succeeded, we're going to want to consume the rest of our input.
 let mut after = input.parse::<TokenStream>()?;
 if let Ok(stream) = maybe_next_impl {
 after = stream;
 }
 assert!(input.is_empty(), "Should've consumed the rest of our input");

 /* Codegen Logic */
 let name = &self.ast.ident;

 // Add the generics from the original struct in, and then add any
 // additional trait bounds which we need on the type.
 if let Err(err) = merge_generics(&mut generics, &self.ast.generics) {
 // Report the merge error as a `compile_error!`, as it may be
 // triggerable by an end-user.
 return Ok(err.to_compile_error());
 }

 self.add_trait_bounds(&bound, &mut generics.where_clause, self.add_bounds);
 let (impl_generics, _, where_clause) = generics.split_for_impl();
 let (_, ty_generics, _) = self.ast.generics.split_for_impl();

 let generated = quote! {
 #(#before)*
 #safety impl #impl_generics #bound for #name #ty_generics #where_clause {
 #body
 }
 #after
 };

 if wrap {
 Ok(quote! {
 const _: () = { #generated };
 })
 } else {
 Ok(generated)
 }
 }
}

/// Dumps an unpretty version of a tokenstream. Takes any type which implements
/// `Display`.
///
/// This is mostly useful for visualizing the output of a procedural macro, as
/// it makes it marginally more readable. It is used in the implementation of
/// `test_derive!` to unprettily print the output.
///
/// # Stability
///
/// The stability of the output of this function is not guaranteed. Do not
/// assert that the output of this function does not change between minor
/// versions.
///
/// # Example
///
/// ```
/// # use quote::quote;
/// assert_eq!(
/// synstructure::unpretty_print(quote! {
/// const _: () = {
/// extern crate krate;
/// impl<T, U> krate::Trait for A<T, U>
/// where
/// Option: krate::Trait,
/// U: krate::Trait
/// {
/// fn a() {}
/// }
/// };
/// }),
/// "const _ : (
/// )
/// = {
/// extern crate krate ;
/// impl < T , U > krate :: Trait for A < T , U > where Option : krate :: Trait , U : krate :: Trait {
/// fn a (
/// )
/// {
/// }
/// }
/// }
/// ;
/// "
/// )
/// ```
pub fn unpretty_print<T: std::fmt::Display>(ts: T) -> String {
 let mut res = String::new();

 let raw_s = ts.to_string();
 let mut s = &raw_s[..];
 let mut indent = 0;
 while let Some(i) = s.find(&['(', '{', '[', ')', '}', ']', ';'][..]) {
 match &s[i..=i] {
 "(" | "{" | "[" => indent += 1,
 ")" | "}" | "]" => indent -= 1,
 _ => {}
 }
 res.push_str(&s[..=i]);
 res.push('\n');
 for _ in 0..indent {
 res.push_str(" ");
 }
 s = trim_start_matches(&s[i + 1..], ' ');
 }
 res.push_str(s);
 res
}

/// `trim_left_matches` has been deprecated in favor of `trim_start_matches`.
/// This helper silences the warning, as we need to continue using
/// `trim_left_matches` for rust 1.15 support.
#[allow(deprecated)]
fn trim_start_matches(s: &str, c: char) -> &str {
 s.trim_left_matches(c)
}

/// Helper trait describing values which may be returned by macro implementation
/// methods used by this crate's macros.
pub trait MacroResult {
 /// Convert this result into a `Result` for further processing / validation.
 fn into_result(self) -> Result<TokenStream>;

 /// Convert this result into a `proc_macro::TokenStream`, ready to return
 /// from a native `proc_macro` implementation.
 ///
 /// If `into_result()` would return an `Err`, this method should instead
 /// generate a `compile_error!` invocation to nicely report the error.
 ///
 /// *This method is available if `synstructure` is built with the
 /// `"proc-macro"` feature.*
 #[cfg(all(
 not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
 feature = "proc-macro"
 ))]
 fn into_stream(self) -> proc_macro::TokenStream
 where
 Self: Sized,
 {
 match self.into_result() {
 Ok(ts) => ts.into(),
 Err(err) => err.to_compile_error().into(),
 }
 }
}

#[cfg(all(
 not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
 feature = "proc-macro"
))]
impl MacroResult for proc_macro::TokenStream {
 fn into_result(self) -> Result<TokenStream> {
 Ok(self.into())
 }

 fn into_stream(self) -> proc_macro::TokenStream {
 self
 }
}

impl MacroResult for TokenStream {
 fn into_result(self) -> Result<TokenStream> {
 Ok(self)
 }
}

impl<T: MacroResult> MacroResult for Result<T> {
 fn into_result(self) -> Result<TokenStream> {
 match self {
 Ok(v) => v.into_result(),
 Err(err) => Err(err),
 }
 }
}

#[cfg(test)]
mod tests {
 use super::*;

 // Regression test for #48
 #[test]
 fn test_each_enum() {
 let di: syn::DeriveInput = syn::parse_quote! {
 enum A {
 Foo(usize, bool),
 Bar(bool, usize),
 Baz(usize, bool, usize),
 Quux(bool, usize, bool)
 }
 };
 let mut s = Structure::new(&di);

 s.filter(|bi| bi.ast().ty.to_token_stream().to_string() == "bool");

 assert_eq!(
 s.each(|bi| quote!(do_something(#bi))).to_string(),
 quote! {
 A::Foo(_, ref __binding_1,) => { { do_something(__binding_1) } }
 A::Bar(ref __binding_0, ..) => { { do_something(__binding_0) } }
 A::Baz(_, ref __binding_1, ..) => { { do_something(__binding_1) } }
 A::Quux(ref __binding_0, _, ref __binding_2,) => {
 {
 do_something(__binding_0)
 }
 {
 do_something(__binding_2)
 }
 }
 }
 .to_string()
 );
 }
}

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