rustc_builtin_macros/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)`)
//! - `EnumDiscr` when `Self` is an enum, for comparing the enum discriminants.
//! - `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.
//!
//! 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
//! 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 the discriminants,
//!
//! ```text
//! EnumDiscr(
//!     &[<ident of self discriminant>, <ident of other discriminant>],
//!     <expr to combine with>,
//! )
//! ```
//!
//! Note that this setup doesn't allow for the brute-force "match every variant
//! against every other variant" approach, which is bad because it produces a
//! quadratic amount of code (see #15375).
//!
//! ## 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>)])),
//!     ],
//! )
//! ```

use std::ops::Not;
use std::{iter, vec};

pub(crate) use SubstructureFields::*;
pub(crate) use rustc_ast as ast;
use rustc_ast::token::{IdentIsRaw, LitKind, Token, TokenKind};
use rustc_ast::tokenstream::{DelimSpan, Spacing, TokenTree};
use rustc_ast::{
    AttrArgs, DelimArgs, EnumDef, Expr, GenericArg, GenericParamKind, Generics, Safety, SelfKind,
    VariantData,
};
use rustc_attr_ir::{Attribute, AttributeKind, ReprPacked};
use rustc_attr_parsing::AttributeParser;
use rustc_expand::base::ExtCtxt;
use rustc_span::{DUMMY_SP, Ident, Span, Symbol, kw, respan, sym};
pub(crate) use smallvec::{SmallVec, smallvec};
use thin_vec::{ThinVec, thin_vec};
use ty::{Path, Ref, Self_, Ty};

use crate::{deriving, diagnostics};

pub(crate) mod ty;

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

    /// Path of the trait, including any type parameters
    pub path: Path,

    /// Whether to skip adding the current trait as a bound to the type parameters of the type.
    pub skip_path_as_bound: bool,

    /// Whether `Copy` is needed as an additional bound on type parameters in a packed struct.
    pub needs_copy_as_bound_if_packed: bool,

    /// Additional bounds required of any type parameters of the type,
    /// other than the current trait
    pub additional_bounds: SmallVec<[Ty; 1]>,

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

    pub methods: SmallVec<[MethodDef<'a>; 1]>,

    pub associated_types: SmallVec<[(Ident, Ty); 1]>,

    pub is_const: bool,

    /// The safety of the `impl`.
    pub safety: Safety,

    /// Whether the added `impl` should appear in rustdoc output.
    pub document: bool,
}

pub(crate) struct MethodDef<'a> {
    /// name of the method
    pub name: Symbol,
    /// List of generics, e.g., `R: rand::Rng`
    pub generics: Generics,

    /// Is there is a `&self` argument? If not, it is a static function.
    pub explicit_self: bool,

    /// Arguments other than the self argument.
    pub nonself_args: SmallVec<[(Ty, Symbol); 1]>,

    /// Returns type
    pub ret_ty: Ty,

    pub attributes: ast::AttrVec,

    pub fieldless_variants_strategy: FieldlessVariantsStrategy,

    pub combine_substructure: CombineSubstructureFunc<'a>,
}

/// How to handle fieldless enum variants.
#[derive(PartialEq)]
pub(crate) enum FieldlessVariantsStrategy {
    /// Combine fieldless variants into a single match arm.
    /// This assumes that relevant information has been handled
    /// by looking at the enum's discriminant.
    Unify,
    /// Don't do anything special about fieldless variants. They are
    /// handled like any other variant.
    Default,
    /// If all variants of the enum are fieldless, expand the special
    /// `AllFieldLessEnum` substructure, so that the entire enum can be handled
    /// at once.
    SpecializeIfAllVariantsFieldless,
}

/// All the data about the data structure/method being derived upon.
pub(crate) struct Substructure<'a> {
    /// ident of self
    pub type_ident: Ident,
    /// Verbatim access to any non-selflike arguments, i.e. arguments that
    /// don't have type `&Self`.
    pub nonselflike_args: &'a [Box<Expr>],
    pub fields: SubstructureFields<'a>,
}

/// Summary of the relevant parts of a struct/enum field.
pub(crate) struct FieldInfo {
    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_expr: Box<Expr>,
    /// The expressions corresponding to references to this field in
    /// the other selflike arguments.
    pub other_selflike_exprs: Vec<Box<Expr>>,
    pub maybe_scalar: bool,
}

/// A summary of the possible sets of fields.
pub(crate) enum SubstructureFields<'a> {
    /// A non-static method where `Self` is a struct.
    Struct(&'a ast::VariantData, Vec<FieldInfo>),

    /// A non-static method handling the entire enum at once
    /// (after it has been determined that none of the enum
    /// variants has any fields).
    AllFieldlessEnum(&'a ast::EnumDef),

    /// Matching variants of the enum: variant index, ast::Variant,
    /// fields: the field name is only non-`None` in the case of a struct
    /// variant.
    EnumMatching(&'a ast::Variant, Vec<FieldInfo>),

    /// The discriminant of an enum. The first field is a `FieldInfo` for the discriminants, as
    /// if they were fields. The second field is the expression to combine the
    /// discriminant expression with; it will be `None` if no match is necessary.
    EnumDiscr(FieldInfo, Option<Box<Expr>>),

    /// A static method where `Self` is a struct.
    StaticStruct(&'a ast::VariantData),

    /// A static method where `Self` is an enum.
    StaticEnum(&'a ast::EnumDef),
}

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

pub(crate) fn combine_substructure<'a>(
    f: impl Fn(&ExtCtxt<'_>, Span, Substructure<'_>) -> BlockOrExpr + 'a,
) -> CombineSubstructureFunc<'a> {
    Box::new(f)
}

struct TypeParameter {
    bound_generic_params: ThinVec<ast::GenericParam>,
    ty: Box<ast::Ty>,
}

/// The code snippets built up for derived code are sometimes used as blocks
/// (e.g. in a function body) and sometimes used as expressions (e.g. in a match
/// arm). This structure avoids committing to either form until necessary,
/// avoiding the insertion of any unnecessary blocks.
///
/// The statements come before the expression.
pub(crate) struct BlockOrExpr(ThinVec<ast::Stmt>, Option<Box<Expr>>);

impl BlockOrExpr {
    pub(crate) fn new_stmts(stmts: ThinVec<ast::Stmt>) -> BlockOrExpr {
        BlockOrExpr(stmts, None)
    }

    pub(crate) fn new_expr(expr: Box<Expr>) -> BlockOrExpr {
        BlockOrExpr(ThinVec::new(), Some(expr))
    }

    pub(crate) fn new_mixed(stmts: ThinVec<ast::Stmt>, expr: Option<Box<Expr>>) -> BlockOrExpr {
        BlockOrExpr(stmts, expr)
    }

    // Converts it into a block.
    fn into_block(mut self, cx: &ExtCtxt<'_>, span: Span) -> Box<ast::Block> {
        if let Some(expr) = self.1 {
            self.0.push(cx.stmt_expr(expr));
        }
        cx.block(span, self.0)
    }

    // Converts it into an expression.
    fn into_expr(self, cx: &ExtCtxt<'_>, span: Span) -> Box<Expr> {
        if self.0.is_empty() {
            match self.1 {
                None => cx.expr_block(cx.block(span, ThinVec::new())),
                Some(expr) => expr,
            }
        } else if let [stmt] = self.0.as_slice()
            && let ast::StmtKind::Expr(expr) = &stmt.kind
            && self.1.is_none()
        {
            // There's only a single statement expression. Pull it out.
            expr.clone()
        } else {
            // Multiple statements and/or expressions.
            cx.expr_block(self.into_block(cx, span))
        }
    }
}

/// 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`.
/// Also include bound generic params from the input type.
fn find_type_parameters(
    ty: &ast::Ty,
    ty_param_names: &[Symbol],
    cx: &ExtCtxt<'_>,
) -> Vec<TypeParameter> {
    use rustc_ast::visit;

    struct Visitor<'a, 'b> {
        cx: &'a ExtCtxt<'b>,
        ty_param_names: &'a [Symbol],
        bound_generic_params_stack: ThinVec<ast::GenericParam>,
        type_params: Vec<TypeParameter>,
    }

    impl<'a, 'b> visit::Visitor<'a> for Visitor<'a, 'b> {
        fn visit_ty(&mut self, ty: &'a ast::Ty) {
            let stack_len = self.bound_generic_params_stack.len();
            if let ast::TyKind::FnPtr(fn_ptr) = &ty.kind
                && !fn_ptr.generic_params.is_empty()
            {
                // Given a field `x: for<'a> fn(T::SomeType<'a>)`, we wan't to account for `'a` so
                // that we generate `where for<'a> T::SomeType<'a>: ::core::clone::Clone`. #122622
                self.bound_generic_params_stack.extend(fn_ptr.generic_params.iter().cloned());
            }

            if let ast::TyKind::Path(_, path) = &ty.kind
                && let Some(segment) = path.segments.first()
                && self.ty_param_names.contains(&segment.ident.name)
            {
                self.type_params.push(TypeParameter {
                    bound_generic_params: self.bound_generic_params_stack.clone(),
                    ty: Box::new(ty.clone()),
                });
            }

            visit::walk_ty(self, ty);
            self.bound_generic_params_stack.truncate(stack_len);
        }

        // Place bound generic params on a stack, to extract them when a type is encountered.
        fn visit_poly_trait_ref(&mut self, trait_ref: &'a ast::PolyTraitRef) {
            let stack_len = self.bound_generic_params_stack.len();
            self.bound_generic_params_stack.extend(trait_ref.bound_generic_params.iter().cloned());

            visit::walk_poly_trait_ref(self, trait_ref);

            self.bound_generic_params_stack.truncate(stack_len);
        }

        fn visit_mac_call(&mut self, mac: &ast::MacCall) {
            self.cx.dcx().emit_err(diagnostics::DeriveMacroCall { span: mac.span() });
        }
    }

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

    visitor.type_params
}

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

    pub(crate) fn expand_ext(
        self,
        cx: &ExtCtxt<'_>,
        item: &'a ast::Item,
        push: &mut dyn FnMut(Box<ast::Item>),
        from_scratch: bool,
    ) {
        let is_packed = matches!(
            AttributeParser::parse_limited_sym(cx.sess, &item.attrs, &[sym::repr]),
            Some(Attribute::Parsed(AttributeKind::Repr { reprs, .. })) if reprs.iter().any(|(x, _)| matches!(x, ReprPacked(..)))
        );

        let mut newitem = match &item.kind {
            ast::ItemKind::Struct(ident, generics, struct_def) => {
                self.expand_struct_def(cx, struct_def, *ident, generics, from_scratch, is_packed)
            }
            ast::ItemKind::Enum(ident, generics, enum_def) => {
                // We can skip generating the impl here, because `repr(packed)`
                // enums cause an error later on and to prevent ICEs like #133025.
                // This can only cause further compilation errors
                // downstream in blatantly illegal code, so it is fine.
                if is_packed {
                    return;
                }
                self.expand_enum_def(cx, enum_def, *ident, generics, from_scratch)
            }
            ast::ItemKind::Union(ident, generics, struct_def) => {
                if self.supports_unions {
                    self.expand_struct_def(
                        cx,
                        struct_def,
                        *ident,
                        generics,
                        from_scratch,
                        is_packed,
                    )
                } else {
                    cx.dcx().emit_err(diagnostics::DeriveUnion { span: self.span });
                    return;
                }
            }
            _ => unreachable!(),
        };
        // Keep the lint attributes of the previous item to control how the
        // generated implementations are linted
        newitem.attrs.extend(
            item.attrs
                .iter()
                .filter(|a| {
                    a.has_any_name(&[
                        sym::allow,
                        sym::warn,
                        sym::deny,
                        sym::forbid,
                        sym::stable,
                        sym::unstable,
                    ])
                })
                .cloned(),
        );
        push(newitem);
    }

    /// 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: &ExtCtxt<'_>,
        type_ident: Ident,
        generics: &Generics,
        field_tys: impl Iterator<Item = &'a ast::Ty>,
        methods: impl Iterator<Item = Box<ast::AssocItem>>,
        is_packed: bool,
    ) -> Box<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)| {
            Box::new(ast::AssocItem {
                id: ast::DUMMY_NODE_ID,
                span: self.span,
                vis: ast::Visibility {
                    span: self.span.shrink_to_lo(),
                    kind: ast::VisibilityKind::Inherited,
                },
                attrs: ast::AttrVec::new(),
                kind: ast::AssocItemKind::Type(Box::new(ast::TyAlias {
                    defaultness: ast::Defaultness::Implicit,
                    ident,
                    generics: Generics::default(),
                    after_where_clause: ast::WhereClause::default(),
                    bounds: ThinVec::new(),
                    ty: Some(type_def.to_ty(cx, self.span, type_ident, generics)),
                })),
                tokens: None,
            })
        });

        let mut where_clause = ast::WhereClause::default();
        where_clause.span = generics.where_clause.span;
        let ctxt = self.span.ctxt();
        let span = generics.span.with_ctxt(ctxt);

        // Create the generic parameters
        let params: ThinVec<_> = generics
            .params
            .iter()
            .map(|param| match &param.kind {
                GenericParamKind::Lifetime => param.clone(),
                GenericParamKind::Type { .. } => {
                    // Extra restrictions on the generics parameters to the
                    // type being derived upon.
                    let span = param.ident.span.with_ctxt(ctxt);
                    let bounds: ThinVec<_> = self
                        .additional_bounds
                        .iter()
                        .map(|p| {
                            cx.trait_bound(p.to_path(cx, span, type_ident, generics), self.is_const)
                        })
                        .chain(
                            // Add a bound for the current trait.
                            self.skip_path_as_bound.not().then(|| {
                                let mut trait_path = trait_path.clone();
                                trait_path.span = span;
                                cx.trait_bound(trait_path, self.is_const)
                            }),
                        )
                        .chain({
                            // Add a `Copy` bound if required.
                            if is_packed && self.needs_copy_as_bound_if_packed {
                                let p = deriving::path_std!(marker::Copy);
                                Some(cx.trait_bound(
                                    p.to_path(cx, span, type_ident, generics),
                                    self.is_const,
                                ))
                            } else {
                                None
                            }
                        })
                        .chain(
                            // Also add in any bounds from the declaration.
                            param.bounds.iter().cloned(),
                        )
                        .collect();

                    cx.typaram(span, param.ident, bounds, None)
                }
                GenericParamKind::Const { ty, span, .. } => {
                    let const_nodefault_kind = GenericParamKind::Const {
                        ty: ty.clone(),
                        span: span.with_ctxt(ctxt),

                        // We can't have default values inside impl block
                        default: None,
                    };
                    let mut param_clone = param.clone();
                    param_clone.kind = const_nodefault_kind;
                    param_clone
                }
            })
            .map(|mut param| {
                // Remove all attributes, because there might be helper attributes
                // from other macros that will not be valid in the expanded implementation.
                param.attrs.clear();
                param
            })
            .collect();

        // and similarly for where clauses
        where_clause.predicates.extend(generics.where_clause.predicates.iter().map(|clause| {
            ast::WherePredicate {
                attrs: clause.attrs.clone(),
                kind: clause.kind.clone(),
                id: ast::DUMMY_NODE_ID,
                span: clause.span.with_ctxt(ctxt),
                is_placeholder: false,
            }
        }));

        let ty_param_names: Vec<Symbol> = params
            .iter()
            .filter(|param| matches!(param.kind, ast::GenericParamKind::Type { .. }))
            .map(|ty_param| ty_param.ident.name)
            .collect();

        if !ty_param_names.is_empty() {
            for field_ty in field_tys {
                let field_ty_params = find_type_parameters(field_ty, &ty_param_names, cx);

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

                    // Require the current trait.
                    if !self.skip_path_as_bound {
                        bounds.push(cx.trait_bound(trait_path.clone(), self.is_const));
                    }

                    // Add a `Copy` bound if required.
                    if is_packed && self.needs_copy_as_bound_if_packed {
                        let p = deriving::path_std!(marker::Copy);
                        bounds.push(cx.trait_bound(
                            p.to_path(cx, self.span, type_ident, generics),
                            self.is_const,
                        ));
                    }

                    if !bounds.is_empty() {
                        let predicate = ast::WhereBoundPredicate {
                            bound_generic_params: field_ty_param.bound_generic_params,
                            bounded_ty: field_ty_param.ty,
                            bounds,
                        };

                        let kind = ast::WherePredicateKind::BoundPredicate(predicate);
                        let predicate = ast::WherePredicate {
                            attrs: ThinVec::new(),
                            kind,
                            id: ast::DUMMY_NODE_ID,
                            span: self.span,
                            is_placeholder: false,
                        };
                        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(param.ident.span.with_ctxt(ctxt), param.ident))
                }
                GenericParamKind::Type { .. } => {
                    GenericArg::Type(cx.ty_ident(param.ident.span.with_ctxt(ctxt), param.ident))
                }
                GenericParamKind::Const { .. } => {
                    GenericArg::Const(cx.const_ident(param.ident.span.with_ctxt(ctxt), param.ident))
                }
            })
            .collect();

        // Create the type of `self`.
        let path =
            cx.path_all(type_ident.span.with_ctxt(ctxt), false, vec![type_ident], self_params);
        let self_type = cx.ty_path(path);
        let rustc_const_unstable =
            cx.path_ident(self.span, Ident::new(sym::rustc_const_unstable, self.span));

        let mut attrs = thin_vec![cx.attr_word(sym::automatically_derived, self.span),];

        // Only add `rustc_const_unstable` attributes if `derive_const` is used within libcore/libstd,
        // Other crates don't need stability attributes, so adding them is not useful, but libcore needs them
        // on all const trait impls.
        if self.is_const && cx.ecfg.features.staged_api() {
            attrs.push(
                cx.attr_nested(
                    rustc_ast::AttrItem {
                        unsafety: Safety::Default,
                        path: rustc_const_unstable,
                        args: AttrArgs::Delimited(DelimArgs {
                            dspan: DelimSpan::from_single(self.span),
                            delim: rustc_ast::token::Delimiter::Parenthesis,
                            tokens: [
                                TokenKind::Ident(sym::feature, IdentIsRaw::No),
                                TokenKind::Eq,
                                TokenKind::lit(LitKind::Str, sym::derive_const, None),
                                TokenKind::Comma,
                                TokenKind::Ident(sym::issue, IdentIsRaw::No),
                                TokenKind::Eq,
                                TokenKind::lit(LitKind::Str, sym::derive_const_issue, None),
                            ]
                            .into_iter()
                            .map(|kind| {
                                TokenTree::Token(Token { kind, span: self.span }, Spacing::Alone)
                            })
                            .collect(),
                        }),
                        span: self.span,
                    },
                    self.span,
                ),
            )
        }

        if !self.document {
            attrs.push(cx.attr_nested_word(sym::doc, sym::hidden, self.span));
        }

        cx.item(
            self.span,
            attrs,
            ast::ItemKind::Impl(ast::Impl {
                generics: trait_generics,
                of_trait: Some(Box::new(ast::TraitImplHeader {
                    safety: self.safety,
                    polarity: ast::ImplPolarity::Positive,
                    defaultness: ast::Defaultness::Implicit,
                    trait_ref,
                })),
                constness: if self.is_const { ast::Const::Yes(DUMMY_SP) } else { ast::Const::No },
                self_ty: self_type,
                items: methods.chain(associated_types).collect(),
            }),
        )
    }

    fn expand_struct_def(
        &self,
        cx: &ExtCtxt<'_>,
        struct_def: &'a VariantData,
        type_ident: Ident,
        generics: &Generics,
        from_scratch: bool,
        is_packed: bool,
    ) -> Box<ast::Item> {
        let field_tys = struct_def.fields().iter().map(|field| &*field.ty);

        let methods = self.methods.iter().map(|method_def| {
            let ArgDetails { explicit_self, selflike_args, nonselflike_args, nonself_arg_tys } =
                method_def.extract_arg_details(cx, self, type_ident, generics);

            let body = if from_scratch || method_def.is_static() {
                method_def.call_substructure_method(
                    cx,
                    self,
                    type_ident,
                    &nonselflike_args,
                    StaticStruct(struct_def),
                )
            } else {
                method_def.expand_struct_method_body(
                    cx,
                    self,
                    struct_def,
                    type_ident,
                    &selflike_args,
                    &nonselflike_args,
                    is_packed,
                )
            };

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

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

    fn expand_enum_def(
        &self,
        cx: &ExtCtxt<'_>,
        enum_def: &'a EnumDef,
        type_ident: Ident,
        generics: &Generics,
        from_scratch: bool,
    ) -> Box<ast::Item> {
        let field_tys = enum_def
            .variants
            .iter()
            .flat_map(|variant| variant.data.fields())
            .map(|field| &*field.ty);

        let methods = self.methods.iter().filter_map(|method_def| {
            let ArgDetails { explicit_self, selflike_args, nonselflike_args, nonself_arg_tys } =
                method_def.extract_arg_details(cx, self, type_ident, generics);

            let body = if from_scratch || method_def.is_static() {
                method_def.call_substructure_method(
                    cx,
                    self,
                    type_ident,
                    &nonselflike_args,
                    StaticEnum(enum_def),
                )
            } else {
                method_def.expand_enum_method_body(
                    cx,
                    self,
                    enum_def,
                    type_ident,
                    selflike_args,
                    &nonselflike_args,
                )
            };

            // `assert_fields_are_eq` has an empty default implementation
            if body.0.is_empty() && body.1.is_none() && method_def.name == sym::assert_fields_are_eq
            {
                return None;
            }

            Some(method_def.create_method(
                cx,
                self,
                type_ident,
                generics,
                explicit_self,
                nonself_arg_tys,
                body,
            ))
        });

        let is_packed = false; // enums are never packed
        self.create_derived_impl(cx, type_ident, generics, field_tys, methods, is_packed)
    }
}

struct ArgDetails {
    /// The `&self` arg, if present.
    explicit_self: Option<ast::ExplicitSelf>,
    /// Expressions for `&self` (if present) and also any other
    /// args with the same type (e.g. the `other` arg in `PartialEq::eq`).
    selflike_args: ThinVec<Box<Expr>>,
    /// Expressions for all the remaining args.
    nonselflike_args: Vec<Box<Expr>>,
    /// Additional information about all the args other than `&self`.
    nonself_arg_tys: Vec<(Ident, Box<ast::Ty>)>,
}

impl<'a> MethodDef<'a> {
    fn call_substructure_method(
        &self,
        cx: &ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        nonselflike_args: &[Box<Expr>],
        fields: SubstructureFields<'_>,
    ) -> BlockOrExpr {
        let span = trait_.span;
        let substructure = Substructure { type_ident, nonselflike_args, fields };
        let f: &CombineSubstructureFunc<'_> = &self.combine_substructure;
        f(cx, span, substructure)
    }

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

    fn extract_arg_details(
        &self,
        cx: &ExtCtxt<'_>,
        trait_: &TraitDef<'_>,
        type_ident: Ident,
        generics: &Generics,
    ) -> ArgDetails {
        let mut selflike_args = ThinVec::new();
        let mut nonselflike_args = Vec::new();
        let mut nonself_arg_tys = Vec::new();
        let span = trait_.span;

        let explicit_self = self.explicit_self.then(|| {
            // This constructs a fresh `self` path.
            selflike_args.push(cx.expr_self(span));
            respan(span, SelfKind::Region(None, ast::Mutability::Not))
        });

        for (ty, name) in self.nonself_args.iter() {
            let ast_ty = ty.to_ty(cx, span, type_ident, generics);
            let ident = Ident::new(*name, span);
            nonself_arg_tys.push((ident, ast_ty));

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

            match ty {
                // Selflike (`&Self`) arguments only occur in non-static methods.
                Ref(Self_, _) if !self.is_static() => selflike_args.push(arg_expr),
                Self_ => cx.dcx().span_bug(span, "`Self` in non-return position"),
                _ => nonselflike_args.push(arg_expr),
            }
        }

        ArgDetails { explicit_self, selflike_args, nonselflike_args, nonself_arg_tys }
    }

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

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

        let ret_type = if let Ty::Unit = &self.ret_ty {
            ast::FnRetTy::Default(span)
        } else {
            ast::FnRetTy::Ty(self.ret_ty.to_ty(cx, span, type_ident, generics))
        };

        let method_ident = Ident::new(self.name, span);
        let fn_decl = cx.fn_decl(args, ret_type);
        let body_block = body.into_block(cx, span);

        let trait_lo_sp = span.shrink_to_lo();

        let sig = ast::FnSig { header: ast::FnHeader::default(), decl: fn_decl, span };
        let defaultness = ast::Defaultness::Implicit;

        // Create the method.
        Box::new(ast::AssocItem {
            id: ast::DUMMY_NODE_ID,
            attrs: self.attributes.clone(),
            span,
            vis: ast::Visibility { span: trait_lo_sp, kind: ast::VisibilityKind::Inherited },
            kind: ast::AssocItemKind::Fn(Box::new(ast::Fn {
                defaultness,
                sig,
                ident: method_ident,
                generics: fn_generics,
                contract: None,
                body: Some(body_block),
                define_opaque: None,
                eii_impl: None,
            })),
            tokens: None,
        })
    }

    /// The normal case uses field access.
    ///
    /// ```
    /// #[derive(PartialEq)]
    /// # struct Dummy;
    /// struct A { x: u8, y: u8 }
    ///
    /// // equivalent to:
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         self.x == other.x && self.y == other.y
    ///     }
    /// }
    /// ```
    ///
    /// But if the struct is `repr(packed)`, we can't use something like
    /// `&self.x` because that might cause an unaligned ref. So for any trait
    /// method that takes a reference, we use a local block to force a copy.
    /// This requires that the field impl `Copy`.
    ///
    /// ```rust,ignore (example)
    /// # struct A { x: u8, y: u8 }
    /// impl PartialEq for A {
    ///     fn eq(&self, other: &A) -> bool {
    ///         // Desugars to `{ self.x }.eq(&{ other.y }) && ...`
    ///         { self.x } == { other.y } && { self.y } == { other.y }
    ///     }
    /// }
    /// impl Hash for A {
    ///     fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) -> () {
    ///         ::core::hash::Hash::hash(&{ self.x }, state);
    ///         ::core::hash::Hash::hash(&{ self.y }, state);
    ///     }
    /// }
    /// ```
    fn expand_struct_method_body<'b>(
        &self,
        cx: &ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        struct_def: &'b VariantData,
        type_ident: Ident,
        selflike_args: &[Box<Expr>],
        nonselflike_args: &[Box<Expr>],
        is_packed: bool,
    ) -> BlockOrExpr {
        assert!(selflike_args.len() == 1 || selflike_args.len() == 2);

        let selflike_fields =
            trait_.create_struct_field_access_fields(cx, selflike_args, struct_def, is_packed);
        self.call_substructure_method(
            cx,
            trait_,
            type_ident,
            nonselflike_args,
            Struct(struct_def, selflike_fields),
        )
    }

    /// ```
    /// #[derive(PartialEq)]
    /// # struct Dummy;
    /// enum A {
    ///     A1,
    ///     A2(i32)
    /// }
    /// ```
    ///
    /// is equivalent to:
    ///
    /// ```
    /// #![feature(core_intrinsics)]
    /// enum A {
    ///     A1,
    ///     A2(i32)
    /// }
    /// impl ::core::cmp::PartialEq for A {
    ///     #[inline]
    ///     fn eq(&self, other: &A) -> bool {
    ///         let __self_discr = ::core::intrinsics::discriminant_value(self);
    ///         let __arg1_discr = ::core::intrinsics::discriminant_value(other);
    ///         __self_discr == __arg1_discr
    ///             && match (self, other) {
    ///                 (A::A2(__self_0), A::A2(__arg1_0)) => *__self_0 == *__arg1_0,
    ///                 _ => true,
    ///             }
    ///     }
    /// }
    /// ```
    ///
    /// Creates a discriminant check combined with a match for a tuple of all
    /// `selflike_args`, with an arm for each variant with fields, possibly an
    /// arm for each fieldless variant (if `unify_fieldless_variants` is not
    /// `Unify`), and possibly a default arm.
    fn expand_enum_method_body<'b>(
        &self,
        cx: &ExtCtxt<'_>,
        trait_: &TraitDef<'b>,
        enum_def: &'b EnumDef,
        type_ident: Ident,
        mut selflike_args: ThinVec<Box<Expr>>,
        nonselflike_args: &[Box<Expr>],
    ) -> BlockOrExpr {
        assert!(
            !selflike_args.is_empty(),
            "static methods must use `expand_static_enum_method_body`",
        );

        let span = trait_.span;
        let variants = &enum_def.variants;

        // Traits that unify fieldless variants always use the discriminant(s).
        let unify_fieldless_variants =
            self.fieldless_variants_strategy == FieldlessVariantsStrategy::Unify;

        // For zero-variant enum, this function body is unreachable. Generate
        // `match *self {}`. This produces machine code identical to `unsafe {
        // core::intrinsics::unreachable() }` while being safe and stable.
        if variants.is_empty() {
            selflike_args.truncate(1);
            let match_arg = cx.expr_deref(span, selflike_args.pop().unwrap());
            let match_arms = ThinVec::new();
            let expr = cx.expr_match(span, match_arg, match_arms);
            return BlockOrExpr(ThinVec::new(), Some(expr));
        }

        let prefixes = iter::once("__self".to_string())
            .chain((1..selflike_args.len()).map(|arg_count| format!("__arg{arg_count}")))
            .collect::<Vec<String>>();

        // Build a series of let statements mapping each selflike_arg
        // to its discriminant value.
        //
        // e.g. for `PartialEq::eq` builds two statements:
        // ```
        // let __self_discr = ::core::intrinsics::discriminant_value(self);
        // let __arg1_discr = ::core::intrinsics::discriminant_value(other);
        // ```
        let get_discr_pieces = |cx: &ExtCtxt<'_>| {
            let discr_idents: Vec<_> = prefixes
                .iter()
                .map(|name| Ident::from_str_and_span(&format!("{name}_discr"), span))
                .collect();

            let mut discr_exprs: Vec<_> = discr_idents
                .iter()
                .map(|&ident| cx.expr_addr_of(span, cx.expr_ident(span, ident)))
                .collect();

            let self_expr = discr_exprs.remove(0);
            let other_selflike_exprs = discr_exprs;
            let discr_field =
                FieldInfo { span, name: None, self_expr, other_selflike_exprs, maybe_scalar: true };

            let discr_let_stmts: ThinVec<_> = iter::zip(&discr_idents, &selflike_args)
                .map(|(&ident, selflike_arg)| {
                    let variant_value = deriving::call_intrinsic(
                        cx,
                        span,
                        sym::discriminant_value,
                        thin_vec![selflike_arg.clone()],
                    );
                    cx.stmt_let(span, false, ident, variant_value)
                })
                .collect();

            (discr_field, discr_let_stmts)
        };

        // There are some special cases involving fieldless enums where no
        // match is necessary.
        let all_fieldless = variants.iter().all(|v| v.data.fields().is_empty());
        if all_fieldless {
            if variants.len() > 1 {
                match self.fieldless_variants_strategy {
                    FieldlessVariantsStrategy::Unify => {
                        // If the type is fieldless and the trait uses the discriminant and
                        // there are multiple variants, we need just an operation on
                        // the discriminant(s).
                        let (discr_field, mut discr_let_stmts) = get_discr_pieces(cx);
                        let mut discr_check = self.call_substructure_method(
                            cx,
                            trait_,
                            type_ident,
                            nonselflike_args,
                            EnumDiscr(discr_field, None),
                        );
                        discr_let_stmts.append(&mut discr_check.0);
                        return BlockOrExpr(discr_let_stmts, discr_check.1);
                    }
                    FieldlessVariantsStrategy::SpecializeIfAllVariantsFieldless => {
                        return self.call_substructure_method(
                            cx,
                            trait_,
                            type_ident,
                            nonselflike_args,
                            AllFieldlessEnum(enum_def),
                        );
                    }
                    FieldlessVariantsStrategy::Default => (),
                }
            } else if let [variant] = variants.as_slice() {
                // If there is a single variant, we don't need an operation on
                // the discriminant(s). Just use the most degenerate result.
                return self.call_substructure_method(
                    cx,
                    trait_,
                    type_ident,
                    nonselflike_args,
                    EnumMatching(variant, Vec::new()),
                );
            }
        }

        // These arms are of the form:
        // (Variant1, Variant1, ...) => Body1
        // (Variant2, Variant2, ...) => Body2
        // ...
        // where each tuple has length = selflike_args.len()
        let mut match_arms: ThinVec<ast::Arm> = variants
            .iter()
            .filter(|&v| !(unify_fieldless_variants && v.data.fields().is_empty()))
            .map(|variant| {
                // A single arm has form (&VariantK, &VariantK, ...) => BodyK
                // (see "Final wrinkle" note below for why.)

                let fields = trait_.create_struct_pattern_fields(cx, &variant.data, &prefixes);

                let sp = variant.span.with_ctxt(trait_.span.ctxt());
                let variant_path = cx.path(sp, vec![type_ident, variant.ident]);
                let mut subpats =
                    trait_.create_struct_patterns(cx, variant_path, &variant.data, &prefixes);

                // `(VariantK, VariantK, ...)` or just `VariantK`.
                let single_pat = if subpats.len() == 1 {
                    subpats.pop().unwrap()
                } else {
                    cx.pat_tuple(span, subpats)
                };

                // For the BodyK, we need to delegate to our caller,
                // passing it an EnumMatching to indicate which case
                // we are in.
                //
                // 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(variant, fields);
                let arm_expr = self
                    .call_substructure_method(
                        cx,
                        trait_,
                        type_ident,
                        nonselflike_args,
                        substructure,
                    )
                    .into_expr(cx, span);

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

        // Add a default arm to the match, if necessary.
        let first_fieldless = variants.iter().find(|v| v.data.fields().is_empty());
        let default = match first_fieldless {
            Some(v) if unify_fieldless_variants => {
                // We need a default case that handles all the fieldless
                // variants. The index and actual variant aren't meaningful in
                // this case, so just use dummy values.
                Some(
                    self.call_substructure_method(
                        cx,
                        trait_,
                        type_ident,
                        nonselflike_args,
                        EnumMatching(v, Vec::new()),
                    )
                    .into_expr(cx, span),
                )
            }
            _ if variants.len() > 1 && selflike_args.len() > 1 => {
                // Because we know that all the arguments will match if we reach
                // the match expression we add the unreachable intrinsics as the
                // result of the default which should help llvm in optimizing it.
                Some(deriving::call_unreachable(cx, span))
            }
            _ => None,
        };
        if let Some(arm) = default {
            match_arms.push(cx.arm(span, cx.pat_wild(span), arm));
        }

        // Create a match expression with one arm per discriminant plus
        // possibly a default arm, e.g.:
        //      match (self, other) {
        //          (Variant1, Variant1, ...) => Body1
        //          (Variant2, Variant2, ...) => Body2,
        //          ...
        //          _ => ::core::intrinsics::unreachable(),
        //      }
        let get_match_expr = |mut selflike_args: ThinVec<Box<Expr>>| {
            let match_arg = if selflike_args.len() == 1 {
                selflike_args.pop().unwrap()
            } else {
                cx.expr(span, ast::ExprKind::Tup(selflike_args))
            };
            cx.expr_match(span, match_arg, match_arms)
        };

        // If the trait uses the discriminant and there are multiple variants, we need
        // to add a discriminant check operation before the match. Otherwise, the match
        // is enough.
        if unify_fieldless_variants && variants.len() > 1 {
            let (discr_field, mut discr_let_stmts) = get_discr_pieces(cx);

            // Combine a discriminant check with the match.
            let mut discr_check_plus_match = self.call_substructure_method(
                cx,
                trait_,
                type_ident,
                nonselflike_args,
                EnumDiscr(discr_field, Some(get_match_expr(selflike_args))),
            );
            discr_let_stmts.append(&mut discr_check_plus_match.0);
            BlockOrExpr(discr_let_stmts, discr_check_plus_match.1)
        } else {
            BlockOrExpr(ThinVec::new(), Some(get_match_expr(selflike_args)))
        }
    }
}

// general helper methods.
impl<'a> TraitDef<'a> {
    fn create_struct_patterns(
        &self,
        cx: &ExtCtxt<'_>,
        struct_path: ast::Path,
        struct_def: &'a VariantData,
        prefixes: &[String],
    ) -> ThinVec<ast::Pat> {
        prefixes
            .iter()
            .map(|prefix| {
                let pieces_iter =
                    struct_def.fields().iter().enumerate().map(|(i, struct_field)| {
                        let sp = struct_field.span.with_ctxt(self.span.ctxt());
                        let ident = self.mk_pattern_ident(prefix, i);
                        let path = ident.with_span_pos(sp);
                        (struct_field.ident, cx.pat_ident(path.span, path))
                    });

                let struct_path = struct_path.clone();
                match *struct_def {
                    VariantData::Struct { .. } => {
                        let field_pats = pieces_iter
                            .map(|(ident, pat)| ast::PatField {
                                ident: ident
                                    .expect("a braced struct with unnamed fields in `derive`"),
                                is_shorthand: false,
                                attrs: ast::AttrVec::new(),
                                id: ast::DUMMY_NODE_ID,
                                span: pat.span.with_ctxt(self.span.ctxt()),
                                pat: Box::new(pat),
                                is_placeholder: false,
                            })
                            .collect();
                        cx.pat_struct(self.span, struct_path, field_pats)
                    }
                    VariantData::Tuple(..) => {
                        let subpats = pieces_iter.map(|(_, subpat)| subpat).collect();
                        cx.pat_tuple_struct(self.span, struct_path, subpats)
                    }
                    VariantData::Unit(..) => cx.pat_path(self.span, struct_path),
                }
            })
            .collect()
    }

    fn create_fields<F>(&self, struct_def: &'a VariantData, mk_exprs: F) -> Vec<FieldInfo>
    where
        F: Fn(usize, &ast::FieldDef, Span) -> Vec<Box<ast::Expr>>,
    {
        struct_def
            .fields()
            .iter()
            .enumerate()
            .map(|(i, struct_field)| {
                // For this field, get an expr for each selflike_arg. E.g. for
                // `PartialEq::eq`, one for each of `&self` and `other`.
                let sp = struct_field.span.with_ctxt(self.span.ctxt());
                let mut exprs: Vec<_> = mk_exprs(i, struct_field, sp);
                let self_expr = exprs.remove(0);
                let other_selflike_exprs = exprs;
                FieldInfo {
                    span: sp.with_ctxt(self.span.ctxt()),
                    name: struct_field.ident,
                    self_expr,
                    other_selflike_exprs,
                    maybe_scalar: struct_field.ty.peel_refs().kind.maybe_scalar(),
                }
            })
            .collect()
    }

    fn mk_pattern_ident(&self, prefix: &str, i: usize) -> Ident {
        Ident::from_str_and_span(&format!("{prefix}_{i}"), self.span)
    }

    fn create_struct_pattern_fields(
        &self,
        cx: &ExtCtxt<'_>,
        struct_def: &'a VariantData,
        prefixes: &[String],
    ) -> Vec<FieldInfo> {
        self.create_fields(struct_def, |i, _struct_field, sp| {
            prefixes
                .iter()
                .map(|prefix| {
                    let ident = self.mk_pattern_ident(prefix, i);
                    cx.expr_path(cx.path_ident(sp, ident))
                })
                .collect()
        })
    }

    fn create_struct_field_access_fields(
        &self,
        cx: &ExtCtxt<'_>,
        selflike_args: &[Box<Expr>],
        struct_def: &'a VariantData,
        is_packed: bool,
    ) -> Vec<FieldInfo> {
        self.create_fields(struct_def, |i, struct_field, sp| {
            selflike_args
                .iter()
                .map(|selflike_arg| {
                    // Note: we must use `struct_field.span` rather than `sp` in the
                    // `unwrap_or_else` case otherwise the hygiene is wrong and we get
                    // "field `0` of struct `Point` is private" errors on tuple
                    // structs.
                    let mut field_expr = cx.expr(
                        sp,
                        ast::ExprKind::Field(
                            selflike_arg.clone(),
                            struct_field.ident.unwrap_or_else(|| {
                                Ident::from_str_and_span(&i.to_string(), struct_field.span)
                            }),
                        ),
                    );
                    if is_packed {
                        // Fields in packed structs are wrapped in a block, e.g. `&{self.0}`,
                        // causing a copy instead of a (potentially misaligned) reference.
                        field_expr = cx.expr_block(
                            cx.block(struct_field.span, thin_vec![cx.stmt_expr(field_expr)]),
                        );
                    }
                    cx.expr_addr_of(sp, field_expr)
                })
                .collect()
        })
    }
}

/// The function passed to `cs_fold` is called repeatedly with a value of this
/// type. It describes one part of the code generation. The result is always an
/// expression.
pub(crate) enum CsFold {
    /// The basic case: a field expression for one or more selflike args. E.g.
    /// for `PartialEq::eq` this is something like `self.x == other.x`.
    Single(FieldInfo),

    /// The combination of two field expressions. E.g. for `PartialEq::eq` this
    /// is something like `<field1 equality> && <field2 equality>`.
    Combine(Span, Box<Expr>, Box<Expr>),

    // The fallback case for a struct or enum variant with no fields.
    Fieldless,
}

/// Folds over fields, combining the expressions for each field in a sequence.
/// Statics may not be folded over.
pub(crate) fn cs_fold<F>(
    use_foldl: bool,
    cx: &ExtCtxt<'_>,
    trait_span: Span,
    substructure: Substructure<'_>,
    mut f: F,
) -> Box<Expr>
where
    F: FnMut(&ExtCtxt<'_>, CsFold) -> Box<Expr>,
{
    match substructure.fields {
        EnumMatching(.., all_fields) | Struct(_, all_fields) => {
            let mut fields = all_fields.into_iter();

            let base_field = if use_foldl { fields.next() } else { fields.next_back() };

            let Some(base_field) = base_field else {
                return f(cx, CsFold::Fieldless);
            };

            let base_expr = f(cx, CsFold::Single(base_field));

            let op = |old, field: FieldInfo| {
                let span = field.span;
                let new = f(cx, CsFold::Single(field));
                f(cx, CsFold::Combine(span, old, new))
            };

            if use_foldl { fields.fold(base_expr, op) } else { fields.rfold(base_expr, op) }
        }
        EnumDiscr(discr_field, match_expr) => {
            let discr_check_expr = f(cx, CsFold::Single(discr_field));
            if let Some(match_expr) = match_expr {
                if use_foldl {
                    f(cx, CsFold::Combine(trait_span, discr_check_expr, match_expr))
                } else {
                    f(cx, CsFold::Combine(trait_span, match_expr, discr_check_expr))
                }
            } else {
                discr_check_expr
            }
        }
        _ => cx.dcx().span_bug(trait_span, "unexpected substructure in `derive`"),
    }
}