path: root/src/libsyntax_ext/deriving/generic/mod.rs

//! Some code that abstracts away much of the boilerplate of writing
//! `derive` instances for traits. Among other things it manages getting
//! access to the fields of the 4 different sorts of structs and enum
//! variants, as well as creating the method and impl ast instances.
//!
//! Supported features (fairly exhaustive):
//!
//! - Methods taking any number of parameters of any type, and returning
//!   any type, other than vectors, bottom and closures.
//! - Generating `impl`s for types with type parameters and lifetimes
//!   (e.g., `Option<T>`), the parameters are automatically given the
//!   current trait as a bound. (This includes separate type parameters
//!   and lifetimes for methods.)
//! - Additional bounds on the type parameters (`TraitDef.additional_bounds`)
//!
//! The most important thing for implementors is the `Substructure` and
//! `SubstructureFields` objects. The latter groups 5 possibilities of the
//! arguments:
//!
//! - `Struct`, when `Self` is a struct (including tuple structs, e.g
//!   `struct T(i32, char)`).
//! - `EnumMatching`, when `Self` is an enum and all the arguments are the
//!   same variant of the enum (e.g., `Some(1)`, `Some(3)` and `Some(4)`)
//! - `EnumNonMatchingCollapsed` when `Self` is an enum and the arguments
//!   are not the same variant (e.g., `None`, `Some(1)` and `None`).
//! - `StaticEnum` and `StaticStruct` for static methods, where the type
//!   being derived upon is either an enum or struct respectively. (Any
//!   argument with type Self is just grouped among the non-self
//!   arguments.)
//!
//! In the first two cases, the values from the corresponding fields in
//! all the arguments are grouped together. For `EnumNonMatchingCollapsed`
//! this isn't possible (different variants have different fields), so the
//! fields are inaccessible. (Previous versions of the deriving infrastructure
//! had a way to expand into code that could access them, at the cost of
//! generating exponential amounts of code; see issue #15375). There are no
//! fields with values in the static cases, so these are treated entirely
//! differently.
//!
//! The non-static cases have `Option<ident>` in several places associated
//! with field `expr`s. This represents the name of the field it is
//! associated with. It is only not `None` when the associated field has
//! an identifier in the source code. For example, the `x`s in the
//! following snippet
//!
//! ```rust
//! # #![allow(dead_code)]
//! struct A { x : i32 }
//!
//! struct B(i32);
//!
//! enum C {
//!     C0(i32),
//!     C1 { x: i32 }
//! }
//! ```
//!
//! The `i32`s in `B` and `C0` don't have an identifier, so the
//! `Option<ident>`s would be `None` for them.
//!
//! In the static cases, the structure is summarized, either into the just
//! spans of the fields or a list of spans and the field idents (for tuple
//! structs and record structs, respectively), or a list of these, for
//! enums (one for each variant). For empty struct and empty enum
//! variants, it is represented as a count of 0.
//!
//! # "`cs`" functions
//!
//! The `cs_...` functions ("combine substructure) are designed to
//! make life easier by providing some pre-made recipes for common
//! threads; mostly calling the function being derived on all the
//! arguments and then combining them back together in some way (or
//! letting the user chose that). They are not meant to be the only
//! way to handle the structures that this code creates.
//!
//! # Examples
//!
//! The following simplified `PartialEq` is used for in-code examples:
//!
//! ```rust
//! trait PartialEq {
//!     fn eq(&self, other: &Self) -> bool;
//! }
//! impl PartialEq for i32 {
//!     fn eq(&self, other: &i32) -> bool {
//!         *self == *other
//!     }
//! }
//! ```
//!
//! Some examples of the values of `SubstructureFields` follow, using the
//! above `PartialEq`, `A`, `B` and `C`.
//!
//! ## Structs
//!
//! When generating the `expr` for the `A` impl, the `SubstructureFields` is
//!
//! ```{.text}
//! Struct(vec![FieldInfo {
//!            span: <span of x>
//!            name: Some(<ident of x>),
//!            self_: <expr for &self.x>,
//!            other: vec![<expr for &other.x]
//!          }])
//! ```
//!
//! For the `B` impl, called with `B(a)` and `B(b)`,
//!
//! ```{.text}
//! Struct(vec![FieldInfo {
//!           span: <span of `i32`>,
//!           name: None,
//!           self_: <expr for &a>
//!           other: vec![<expr for &b>]
//!          }])
//! ```
//!
//! ## Enums
//!
//! When generating the `expr` for a call with `self == C0(a)` and `other
//! == C0(b)`, the SubstructureFields is
//!
//! ```{.text}
//! EnumMatching(0, <ast::Variant for C0>,
//!              vec![FieldInfo {
//!                 span: <span of i32>
//!                 name: None,
//!                 self_: <expr for &a>,
//!                 other: vec![<expr for &b>]
//!               }])
//! ```
//!
//! For `C1 {x}` and `C1 {x}`,
//!
//! ```{.text}
//! EnumMatching(1, <ast::Variant for C1>,
//!              vec![FieldInfo {
//!                 span: <span of x>
//!                 name: Some(<ident of x>),
//!                 self_: <expr for &self.x>,
//!                 other: vec![<expr for &other.x>]
//!                }])
//! ```
//!
//! For `C0(a)` and `C1 {x}` ,
//!
//! ```{.text}
//! EnumNonMatchingCollapsed(
//!     vec![<ident of self>, <ident of __arg_1>],
//!     &[<ast::Variant for C0>, <ast::Variant for C1>],
//!     &[<ident for self index value>, <ident of __arg_1 index value>])
//! ```
//!
//! It is the same for when the arguments are flipped to `C1 {x}` and
//! `C0(a)`; the only difference is what the values of the identifiers
//! <ident for self index value> and <ident of __arg_1 index value> will
//! be in the generated code.
//!
//! `EnumNonMatchingCollapsed` deliberately provides far less information
//! than is generally available for a given pair of variants; see #15375
//! for discussion.
//!
//! ## Static
//!
//! A static method on the types above would result in,
//!
//! ```{.text}
//! StaticStruct(<ast::VariantData of A>, Named(vec![(<ident of x>, <span of x>)]))
//!
//! StaticStruct(<ast::VariantData of B>, Unnamed(vec![<span of x>]))
//!
//! StaticEnum(<ast::EnumDef of C>,
//!            vec![(<ident of C0>, <span of C0>, Unnamed(vec![<span of i32>])),
//!                 (<ident of C1>, <span of C1>, Named(vec![(<ident of x>, <span of x>)]))])
//! ```

pub use StaticFields::*;
pub use SubstructureFields::*;

use std::cell::RefCell;
use std::iter;
use std::vec;

use syntax::ast::{self, BinOpKind, EnumDef, Expr, Generics, Ident, PatKind};
use syntax::ast::{GenericArg, GenericParamKind, VariantData};
use syntax::attr;
use syntax::ptr::P;
use syntax::sess::ParseSess;
use syntax::source_map::respan;
use syntax::symbol::{kw, sym, Symbol};
use syntax::util::map_in_place::MapInPlace;
use syntax_expand::base::{Annotatable, ExtCtxt};
use syntax_pos::Span;

use ty::{LifetimeBounds, Path, Ptr, PtrTy, Self_, Ty};

use crate::deriving;

pub mod ty;

pub struct TraitDef<'a> {
    /// The span for the current #[derive(Foo)] header.
    pub span: Span,

    pub attributes: Vec<ast::Attribute>,

    /// Path of the trait, including any type parameters
    pub path: Path<'a>,

    /// Additional bounds required of any type parameters of the type,
    /// other than the current trait
    pub additional_bounds: Vec<Ty<'a>>,

    /// Any extra lifetimes and/or bounds, e.g., `D: serialize::Decoder`
    pub generics: LifetimeBounds<'a>,

    /// Is it an `unsafe` trait?
    pub is_unsafe: bool,

    /// Can this trait be derived for unions?
    pub supports_unions: bool,

    pub methods: Vec<MethodDef<'a>>,

    pub associated_types: Vec<(ast::Ident, Ty<'a>)>,
}

pub struct MethodDef<'a> {
    /// name of the method
    pub name: &'a str,
    /// List of generics, e.g., `R: rand::Rng`
    pub generics: LifetimeBounds<'a>,

    /// Whether there is a self argument (outer Option) i.e., whether
    /// this is a static function, and whether it is a pointer (inner
    /// Option)
    pub explicit_self: Option<Option<PtrTy>>,

    /// Arguments other than the self argument
    pub args: Vec<(Ty<'a>, &'a str)>,

    /// Returns type
    pub ret_ty: Ty<'a>,

    pub attributes: Vec<ast::Attribute>,

    // Is it an `unsafe fn`?
    pub is_unsafe: bool,

    /// Can we combine fieldless variants for enums into a single match arm?
    pub unify_fieldless_variants: bool,

    pub combine_substructure: RefCell<CombineSubstructureFunc<'a>>,
}

/// All the data about the data structure/method being derived upon.
pub struct Substructure<'a> {
    /// ident of self
    pub type_ident: Ident,
    /// ident of the method
    pub method_ident: Ident,
    /// dereferenced access to any `Self_` or `Ptr(Self_, _)` arguments
    pub self_args: &'a [P<Expr>],
    /// verbatim access to any other arguments
    pub nonself_args: &'a [P<Expr>],
    pub fields: &'a SubstructureFields<'a>,
}

/// Summary of the relevant parts of a struct/enum field.
pub struct FieldInfo<'a> {
    pub span: Span,
    /// None for tuple structs/normal enum variants, Some for normal
    /// structs/struct enum variants.
    pub name: Option<Ident>,
    /// The expression corresponding to this field of `self`
    /// (specifically, a reference to it).
    pub self_: P<Expr>,
    /// The expressions corresponding to references to this field in
    /// the other `Self` arguments.
    pub other: Vec<P<Expr>>,
    /// The attributes on the field
    pub attrs: &'a [ast::Attribute],
}

/// Fields for a static method
pub enum StaticFields {
    /// Tuple and unit structs/enum variants like this.
    Unnamed(Vec<Span>, bool /*is tuple*/),
    /// Normal structs/struct variants.
    Named(Vec<(Ident, Span)>),
}

/// A summary of the possible sets of fields.
pub enum SubstructureFields<'a> {
    Struct(&'a ast::VariantData, Vec<FieldInfo<'a>>),
    /// Matching variants of the enum: variant index, variant count, ast::Variant,
    /// fields: the field name is only non-`None` in the case of a struct
    /// variant.
    EnumMatching(usize, usize, &'a ast::Variant, Vec<FieldInfo<'a>>),

    /// Non-matching variants of the enum, but with all state hidden from
    /// the consequent code. The first component holds `Ident`s for all of
    /// the `Self` arguments; the second component is a slice of all of the
    /// variants for the enum itself, and the third component is a list of
    /// `Ident`s bound to the variant index values for each of the actual
    /// input `Self` arguments.
    EnumNonMatchingCollapsed(Vec<Ident>, &'a [ast::Variant], &'a [Ident]),

    /// A static method where `Self` is a struct.
    StaticStruct(&'a ast::VariantData, StaticFields),
    /// A static method where `Self` is an enum.
    StaticEnum(&'a ast::EnumDef, Vec<(Ident, Span, StaticFields)>),
}

/// Combine the values of all the fields together. The last argument is
/// all the fields of all the structures.
pub type CombineSubstructureFunc<'a> =
    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, &Substructure<'_>) -> P<Expr> + 'a>;

/// Deal with non-matching enum variants. The tuple is a list of
/// identifiers (one for each `Self` argument, which could be any of the
/// variants since they have been collapsed together) and the identifiers
/// holding the variant index value for each of the `Self` arguments. The
/// last argument is all the non-`Self` args of the method being derived.
pub type EnumNonMatchCollapsedFunc<'a> =
    Box<dyn FnMut(&mut ExtCtxt<'_>, Span, (&[Ident], &[Ident]), &[P<Expr>]) -> P<Expr> + 'a>;

pub fn combine_substructure(
    f: CombineSubstructureFunc<'_>,
) -> RefCell<CombineSubstructureFunc<'_>> {
    RefCell::new(f)
}

/// This method helps to extract all the type parameters referenced from a
/// type. For a type parameter `<T>`, it looks for either a `TyPath` that
/// is not global and starts with `T`, or a `TyQPath`.
fn find_type_parameters(
    ty: &ast::Ty,
    ty_param_names: &[ast::Name],
    cx: &ExtCtxt<'_>,
) -> Vec<P<ast::Ty>> {
    use syntax::visit;

    struct Visitor<'a, 'b> {
        cx: &'a ExtCtxt<'b>,
        ty_param_names: &'a [ast::Name],
        types: Vec<P<ast::Ty>>,
    }

    impl<'a, 'b> visit::Visitor<'a> for Visitor<'a, 'b> {
        fn visit_ty(&mut self, ty: &'a ast::Ty) {
            if let ast::TyKind::Path(_, ref path) = ty.kind {
                if let Some(segment) = path.segments.first() {
                    if self.ty_param_names.contains(&segment.ident.name) {
                        self.types.push(P(ty.clone()));
                    }
                }
            }

            visit::walk_ty(self, ty)
        }

        fn visit_mac(&mut self, mac: &ast::Mac) {
            self.cx.span_err(mac.span(), "`derive` cannot be used on items with type macros");
        }
    }

    let mut visitor = Visitor { cx, ty_param_names, types: Vec::new() };
    visit::Visitor::visit_ty(&mut visitor, ty);

    visitor.types
}

impl<'a> TraitDef<'a> {
    pub fn expand(
        self,
        cx: &mut ExtCtxt<'_>,
        mitem: &ast::MetaItem,
        item: &'a Annotatable,
        push: &mut dyn FnMut(Annotatable),
    ) {
        self.expand_ext(cx, mitem, item, push, false);
    }

    pub fn expand_ext(
        self,
        cx: &mut ExtCtxt<'_>,
        mitem: &ast::MetaItem,
        item: &'a Annotatable,
        push: &mut dyn FnMut(Annotatable),
        from_scratch: bool,
    ) {
        match *item {
            Annotatable::Item(ref item) => {
                let is_packed = item.attrs.iter().any(|attr| {
                    for r in attr::find_repr_attrs(&cx.parse_sess, attr) {
                        if let attr::ReprPacked(_) = r {
                            return true;
                        }
                    }
                    false
                });
                let has_no_type_params = match item.kind {
                    ast::ItemKind::Struct(_, ref generics)
                    | ast::ItemKind::Enum(_, ref generics)
                    | ast::ItemKind::Union(_, ref generics) => {
                        !generics.params.iter().any(|param| match param.kind {
                            ast::GenericParamKind::Type { .. } => true,
                            _ => false,
                        })
                    }
                    _ => {
                        // Non-ADT derive is an error, but it should have been
                        // set earlier; see
                        // libsyntax/ext/expand.rs:MacroExpander::expand()
                        return;
                    }
                };
                let container_id = cx.current_expansion.id.expn_data().parent;
                let always_copy = has_no_type_params && cx.resolver.has_derive_copy(container_id);
                let use_temporaries = is_packed && always_copy;

                let newitem = match item.kind {
                    ast::ItemKind::Struct(ref struct_def, ref generics) => self.expand_struct_def(
                        cx,
                        &struct_def,
                        item.ident,
                        generics,
                        from_scratch,
                        use_temporaries,
                    ),
                    ast::ItemKind::Enum(ref enum_def, ref generics) => {
                        // We ignore `use_temporaries` here, because
                        // `repr(packed)` enums cause an error later on.
                        //
                        // This can only cause further compilation errors
                        // downstream in blatantly illegal code, so it
                        // is fine.
                        self.expand_enum_def(
                            cx,
                            enum_def,
                            &item.attrs,
                            item.ident,
                            generics,
                            from_scratch,
                        )
                    }
                    ast::ItemKind::Union(ref struct_def, ref generics) => {
                        if self.supports_unions {
                            self.expand_struct_def(
                                cx,
                                &struct_def,
                                item.ident,
                                generics,
                                from_scratch,
                                use_temporaries,
                            )
                        } else {
                            cx.span_err(mitem.span, "this trait cannot be derived for unions");
                            return;
                        }
                    }
                    _ => unreachable!(),
                };
                // Keep the lint attributes of the previous item to control how the
                // generated implementations are linted
                let mut attrs = newitem.attrs.clone();
                attrs.extend(
                    item.attrs
                        .iter()
                        .filter(|a| {
                            [
                                sym::allow,
                                sym::warn,
                                sym::deny,
                                sym::forbid,
                                sym::stable,
                                sym::unstable,
                            ]
                            .contains(&a.name_or_empty())
                        })
                        .cloned(),
                );
                push(Annotatable::Item(P(ast::Item { attrs: attrs, ..(*newitem).clone() })))
            }
            _ => {
                // Non-Item derive is an error, but it should have been
                // set earlier; see
                // libsyntax/ext/expand.rs:MacroExpander::expand()
                return;
            }
        }
    }

    /// Given that we are deriving a trait `DerivedTrait` for a type like:
    ///
    /// ```ignore (only-for-syntax-highlight)
    /// struct Struct<'a, ..., 'z, A, B: DeclaredTrait, C, ..., Z> where C: WhereTrait {
    ///     a: A,
    ///     b: B::Item,
    ///     b1: <B as DeclaredTrait>::Item,
    ///     c1: <C as WhereTrait>::Item,
    ///     c2: Option<<C as WhereTrait>::Item>,
    ///     ...
    /// }
    /// ```
    ///
    /// create an impl like:
    ///
    /// ```ignore (only-for-syntax-highlight)
    /// impl<'a, ..., 'z, A, B: DeclaredTrait, C, ... Z> where
    ///     C:                       WhereTrait,
    ///     A: DerivedTrait + B1 + ... + BN,
    ///     B: DerivedTrait + B1 + ... + BN,
    ///     C: DerivedTrait + B1 + ... + BN,
    ///     B::Item:                 DerivedTrait + B1 + ... + BN,
    ///     <C as WhereTrait>::Item: DerivedTrait + B1 + ... + BN,
    ///     ...
    /// {
    ///     ...
    /// }
    /// ```
    ///
    /// where B1, ..., BN are the bounds given by `bounds_paths`.'. Z is a phantom type, and
    /// therefore does not get bound by the derived trait.
    fn create_derived_impl(
        &self,
        cx: &mut ExtCtxt<'_>,
        type_ident: Ident,
        generics: &Generics,
        field_tys: Vec<P<ast::Ty>>,
        methods: Vec<ast::AssocItem>,
    ) -> P<ast::Item> {
        let trait_path = self.path.to_path(cx, self.span, type_ident, generics);

        // Transform associated types from `deriving::ty::Ty` into `ast::AssocItem`
        let associated_types =
            self.associated_types.iter().map(|&(ident, ref type_def)| ast::AssocItem {
                id: ast::DUMMY_NODE_ID,
                span: self.span,
                ident,
                vis: respan(self.span.shrink_to_lo(), ast::VisibilityKind::Inherited),
                defaultness: ast::Defaultness::Final,
                attrs: Vec::new(),
                generics: Generics::default(),
                kind: ast::AssocItemKind::TyAlias(
                    Vec::new(),
                    Some(type_def.to_ty(cx, self.span, type_ident, generics)),
                ),
                tokens: None,
            });

        let Generics { mut params, mut where_clause, span } =
            self.generics.to_generics(cx, self.span, type_ident, generics);

        // Create the generic parameters
        params.extend(generics.params.iter().map(|param| match param.kind {
            GenericParamKind::Lifetime { .. } => param.clone(),
            GenericParamKind::Type { .. } => {
                // I don't think this can be moved out of the loop, since
                // a GenericBound requires an ast id
                let bounds: Vec<_> =
                    // extra restrictions on the generics parameters to the
                    // type being derived upon
                    self.additional_bounds.iter().map(|p| {
                        cx.trait_bound(p.to_path(cx, self.span, type_ident, generics))
                    }).chain(
                        // require the current trait
                        iter::once(cx.trait_bound(trait_path.clone()))
                    ).chain(
                        // also add in any bounds from the declaration
                        param.bounds.iter().cloned()
                    ).collect();

                cx.typaram(self.span, param.ident, vec![], bounds, None)
            }
            GenericParamKind::Const { .. } => param.clone(),
        }));

        // and similarly for where clauses
        where_clause.predicates.extend(generics.where_clause.predicates.iter().map(|clause| {
            match *clause {
                ast::WherePredicate::BoundPredicate(ref wb) => {
                    ast::WherePredicate::BoundPredicate(ast::WhereBoundPredicate {
                        span: self.span,
                        bound_generic_params: wb.bound_generic_params.clone(),
                        bounded_ty: wb.bounded_ty.clone(),
                        bounds: wb.bounds.iter().cloned().collect(),
                    })
                }
                ast::WherePredicate::RegionPredicate(ref rb) => {
                    ast::WherePredicate::RegionPredicate(ast::WhereRegionPredicate {
                        span: self.span,
                        lifetime: rb.lifetime,
                        bounds: rb.bounds.iter().cloned().collect(),
                    })
                }
                ast::WherePredicate::EqPredicate(ref we) => {
                    ast::WherePredicate::EqPredicate(ast::WhereEqPredicate {
                        id: ast::DUMMY_NODE_ID,
                        span: self.span,
                        lhs_ty: we.lhs_ty.clone(),
                        rhs_ty: we.rhs_ty.clone(),
                    })
                }
            }
        }));

        {
            // Extra scope required here so ty_params goes out of scope before params is moved

            let mut ty_params = params
                .iter()
                .filter_map(|param| match param.kind {
                    ast::GenericParamKind::Type { .. } => Some(param),
                    _ => None,
                })
                .peekable();

            if ty_params.peek().is_some() {
                let ty_param_names: Vec<ast::Name> =
                    ty_params.map(|ty_param| ty_param.ident.name).collect();

                for field_ty in field_tys {
                    let tys = find_type_parameters(&field_ty, &ty_param_names, cx);

                    for ty in tys {
                        // if we have already handled this type, skip it
                        if let ast::TyKind::Path(_, ref p) = ty.kind {
                            if p.segments.len() == 1
                                && ty_param_names.contains(&p.segments[0].ident.name)
                            {
                                continue;
                            };
                        }
                        let mut bounds: Vec<_> = self
                            .additional_bounds
                            .iter()
                            .map(|p| cx.trait_bound(p.to_path(cx, self.span, type_ident, generics)))
                            .collect();

                        // require the current trait
                        bounds.push(cx.trait_bound(trait_path.clone()));

                        let predicate = ast::WhereBoundPredicate {
                            span: self.span,
                            bound_generic_params: Vec::new(),
                            bounded_ty: ty,
                            bounds,
                        };

                        let predicate = ast::WherePredicate::BoundPredicate(predicate);
                        where_clause.predicates.push(predicate);
                    }
                }
            }
        }

        let trait_generics = Generics { params, where_clause, span };

        // Create the reference to the trait.
        let trait_ref = cx.trait_ref(trait_path);

        let self_params: Vec<_> = generics
            .params
            .iter()
            .map(|param| match param.kind {
                GenericParamKind::Lifetime { .. } => {
                    GenericArg::Lifetime(cx.lifetime(self.span, param.ident))
                }
                GenericParamKind::Type { .. } => {
                    GenericArg::Type(cx.ty_ident(self.span, param.ident))
                }
                GenericParamKind::Const { .. } => {
                    GenericArg::Const(cx.const_ident(self.span, param.ident))
                }
            })
            .collect();

        // Create the type of `self`.
        let path = cx.path_all(self.span, false, vec![type_ident], self_params);
        let self_type = cx.ty_path(path);

        let attr = cx.attribute(cx.meta_word(self.span, sym::automatically_derived));
        // Just mark it now since we know that it'll end up used downstream
        attr::mark_used(&attr);
        let opt_trait_ref = Some(trait_ref);
        let unused_qual = {
            let word = syntax::attr::mk_nested_word_item(Ident::new(
                Symbol::intern("unused_qualifications"),
                self.span,
            ));
            let list = syntax::attr::mk_list_item(Ident::new(sym::allow, self.span), vec![word]);
            cx.attribute(list)
        };

        let mut a = vec![attr, unused_qual];
        a.extend(self.attributes.iter().cloned());

        let unsafety = if self.is_unsafe { ast::Unsafety::Unsafe } else { ast::Unsafety::Normal };

        cx.item(
            self.span,
            Ident::invalid(),
            a,
            ast::ItemKind::Impl(
                unsafety,
                ast::ImplPolarity::Positive,
                ast::Defaultness::Final,
                trait_generics,
                opt_trait_ref,
                self_type,
                methods.into_iter().chain(associated_types).collect(),
            ),
        )
    }

    fn expand_struct_def(
        &self,
        cx: &mut ExtCtxt<'_>,
        struct_def: &'a VariantData,
        type_ident: Ident,
        generics: &Generics,
        from_scratch: bool,
        use_temporaries: bool,
    ) -> P<ast::Item> {
        let field_tys: Vec<P<ast::Ty>> =
            struct_def.fields().iter().map(|field| field.ty.clone()).collect();

        let methods = self
            .methods
            .iter()
            .map(|method_def| {
                let (explicit_self, self_args, nonself_args, tys) =
                    method_def.split_self_nonself_args(cx, self, type_ident, generics);

                let body = if from_scratch || method_def.is_static() {
                    method_def.expand_static_struct_method_body(
                        cx,
                        self,
                        struct_def,
                        type_ident,
                        &self_args[..],
                        &nonself_args[..],
                    )
                } else {
                    method_def.expand_struct_method_body(
                        cx,
                        self,
                        struct_def,
                        type_ident,
                        &self_args[..],
                        &nonself_args[..],
                        use_temporaries,
                    )
                };

                method_def.create_method(cx, self, type_ident, generics, explicit_self, tys, body)
            })
            .collect();

        self.create_derived_impl(cx, type_ident, generics, field_tys, methods)
    }

    fn expand_enum_def(
        &self,
        cx: &mut ExtCtxt<'_>,
        enum_def: &'a EnumDef,
        type_attrs: &[ast::Attribute],
        type_ident: Ident,
        generics: &Generics,
        from_scratch: bool,
    ) -> P<ast::Item> {
        let mut field_tys = Vec::new();

        for variant in &enum_def.variants {
            field_tys.extend(variant.data.fields().iter().map(|field| field.ty.clone()));
        }

        let methods = self
            .methods
            .iter()
            .map(|method_def| {
                let (explicit_self, self_args, nonself_args, tys) =
                    method_def.split_self_nonself_args(cx, self, type_ident, generics);

                let body = if from_scratch || method_def.is_static() {
                    method_def.expand_static_enum_method_body(
                        cx,
                        self,
                        enum_def,
                        type_ident,
                        &self_args[..],
                        &nonself_args[..],
                    )
                } else {
                    method_def.expand_enum_method_body(
                        cx,
                        self,
                        enum_def,
                        type_attrs,
                        type_ident,
                        self_args,
                        &nonself_args[..],
                    )
                };

                method_def.create_method(cx, self, type_ident, generics, explicit_self, tys, body)
            })
            .collect();

        self.create_derived_impl(cx, type_ident, generics, field_tys, methods)
    }
}

fn find_repr_type_name(sess: &ParseSess, type_attrs: &[ast::Attribute]) -> &'static str {
    let mut repr_type_name = "isize";
    for a in type_attrs {
        for r in &attr::find_repr_attrs(sess, a) {
            repr_type_name = match *r {
                attr::ReprPacked(_)
                | attr::ReprSimd
                | attr::ReprAlign(_)
                | attr::ReprTransparent => continue,

                attr::ReprC => "i32",

                attr::ReprInt(attr::SignedInt(ast::IntTy::Isize)) => "isize",
                attr::ReprInt(attr::SignedInt(ast::IntTy::I8)) => "i8",
                attr::ReprInt(attr::SignedInt(ast::IntTy::I16)) => "i16",
                attr::ReprInt(attr::SignedInt(ast::IntTy::I32)) => "i32",
                attr::ReprInt(attr::SignedInt(ast::IntTy::I64)) => "i64",
                attr::ReprInt(attr::SignedInt(ast::IntTy::I128)) => "i128",

                attr::ReprInt(attr::UnsignedInt(ast::UintTy::Usize)) => "usize",
                attr::ReprInt(attr::UnsignedInt(ast::UintTy::U8)) => "u8",
                attr::ReprInt(attr::UnsignedInt(ast::UintTy::U16)) => "u16",
                attr::ReprInt(attr::UnsignedInt(ast::UintTy::U32)) => "u32",
                attr::ReprInt(attr::UnsignedInt(ast::UintTy::U64)) => "u64",
                attr::ReprInt(attr::UnsignedInt(ast::UintTy::U128)) => "u128",
            }
        }
    }
    repr_type_name
}

impl<'a> MethodDef<'a> {
    fn call_substructure_method(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        self_args: &[P<Expr>],
        nonself_args: &[P<Expr>],
        fields: &SubstructureFields<'_>,
    ) -> P<Expr> {
        let substructure = Substructure {
            type_ident,
            method_ident: cx.ident_of(self.name, trait_.span),
            self_args,
            nonself_args,
            fields,
        };
        let mut f = self.combine_substructure.borrow_mut();
        let f: &mut CombineSubstructureFunc<'_> = &mut *f;
        f(cx, trait_.span, &substructure)
    }

    fn get_ret_ty(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        generics: &Generics,
        type_ident: Ident,
    ) -> P<ast::Ty> {
        self.ret_ty.to_ty(cx, trait_.span, type_ident, generics)
    }

    fn is_static(&self) -> bool {
        self.explicit_self.is_none()
    }

    fn split_self_nonself_args(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        generics: &Generics,
    ) -> (Option<ast::ExplicitSelf>, Vec<P<Expr>>, Vec<P<Expr>>, Vec<(Ident, P<ast::Ty>)>) {
        let mut self_args = Vec::new();
        let mut nonself_args = Vec::new();
        let mut arg_tys = Vec::new();
        let mut nonstatic = false;

        let ast_explicit_self = self.explicit_self.as_ref().map(|self_ptr| {
            let (self_expr, explicit_self) = ty::get_explicit_self(cx, trait_.span, self_ptr);

            self_args.push(self_expr);
            nonstatic = true;

            explicit_self
        });

        for (ty, name) in self.args.iter() {
            let ast_ty = ty.to_ty(cx, trait_.span, type_ident, generics);
            let ident = cx.ident_of(name, trait_.span);
            arg_tys.push((ident, ast_ty));

            let arg_expr = cx.expr_ident(trait_.span, ident);

            match *ty {
                // for static methods, just treat any Self
                // arguments as a normal arg
                Self_ if nonstatic => {
                    self_args.push(arg_expr);
                }
                Ptr(ref ty, _) if (if let Self_ = **ty { true } else { false }) && nonstatic => {
                    self_args.push(cx.expr_deref(trait_.span, arg_expr))
                }
                _ => {
                    nonself_args.push(arg_expr);
                }
            }
        }

        (ast_explicit_self, self_args, nonself_args, arg_tys)
    }

    fn create_method(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        generics: &Generics,
        explicit_self: Option<ast::ExplicitSelf>,
        arg_types: Vec<(Ident, P<ast::Ty>)>,
        body: P<Expr>,
    ) -> ast::AssocItem {
        // Create the generics that aren't for `Self`.
        let fn_generics = self.generics.to_generics(cx, trait_.span, type_ident, generics);

        let args = {
            let self_args = explicit_self.map(|explicit_self| {
                let ident = Ident::with_dummy_span(kw::SelfLower).with_span_pos(trait_.span);
                ast::Param::from_self(ast::AttrVec::default(), explicit_self, ident)
            });
            let nonself_args =
                arg_types.into_iter().map(|(name, ty)| cx.param(trait_.span, name, ty));
            self_args.into_iter().chain(nonself_args).collect()
        };

        let ret_type = self.get_ret_ty(cx, trait_, generics, type_ident);

        let method_ident = cx.ident_of(self.name, trait_.span);
        let fn_decl = cx.fn_decl(args, ast::FunctionRetTy::Ty(ret_type));
        let body_block = cx.block_expr(body);

        let unsafety = if self.is_unsafe { ast::Unsafety::Unsafe } else { ast::Unsafety::Normal };

        let trait_lo_sp = trait_.span.shrink_to_lo();

        let sig = ast::FnSig {
            header: ast::FnHeader { unsafety, ext: ast::Extern::None, ..ast::FnHeader::default() },
            decl: fn_decl,
        };

        // Create the method.
        ast::AssocItem {
            id: ast::DUMMY_NODE_ID,
            attrs: self.attributes.clone(),
            generics: fn_generics,
            span: trait_.span,
            vis: respan(trait_lo_sp, ast::VisibilityKind::Inherited),
            defaultness: ast::Defaultness::Final,
            ident: method_ident,
            kind: ast::AssocItemKind::Fn(sig, Some(body_block)),
            tokens: None,
        }
    }

    /// ```
    /// #[derive(PartialEq)]
    /// # struct Dummy;
    /// struct A { x: i32, y: i32 }
    ///
    /// // equivalent to:
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         match *self {
    ///             A {x: ref __self_0_0, y: ref __self_0_1} => {
    ///                 match *other {
    ///                     A {x: ref __self_1_0, y: ref __self_1_1} => {
    ///                         __self_0_0.eq(__self_1_0) && __self_0_1.eq(__self_1_1)
    ///                     }
    ///                 }
    ///             }
    ///         }
    ///     }
    /// }
    ///
    /// // or if A is repr(packed) - note fields are matched by-value
    /// // instead of by-reference.
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         match *self {
    ///             A {x: __self_0_0, y: __self_0_1} => {
    ///                 match other {
    ///                     A {x: __self_1_0, y: __self_1_1} => {
    ///                         __self_0_0.eq(&__self_1_0) && __self_0_1.eq(&__self_1_1)
    ///                     }
    ///                 }
    ///             }
    ///         }
    ///     }
    /// }
    /// ```
    fn expand_struct_method_body<'b>(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        struct_def: &'b VariantData,
        type_ident: Ident,
        self_args: &[P<Expr>],
        nonself_args: &[P<Expr>],
        use_temporaries: bool,
    ) -> P<Expr> {
        let mut raw_fields = Vec::new(); // Vec<[fields of self],
        // [fields of next Self arg], [etc]>
        let mut patterns = Vec::new();
        for i in 0..self_args.len() {
            let struct_path = cx.path(trait_.span, vec![type_ident]);
            let (pat, ident_expr) = trait_.create_struct_pattern(
                cx,
                struct_path,
                struct_def,
                &format!("__self_{}", i),
                ast::Mutability::Not,
                use_temporaries,
            );
            patterns.push(pat);
            raw_fields.push(ident_expr);
        }

        // transpose raw_fields
        let fields = if !raw_fields.is_empty() {
            let mut raw_fields = raw_fields.into_iter().map(|v| v.into_iter());
            let first_field = raw_fields.next().unwrap();
            let mut other_fields: Vec<vec::IntoIter<_>> = raw_fields.collect();
            first_field
                .map(|(span, opt_id, field, attrs)| FieldInfo {
                    span,
                    name: opt_id,
                    self_: field,
                    other: other_fields
                        .iter_mut()
                        .map(|l| match l.next().unwrap() {
                            (.., ex, _) => ex,
                        })
                        .collect(),
                    attrs,
                })
                .collect()
        } else {
            cx.span_bug(trait_.span, "no `self` parameter for method in generic `derive`")
        };

        // body of the inner most destructuring match
        let mut body = self.call_substructure_method(
            cx,
            trait_,
            type_ident,
            self_args,
            nonself_args,
            &Struct(struct_def, fields),
        );

        // make a series of nested matches, to destructure the
        // structs. This is actually right-to-left, but it shouldn't
        // matter.
        for (arg_expr, pat) in self_args.iter().zip(patterns) {
            body = cx.expr_match(
                trait_.span,
                arg_expr.clone(),
                vec![cx.arm(trait_.span, pat.clone(), body)],
            )
        }

        body
    }

    fn expand_static_struct_method_body(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        struct_def: &VariantData,
        type_ident: Ident,
        self_args: &[P<Expr>],
        nonself_args: &[P<Expr>],
    ) -> P<Expr> {
        let summary = trait_.summarise_struct(cx, struct_def);

        self.call_substructure_method(
            cx,
            trait_,
            type_ident,
            self_args,
            nonself_args,
            &StaticStruct(struct_def, summary),
        )
    }

    /// ```
    /// #[derive(PartialEq)]
    /// # struct Dummy;
    /// enum A {
    ///     A1,
    ///     A2(i32)
    /// }
    ///
    /// // is equivalent to
    ///
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> ::bool {
    ///         match (&*self, &*other) {
    ///             (&A1, &A1) => true,
    ///             (&A2(ref self_0),
    ///              &A2(ref __arg_1_0)) => (*self_0).eq(&(*__arg_1_0)),
    ///             _ => {
    ///                 let __self_vi = match *self { A1(..) => 0, A2(..) => 1 };
    ///                 let __arg_1_vi = match *other { A1(..) => 0, A2(..) => 1 };
    ///                 false
    ///             }
    ///         }
    ///     }
    /// }
    /// ```
    ///
    /// (Of course `__self_vi` and `__arg_1_vi` are unused for
    /// `PartialEq`, and those subcomputations will hopefully be removed
    /// as their results are unused. The point of `__self_vi` and
    /// `__arg_1_vi` is for `PartialOrd`; see #15503.)
    fn expand_enum_method_body<'b>(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        enum_def: &'b EnumDef,
        type_attrs: &[ast::Attribute],
        type_ident: Ident,
        self_args: Vec<P<Expr>>,
        nonself_args: &[P<Expr>],
    ) -> P<Expr> {
        self.build_enum_match_tuple(
            cx,
            trait_,
            enum_def,
            type_attrs,
            type_ident,
            self_args,
            nonself_args,
        )
    }

    /// Creates a match for a tuple of all `self_args`, where either all
    /// variants match, or it falls into a catch-all for when one variant
    /// does not match.

    /// There are N + 1 cases because is a case for each of the N
    /// variants where all of the variants match, and one catch-all for
    /// when one does not match.

    /// As an optimization we generate code which checks whether all variants
    /// match first which makes llvm see that C-like enums can be compiled into
    /// a simple equality check (for PartialEq).

    /// The catch-all handler is provided access the variant index values
    /// for each of the self-args, carried in precomputed variables.

    /// ```{.text}
    /// let __self0_vi = unsafe {
    ///     std::intrinsics::discriminant_value(&self) } as i32;
    /// let __self1_vi = unsafe {
    ///     std::intrinsics::discriminant_value(&arg1) } as i32;
    /// let __self2_vi = unsafe {
    ///     std::intrinsics::discriminant_value(&arg2) } as i32;
    ///
    /// if __self0_vi == __self1_vi && __self0_vi == __self2_vi && ... {
    ///     match (...) {
    ///         (Variant1, Variant1, ...) => Body1
    ///         (Variant2, Variant2, ...) => Body2,
    ///         ...
    ///         _ => ::core::intrinsics::unreachable()
    ///     }
    /// }
    /// else {
    ///     ... // catch-all remainder can inspect above variant index values.
    /// }
    /// ```
    fn build_enum_match_tuple<'b>(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        enum_def: &'b EnumDef,
        type_attrs: &[ast::Attribute],
        type_ident: Ident,
        mut self_args: Vec<P<Expr>>,
        nonself_args: &[P<Expr>],
    ) -> P<Expr> {
        let sp = trait_.span;
        let variants = &enum_def.variants;

        let self_arg_names = iter::once("__self".to_string())
            .chain(
                self_args
                    .iter()
                    .enumerate()
                    .skip(1)
                    .map(|(arg_count, _self_arg)| format!("__arg_{}", arg_count)),
            )
            .collect::<Vec<String>>();

        let self_arg_idents =
            self_arg_names.iter().map(|name| cx.ident_of(name, sp)).collect::<Vec<ast::Ident>>();

        // The `vi_idents` will be bound, solely in the catch-all, to
        // a series of let statements mapping each self_arg to an int
        // value corresponding to its discriminant.
        let vi_idents = self_arg_names
            .iter()
            .map(|name| {
                let vi_suffix = format!("{}_vi", &name[..]);
                cx.ident_of(&vi_suffix[..], trait_.span)
            })
            .collect::<Vec<ast::Ident>>();

        // Builds, via callback to call_substructure_method, the
        // delegated expression that handles the catch-all case,
        // using `__variants_tuple` to drive logic if necessary.
        let catch_all_substructure =
            EnumNonMatchingCollapsed(self_arg_idents, &variants[..], &vi_idents[..]);

        let first_fieldless = variants.iter().find(|v| v.data.fields().is_empty());

        // These arms are of the form:
        // (Variant1, Variant1, ...) => Body1
        // (Variant2, Variant2, ...) => Body2
        // ...
        // where each tuple has length = self_args.len()
        let mut match_arms: Vec<ast::Arm> = variants
            .iter()
            .enumerate()
            .filter(|&(_, v)| !(self.unify_fieldless_variants && v.data.fields().is_empty()))
            .map(|(index, variant)| {
                let mk_self_pat = |cx: &mut ExtCtxt<'_>, self_arg_name: &str| {
                    let (p, idents) = trait_.create_enum_variant_pattern(
                        cx,
                        type_ident,
                        variant,
                        self_arg_name,
                        ast::Mutability::Not,
                    );
                    (cx.pat(sp, PatKind::Ref(p, ast::Mutability::Not)), idents)
                };

                // A single arm has form (&VariantK, &VariantK, ...) => BodyK
                // (see "Final wrinkle" note below for why.)
                let mut subpats = Vec::with_capacity(self_arg_names.len());
                let mut self_pats_idents = Vec::with_capacity(self_arg_names.len() - 1);
                let first_self_pat_idents = {
                    let (p, idents) = mk_self_pat(cx, &self_arg_names[0]);
                    subpats.push(p);
                    idents
                };
                for self_arg_name in &self_arg_names[1..] {
                    let (p, idents) = mk_self_pat(cx, &self_arg_name[..]);
                    subpats.push(p);
                    self_pats_idents.push(idents);
                }

                // Here is the pat = `(&VariantK, &VariantK, ...)`
                let single_pat = cx.pat_tuple(sp, subpats);

                // For the BodyK, we need to delegate to our caller,
                // passing it an EnumMatching to indicate which case
                // we are in.

                // All of the Self args have the same variant in these
                // cases.  So we transpose the info in self_pats_idents
                // to gather the getter expressions together, in the
                // form that EnumMatching expects.

                // The transposition is driven by walking across the
                // arg fields of the variant for the first self pat.
                let field_tuples = first_self_pat_idents
                    .into_iter()
                    .enumerate()
                    // For each arg field of self, pull out its getter expr ...
                    .map(|(field_index, (sp, opt_ident, self_getter_expr, attrs))| {
                        // ... but FieldInfo also wants getter expr
                        // for matching other arguments of Self type;
                        // so walk across the *other* self_pats_idents
                        // and pull out getter for same field in each
                        // of them (using `field_index` tracked above).
                        // That is the heart of the transposition.
                        let others = self_pats_idents
                            .iter()
                            .map(|fields| {
                                let (_, _opt_ident, ref other_getter_expr, _) = fields[field_index];

                                // All Self args have same variant, so
                                // opt_idents are the same.  (Assert
                                // here to make it self-evident that
                                // it is okay to ignore `_opt_ident`.)
                                assert!(opt_ident == _opt_ident);

                                other_getter_expr.clone()
                            })
                            .collect::<Vec<P<Expr>>>();

                        FieldInfo {
                            span: sp,
                            name: opt_ident,
                            self_: self_getter_expr,
                            other: others,
                            attrs,
                        }
                    })
                    .collect::<Vec<FieldInfo<'_>>>();

                // Now, for some given VariantK, we have built up
                // expressions for referencing every field of every
                // Self arg, assuming all are instances of VariantK.
                // Build up code associated with such a case.
                let substructure = EnumMatching(index, variants.len(), variant, field_tuples);
                let arm_expr = self.call_substructure_method(
                    cx,
                    trait_,
                    type_ident,
                    &self_args[..],
                    nonself_args,
                    &substructure,
                );

                cx.arm(sp, single_pat, arm_expr)
            })
            .collect();

        let default = match first_fieldless {
            Some(v) if self.unify_fieldless_variants => {
                // We need a default case that handles the fieldless variants.
                // The index and actual variant aren't meaningful in this case,
                // so just use whatever
                let substructure = EnumMatching(0, variants.len(), v, Vec::new());
                Some(self.call_substructure_method(
                    cx,
                    trait_,
                    type_ident,
                    &self_args[..],
                    nonself_args,
                    &substructure,
                ))
            }
            _ if variants.len() > 1 && self_args.len() > 1 => {
                // Since we know that all the arguments will match if we reach
                // the match expression we add the unreachable intrinsics as the
                // result of the catch all which should help llvm in optimizing it
                Some(deriving::call_intrinsic(cx, sp, "unreachable", vec![]))
            }
            _ => None,
        };
        if let Some(arm) = default {
            match_arms.push(cx.arm(sp, cx.pat_wild(sp), arm));
        }

        // We will usually need the catch-all after matching the
        // tuples `(VariantK, VariantK, ...)` for each VariantK of the
        // enum.  But:
        //
        // * when there is only one Self arg, the arms above suffice
        // (and the deriving we call back into may not be prepared to
        // handle EnumNonMatchCollapsed), and,
        //
        // * when the enum has only one variant, the single arm that
        // is already present always suffices.
        //
        // * In either of the two cases above, if we *did* add a
        //   catch-all `_` match, it would trigger the
        //   unreachable-pattern error.
        //
        if variants.len() > 1 && self_args.len() > 1 {
            // Build a series of let statements mapping each self_arg
            // to its discriminant value. If this is a C-style enum
            // with a specific repr type, then casts the values to
            // that type.  Otherwise casts to `i32` (the default repr
            // type).
            //
            // i.e., for `enum E<T> { A, B(1), C(T, T) }`, and a deriving
            // with three Self args, builds three statements:
            //
            // ```
            // let __self0_vi = unsafe {
            //     std::intrinsics::discriminant_value(&self) } as i32;
            // let __self1_vi = unsafe {
            //     std::intrinsics::discriminant_value(&arg1) } as i32;
            // let __self2_vi = unsafe {
            //     std::intrinsics::discriminant_value(&arg2) } as i32;
            // ```
            let mut index_let_stmts: Vec<ast::Stmt> = Vec::with_capacity(vi_idents.len() + 1);

            // We also build an expression which checks whether all discriminants are equal
            // discriminant_test = __self0_vi == __self1_vi && __self0_vi == __self2_vi && ...
            let mut discriminant_test = cx.expr_bool(sp, true);

            let target_type_name = find_repr_type_name(&cx.parse_sess, type_attrs);

            let mut first_ident = None;
            for (&ident, self_arg) in vi_idents.iter().zip(&self_args) {
                let self_addr = cx.expr_addr_of(sp, self_arg.clone());
                let variant_value =
                    deriving::call_intrinsic(cx, sp, "discriminant_value", vec![self_addr]);

                let target_ty = cx.ty_ident(sp, cx.ident_of(target_type_name, sp));
                let variant_disr = cx.expr_cast(sp, variant_value, target_ty);
                let let_stmt = cx.stmt_let(sp, false, ident, variant_disr);
                index_let_stmts.push(let_stmt);

                match first_ident {
                    Some(first) => {
                        let first_expr = cx.expr_ident(sp, first);
                        let id = cx.expr_ident(sp, ident);
                        let test = cx.expr_binary(sp, BinOpKind::Eq, first_expr, id);
                        discriminant_test =
                            cx.expr_binary(sp, BinOpKind::And, discriminant_test, test)
                    }
                    None => {
                        first_ident = Some(ident);
                    }
                }
            }

            let arm_expr = self.call_substructure_method(
                cx,
                trait_,
                type_ident,
                &self_args[..],
                nonself_args,
                &catch_all_substructure,
            );

            // Final wrinkle: the self_args are expressions that deref
            // down to desired places, but we cannot actually deref
            // them when they are fed as r-values into a tuple
            // expression; here add a layer of borrowing, turning
            // `(*self, *__arg_0, ...)` into `(&*self, &*__arg_0, ...)`.
            self_args.map_in_place(|self_arg| cx.expr_addr_of(sp, self_arg));
            let match_arg = cx.expr(sp, ast::ExprKind::Tup(self_args));

            // Lastly we create an expression which branches on all discriminants being equal
            //  if discriminant_test {
            //      match (...) {
            //          (Variant1, Variant1, ...) => Body1
            //          (Variant2, Variant2, ...) => Body2,
            //          ...
            //          _ => ::core::intrinsics::unreachable()
            //      }
            //  }
            //  else {
            //      <delegated expression referring to __self0_vi, et al.>
            //  }
            let all_match = cx.expr_match(sp, match_arg, match_arms);
            let arm_expr = cx.expr_if(sp, discriminant_test, all_match, Some(arm_expr));
            index_let_stmts.push(cx.stmt_expr(arm_expr));
            cx.expr_block(cx.block(sp, index_let_stmts))
        } else if variants.is_empty() {
            // As an additional wrinkle, For a zero-variant enum A,
            // currently the compiler
            // will accept `fn (a: &Self) { match   *a   { } }`
            // but rejects `fn (a: &Self) { match (&*a,) { } }`
            // as well as  `fn (a: &Self) { match ( *a,) { } }`
            //
            // This means that the strategy of building up a tuple of
            // all Self arguments fails when Self is a zero variant
            // enum: rustc rejects the expanded program, even though
            // the actual code tends to be impossible to execute (at
            // least safely), according to the type system.
            //
            // The most expedient fix for this is to just let the
            // code fall through to the catch-all.  But even this is
            // error-prone, since the catch-all as defined above would
            // generate code like this:
            //
            //     _ => { let __self0 = match *self { };
            //            let __self1 = match *__arg_0 { };
            //            <catch-all-expr> }
            //
            // Which is yields bindings for variables which type
            // inference cannot resolve to unique types.
            //
            // One option to the above might be to add explicit type
            // annotations.  But the *only* reason to go down that path
            // would be to try to make the expanded output consistent
            // with the case when the number of enum variants >= 1.
            //
            // That just isn't worth it.  In fact, trying to generate
            // sensible code for *any* deriving on a zero-variant enum
            // does not make sense.  But at the same time, for now, we
            // do not want to cause a compile failure just because the
            // user happened to attach a deriving to their
            // zero-variant enum.
            //
            // Instead, just generate a failing expression for the
            // zero variant case, skipping matches and also skipping
            // delegating back to the end user code entirely.
            //
            // (See also #4499 and #12609; note that some of the
            // discussions there influence what choice we make here;
            // e.g., if we feature-gate `match x { ... }` when x refers
            // to an uninhabited type (e.g., a zero-variant enum or a
            // type holding such an enum), but do not feature-gate
            // zero-variant enums themselves, then attempting to
            // derive Debug on such a type could here generate code
            // that needs the feature gate enabled.)

            deriving::call_intrinsic(cx, sp, "unreachable", vec![])
        } else {
            // Final wrinkle: the self_args are expressions that deref
            // down to desired places, but we cannot actually deref
            // them when they are fed as r-values into a tuple
            // expression; here add a layer of borrowing, turning
            // `(*self, *__arg_0, ...)` into `(&*self, &*__arg_0, ...)`.
            self_args.map_in_place(|self_arg| cx.expr_addr_of(sp, self_arg));
            let match_arg = cx.expr(sp, ast::ExprKind::Tup(self_args));
            cx.expr_match(sp, match_arg, match_arms)
        }
    }

    fn expand_static_enum_method_body(
        &self,
        cx: &mut ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        enum_def: &EnumDef,
        type_ident: Ident,
        self_args: &[P<Expr>],
        nonself_args: &[P<Expr>],
    ) -> P<Expr> {
        let summary = enum_def
            .variants
            .iter()
            .map(|v| {
                let sp = v.span.with_ctxt(trait_.span.ctxt());
                let summary = trait_.summarise_struct(cx, &v.data);
                (v.ident, sp, summary)
            })
            .collect();
        self.call_substructure_method(
            cx,
            trait_,
            type_ident,
            self_args,
            nonself_args,
            &StaticEnum(enum_def, summary),
        )
    }
}

// general helper methods.
impl<'a> TraitDef<'a> {
    fn summarise_struct(&self, cx: &mut ExtCtxt<'_>, struct_def: &VariantData) -> StaticFields {
        let mut named_idents = Vec::new();
        let mut just_spans = Vec::new();
        for field in struct_def.fields() {
            let sp = field.span.with_ctxt(self.span.ctxt());
            match field.ident {
                Some(ident) => named_idents.push((ident, sp)),
                _ => just_spans.push(sp),
            }
        }

        let is_tuple = if let ast::VariantData::Tuple(..) = struct_def { true } else { false };
        match (just_spans.is_empty(), named_idents.is_empty()) {
            (false, false) => cx.span_bug(
                self.span,
                "a struct with named and unnamed \
                                          fields in generic `derive`",
            ),
            // named fields
            (_, false) => Named(named_idents),
            // unnamed fields
            (false, _) => Unnamed(just_spans, is_tuple),
            // empty
            _ => Named(Vec::new()),
        }
    }

    fn create_subpatterns(
        &self,
        cx: &mut ExtCtxt<'_>,
        field_paths: Vec<ast::Ident>,
        mutbl: ast::Mutability,
        use_temporaries: bool,
    ) -> Vec<P<ast::Pat>> {
        field_paths
            .iter()
            .map(|path| {
                let binding_mode = if use_temporaries {
                    ast::BindingMode::ByValue(ast::Mutability::Not)
                } else {
                    ast::BindingMode::ByRef(mutbl)
                };
                cx.pat(path.span, PatKind::Ident(binding_mode, (*path).clone(), None))
            })
            .collect()
    }

    fn create_struct_pattern(
        &self,
        cx: &mut ExtCtxt<'_>,
        struct_path: ast::Path,
        struct_def: &'a VariantData,
        prefix: &str,
        mutbl: ast::Mutability,
        use_temporaries: bool,
    ) -> (P<ast::Pat>, Vec<(Span, Option<Ident>, P<Expr>, &'a [ast::Attribute])>) {
        let mut paths = Vec::new();
        let mut ident_exprs = Vec::new();
        for (i, struct_field) in struct_def.fields().iter().enumerate() {
            let sp = struct_field.span.with_ctxt(self.span.ctxt());
            let ident = cx.ident_of(&format!("{}_{}", prefix, i), self.span);
            paths.push(ident.with_span_pos(sp));
            let val = cx.expr_path(cx.path_ident(sp, ident));
            let val = if use_temporaries { val } else { cx.expr_deref(sp, val) };
            let val = cx.expr(sp, ast::ExprKind::Paren(val));

            ident_exprs.push((sp, struct_field.ident, val, &struct_field.attrs[..]));
        }

        let subpats = self.create_subpatterns(cx, paths, mutbl, use_temporaries);
        let pattern = match *struct_def {
            VariantData::Struct(..) => {
                let field_pats = subpats
                    .into_iter()
                    .zip(&ident_exprs)
                    .map(|(pat, &(sp, ident, ..))| {
                        if ident.is_none() {
                            cx.span_bug(sp, "a braced struct with unnamed fields in `derive`");
                        }
                        ast::FieldPat {
                            ident: ident.unwrap(),
                            is_shorthand: false,
                            attrs: ast::AttrVec::new(),
                            id: ast::DUMMY_NODE_ID,
                            span: pat.span.with_ctxt(self.span.ctxt()),
                            pat,
                            is_placeholder: false,
                        }
                    })
                    .collect();
                cx.pat_struct(self.span, struct_path, field_pats)
            }
            VariantData::Tuple(..) => cx.pat_tuple_struct(self.span, struct_path, subpats),
            VariantData::Unit(..) => cx.pat_path(self.span, struct_path),
        };

        (pattern, ident_exprs)
    }

    fn create_enum_variant_pattern(
        &self,
        cx: &mut ExtCtxt<'_>,
        enum_ident: ast::Ident,
        variant: &'a ast::Variant,
        prefix: &str,
        mutbl: ast::Mutability,
    ) -> (P<ast::Pat>, Vec<(Span, Option<Ident>, P<Expr>, &'a [ast::Attribute])>) {
        let sp = variant.span.with_ctxt(self.span.ctxt());
        let variant_path = cx.path(sp, vec![enum_ident, variant.ident]);
        let use_temporaries = false; // enums can't be repr(packed)
        self.create_struct_pattern(cx, variant_path, &variant.data, prefix, mutbl, use_temporaries)
    }
}

// helpful premade recipes

pub fn cs_fold_fields<'a, F>(
    use_foldl: bool,
    mut f: F,
    base: P<Expr>,
    cx: &mut ExtCtxt<'_>,
    all_fields: &[FieldInfo<'a>],
) -> P<Expr>
where
    F: FnMut(&mut ExtCtxt<'_>, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>,
{
    if use_foldl {
        all_fields
            .iter()
            .fold(base, |old, field| f(cx, field.span, old, field.self_.clone(), &field.other))
    } else {
        all_fields
            .iter()
            .rev()
            .fold(base, |old, field| f(cx, field.span, old, field.self_.clone(), &field.other))
    }
}

pub fn cs_fold_enumnonmatch(
    mut enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
    cx: &mut ExtCtxt<'_>,
    trait_span: Span,
    substructure: &Substructure<'_>,
) -> P<Expr> {
    match *substructure.fields {
        EnumNonMatchingCollapsed(ref all_args, _, tuple) => {
            enum_nonmatch_f(cx, trait_span, (&all_args[..], tuple), substructure.nonself_args)
        }
        _ => cx.span_bug(trait_span, "cs_fold_enumnonmatch expected an EnumNonMatchingCollapsed"),
    }
}

pub fn cs_fold_static(cx: &mut ExtCtxt<'_>, trait_span: Span) -> P<Expr> {
    cx.span_bug(trait_span, "static function in `derive`")
}

/// Fold the fields. `use_foldl` controls whether this is done
/// left-to-right (`true`) or right-to-left (`false`).
pub fn cs_fold<F>(
    use_foldl: bool,
    f: F,
    base: P<Expr>,
    enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
    cx: &mut ExtCtxt<'_>,
    trait_span: Span,
    substructure: &Substructure<'_>,
) -> P<Expr>
where
    F: FnMut(&mut ExtCtxt<'_>, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>,
{
    match *substructure.fields {
        EnumMatching(.., ref all_fields) | Struct(_, ref all_fields) => {
            cs_fold_fields(use_foldl, f, base, cx, all_fields)
        }
        EnumNonMatchingCollapsed(..) => {
            cs_fold_enumnonmatch(enum_nonmatch_f, cx, trait_span, substructure)
        }
        StaticEnum(..) | StaticStruct(..) => cs_fold_static(cx, trait_span),
    }
}

/// Function to fold over fields, with three cases, to generate more efficient and concise code.
/// When the `substructure` has grouped fields, there are two cases:
/// Zero fields: call the base case function with `None` (like the usual base case of `cs_fold`).
/// One or more fields: call the base case function on the first value (which depends on
/// `use_fold`), and use that as the base case. Then perform `cs_fold` on the remainder of the
/// fields.
/// When the `substructure` is a `EnumNonMatchingCollapsed`, the result of `enum_nonmatch_f`
/// is returned. Statics may not be folded over.
/// See `cs_op` in `partial_ord.rs` for a model example.
pub fn cs_fold1<F, B>(
    use_foldl: bool,
    f: F,
    mut b: B,
    enum_nonmatch_f: EnumNonMatchCollapsedFunc<'_>,
    cx: &mut ExtCtxt<'_>,
    trait_span: Span,
    substructure: &Substructure<'_>,
) -> P<Expr>
where
    F: FnMut(&mut ExtCtxt<'_>, Span, P<Expr>, P<Expr>, &[P<Expr>]) -> P<Expr>,
    B: FnMut(&mut ExtCtxt<'_>, Option<(Span, P<Expr>, &[P<Expr>])>) -> P<Expr>,
{
    match *substructure.fields {
        EnumMatching(.., ref all_fields) | Struct(_, ref all_fields) => {
            let (base, all_fields) = match (all_fields.is_empty(), use_foldl) {
                (false, true) => {
                    let field = &all_fields[0];
                    let args = (field.span, field.self_.clone(), &field.other[..]);
                    (b(cx, Some(args)), &all_fields[1..])
                }
                (false, false) => {
                    let idx = all_fields.len() - 1;
                    let field = &all_fields[idx];
                    let args = (field.span, field.self_.clone(), &field.other[..]);
                    (b(cx, Some(args)), &all_fields[..idx])
                }
                (true, _) => (b(cx, None), &all_fields[..]),
            };

            cs_fold_fields(use_foldl, f, base, cx, all_fields)
        }
        EnumNonMatchingCollapsed(..) => {
            cs_fold_enumnonmatch(enum_nonmatch_f, cx, trait_span, substructure)
        }
        StaticEnum(..) | StaticStruct(..) => cs_fold_static(cx, trait_span),
    }
}

/// Returns `true` if the type has no value fields
/// (for an enum, no variant has any fields)
pub fn is_type_without_fields(item: &Annotatable) -> bool {
    if let Annotatable::Item(ref item) = *item {
        match item.kind {
            ast::ItemKind::Enum(ref enum_def, _) => {
                enum_def.variants.iter().all(|v| v.data.fields().is_empty())
            }
            ast::ItemKind::Struct(ref variant_data, _) => variant_data.fields().is_empty(),
            _ => false,
        }
    } else {
        false
    }
}