xmpp-rs/xso-proc/src/compound.rs

// Copyright (c) 2024 Jonas Schäfer <jonas@zombofant.net>
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at http://mozilla.org/MPL/2.0/.

//! Handling of the insides of compound structures (structs and enum variants)

use proc_macro2::{Span, TokenStream};
use quote::{quote, quote_spanned, ToTokens};
use syn::{spanned::Spanned, *};

use std::collections::{hash_map::Entry, HashMap};

use crate::error_message::{FieldName, ParentRef};
use crate::field::{FieldBuilderPart, FieldDef, FieldIteratorPart, FieldTempInit, NestedMatcher};
use crate::meta::{DiscardSpec, Flag, NameRef, NamespaceRef, QNameRef};
use crate::scope::{mangle_member, AsItemsScope, FromEventsScope};
use crate::state::{AsItemsSubmachine, FromEventsSubmachine, State};
use crate::types::{
    default_fn, discard_builder_ty, feed_fn, namespace_ty, ncnamestr_cow_ty, phantom_lifetime_ty,
    ref_ty, unknown_attribute_policy_path, unknown_child_policy_path,
};

fn resolve_policy(policy: Option<Ident>, mut enum_ref: Path) -> Expr {
    match policy {
        Some(ident) => {
            enum_ref.segments.push(ident.into());
            Expr::Path(ExprPath {
                attrs: Vec::new(),
                qself: None,
                path: enum_ref,
            })
        }
        None => {
            let default_fn = default_fn(Type::Path(TypePath {
                qself: None,
                path: enum_ref,
            }));
            Expr::Call(ExprCall {
                attrs: Vec::new(),
                func: Box::new(default_fn),
                paren_token: token::Paren::default(),
                args: punctuated::Punctuated::new(),
            })
        }
    }
}

/// A struct or enum variant's contents.
pub(crate) struct Compound {
    /// The fields of this compound.
    fields: Vec<FieldDef>,

    /// Policy defining how to handle unknown attributes.
    unknown_attribute_policy: Expr,

    /// Policy defining how to handle unknown children.
    unknown_child_policy: Expr,

    /// Attributes to discard.
    discard_attr: Vec<(Option<NamespaceRef>, NameRef)>,

    /// Text to discard.
    discard_text: Flag,

    /// Attribute qualified names which are selected by fields.
    ///
    /// This is used to generate code which asserts, at compile time, that no
    /// two fields select the same XML attribute.
    selected_attributes: Vec<(QNameRef, Member)>,
}

impl Compound {
    /// Construct a compound from processed field definitions.
    pub(crate) fn from_field_defs<I: IntoIterator<Item = Result<FieldDef>>>(
        compound_fields: I,
        unknown_attribute_policy: Option<Ident>,
        unknown_child_policy: Option<Ident>,
        discard: Vec<DiscardSpec>,
    ) -> Result<Self> {
        let unknown_attribute_policy = resolve_policy(
            unknown_attribute_policy,
            unknown_attribute_policy_path(Span::call_site()),
        );
        let unknown_child_policy = resolve_policy(
            unknown_child_policy,
            unknown_child_policy_path(Span::call_site()),
        );
        let compound_fields = compound_fields.into_iter();
        let size_hint = compound_fields.size_hint();
        let mut fields = Vec::with_capacity(size_hint.1.unwrap_or(size_hint.0));
        let mut text_field = None;
        let mut selected_attributes: HashMap<QNameRef, Member> = HashMap::new();
        for field in compound_fields {
            let field = field?;

            if field.is_text_field() {
                if let Some(other_field) = text_field.as_ref() {
                    let mut err = Error::new_spanned(
                        field.member(),
                        "only one `#[xml(text)]` field allowed per compound",
                    );
                    err.combine(Error::new(
                        *other_field,
                        "the other `#[xml(text)]` field is here",
                    ));
                    return Err(err);
                }
                text_field = Some(field.member().span())
            }

            if let Some(qname) = field.captures_attribute() {
                let span = field.span();
                match selected_attributes.entry(qname) {
                    Entry::Occupied(o) => {
                        let mut err = Error::new(
                            span,
                            "this field XML field matches the same attribute as another field",
                        );
                        err.combine(Error::new(
                            o.get().span(),
                            "the other field matching the same attribute is here",
                        ));
                        return Err(err);
                    }
                    Entry::Vacant(v) => {
                        v.insert(field.member().clone());
                    }
                }
            }

            fields.push(field);
        }

        let mut discard_text = Flag::Absent;
        let mut discard_attr = Vec::new();
        for spec in discard {
            match spec {
                DiscardSpec::Text { span } => {
                    if let Some(field) = text_field.as_ref() {
                        let mut err = Error::new(
                            *field,
                            "cannot combine `#[xml(text)]` field with `discard(text)`",
                        );
                        err.combine(Error::new(
                            spec.span(),
                            "the discard(text) attribute is here",
                        ));
                        return Err(err);
                    }
                    if let Flag::Present(other) = discard_text {
                        let mut err = Error::new(
                            span,
                            "only one `discard(text)` meta is allowed per compound",
                        );
                        err.combine(Error::new(other, "the discard(text) meta is here"));
                        return Err(err);
                    }

                    discard_text = Flag::Present(span);
                }

                DiscardSpec::Attribute {
                    qname: QNameRef { namespace, name },
                    span,
                } => {
                    let xml_namespace = namespace;
                    let xml_name = match name {
                        Some(v) => v,
                        None => {
                            return Err(Error::new(
                                span,
                                "discard(attribute) must specify a name, e.g. via discard(attribute = \"some-name\")",
                            ));
                        }
                    };
                    discard_attr.push((xml_namespace, xml_name));
                }
            }
        }

        Ok(Self {
            fields,
            unknown_attribute_policy,
            unknown_child_policy,
            discard_attr,
            discard_text,
            selected_attributes: selected_attributes.into_iter().collect(),
        })
    }

    /// Construct a compound from fields.
    pub(crate) fn from_fields(
        compound_fields: &Fields,
        container_namespace: &NamespaceRef,
        unknown_attribute_policy: Option<Ident>,
        unknown_child_policy: Option<Ident>,
        discard: Vec<DiscardSpec>,
    ) -> Result<Self> {
        Self::from_field_defs(
            compound_fields.iter().enumerate().map(|(i, field)| {
                let index = match i.try_into() {
                    Ok(v) => v,
                    // we are converting to u32, are you crazy?!
                    // (u32, because syn::Member::Index needs that.)
                    Err(_) => {
                        return Err(Error::new_spanned(
                            field,
                            "okay, mate, that are way too many fields. get your life together.",
                        ))
                    }
                };
                FieldDef::from_field(field, index, container_namespace)
            }),
            unknown_attribute_policy,
            unknown_child_policy,
            discard,
        )
    }

    /// Generate code which, at compile time, asserts that all attributes
    /// which are selected by this compound are disjunct.
    ///
    /// NOTE: this needs rustc 1.83 or newer for `const_refs_to_static`.
    fn assert_disjunct_attributes(&self) -> TokenStream {
        let mut checks = TokenStream::default();

        // Comparison is commutative, so we *could* reduce this to n^2/2
        // comparisons instead of n*(n-1). However, by comparing every field
        // with every other field and emitting check code for that, we can
        // point at both fields in the error messages.
        for (i, (qname_a, member_a)) in self.selected_attributes.iter().enumerate() {
            for (j, (qname_b, member_b)) in self.selected_attributes.iter().enumerate() {
                if i == j {
                    continue;
                }
                // Flip a and b around if a is later than b.
                // This way, the error message is the same for both
                // conflicting fields. Note that we always take the span of
                // `a` though, so that the two errors point at different
                // fields.
                let span = member_a.span();
                let (member_a, member_b) = if i > j {
                    (member_b, member_a)
                } else {
                    (member_a, member_b)
                };
                if qname_a.namespace.is_some() != qname_b.namespace.is_some() {
                    // cannot ever match.
                    continue;
                }
                let Some((name_a, name_b)) = qname_a.name.as_ref().zip(qname_b.name.as_ref())
                else {
                    panic!("selected attribute has no XML local name");
                };

                let mut check = quote! {
                    ::xso::exports::const_str_eq(#name_a.as_str(), #name_b.as_str())
                };

                let namespaces = qname_a.namespace.as_ref().zip(qname_b.namespace.as_ref());
                if let Some((ns_a, ns_b)) = namespaces {
                    check.extend(quote! {
                        && ::xso::exports::const_str_eq(#ns_a, #ns_b)
                    });
                };

                let attr_a = if let Some(namespace_a) = qname_a.namespace.as_ref() {
                    format!("{{{}}}{}", namespace_a, name_a)
                } else {
                    format!("{}", name_a)
                };

                let attr_b = if let Some(namespace_b) = qname_b.namespace.as_ref() {
                    format!("{{{}}}{}", namespace_b, name_b)
                } else {
                    format!("{}", name_b)
                };

                // See TODO below for why we do the extra replace calls.
                let attr_a = attr_a.replace('{', "{{").replace('}', "}}");
                let attr_b = attr_b.replace('{', "{{").replace('}', "}}");
                let field_a = FieldName(&member_a)
                    .to_string()
                    .replace('{', "{{")
                    .replace('}', "}}");
                let field_b = FieldName(&member_b)
                    .to_string()
                    .replace('{', "{{")
                    .replace('}', "}}");

                // By assigning the checks to a `const`, we ensure that they
                // are in fact evaluated at compile time, even if that constant
                // is never used.
                checks.extend(quote_spanned! {span=>
                    const _: () = { if #check {
                        // TODO: Rust nightly from 2025-05-02 (or around that date) didn't like our fancy panic, so we use something simpler.
                        // Filed bug upstream: https://github.com/rust-lang/rust/issues/140585
                        // For now, we work around this because it breaks our docs builds...
                        //panic!("member {} and member {} match the same XML attribute: {} == {}", #field_a, #field_b, #attr_a, #attr_b);
                        // Note that we cannot replace it with concat!(..), because we may have `{` and `}` in the strings...
                        panic!(concat!("member ", #field_a, " and member ", #field_b, " match the same XML attribute: ", #attr_a, " == ", #attr_b));
                    } };
                })
            }
        }

        checks
    }

    /// Make and return a set of states which is used to construct the target
    /// type from XML events.
    ///
    /// The states are returned as partial state machine. See the return
    /// type's documentation for details.
    pub(crate) fn make_from_events_statemachine(
        &self,
        state_ty_ident: &Ident,
        output_name: &ParentRef,
        state_prefix: &str,
    ) -> Result<FromEventsSubmachine> {
        let scope = FromEventsScope::new(state_ty_ident.clone());
        let FromEventsScope {
            ref attrs,
            ref builder_data_ident,
            ref text,
            ref substate_data,
            ref substate_result,
            ..
        } = scope;

        let default_state_ident = quote::format_ident!("{}Default", state_prefix);
        let discard_state_ident = quote::format_ident!("{}Discard", state_prefix);
        let builder_data_ty: Type = TypePath {
            qself: None,
            path: quote::format_ident!("{}Data{}", state_ty_ident, state_prefix).into(),
        }
        .into();
        let mut states = Vec::new();

        let mut builder_data_def = TokenStream::default();
        let mut builder_data_init = TokenStream::default();
        let mut output_cons = TokenStream::default();
        let mut child_matchers = TokenStream::default();
        let mut fallback_child_matcher = None;
        let mut text_handler = if self.discard_text.is_set() {
            Some(quote! {
                ::core::result::Result::Ok(::core::ops::ControlFlow::Break(
                    Self::#default_state_ident { #builder_data_ident }
                ))
            })
        } else {
            None
        };
        let mut extra_defs = TokenStream::default();
        let is_tuple = !output_name.is_path();

        for (i, field) in self.fields.iter().enumerate() {
            let member = field.member();
            let builder_field_name = mangle_member(member);
            let part = field.make_builder_part(&scope, output_name)?;
            let state_name = quote::format_ident!("{}Field{}", state_prefix, i);

            match part {
                FieldBuilderPart::Init {
                    value: FieldTempInit { ty, init },
                } => {
                    builder_data_def.extend(quote! {
                        #builder_field_name: #ty,
                    });

                    builder_data_init.extend(quote! {
                        #builder_field_name: #init,
                    });

                    if is_tuple {
                        output_cons.extend(quote! {
                            #builder_data_ident.#builder_field_name,
                        });
                    } else {
                        output_cons.extend(quote! {
                            #member: #builder_data_ident.#builder_field_name,
                        });
                    }
                }

                FieldBuilderPart::Text {
                    value: FieldTempInit { ty, init },
                    collect,
                    finalize,
                } => {
                    if text_handler.is_some() {
                        // the existence of only one text handler is enforced
                        // by Compound's constructor(s).
                        panic!("more than one field attempts to collect text data");
                    }

                    builder_data_def.extend(quote! {
                        #builder_field_name: #ty,
                    });
                    builder_data_init.extend(quote! {
                        #builder_field_name: #init,
                    });
                    text_handler = Some(quote! {
                        #collect
                        ::core::result::Result::Ok(::core::ops::ControlFlow::Break(
                            Self::#default_state_ident { #builder_data_ident }
                        ))
                    });

                    if is_tuple {
                        output_cons.extend(quote! {
                            #finalize,
                        });
                    } else {
                        output_cons.extend(quote! {
                            #member: #finalize,
                        });
                    }
                }

                FieldBuilderPart::Nested {
                    extra_defs: field_extra_defs,
                    value: FieldTempInit { ty, init },
                    matcher,
                    builder,
                    collect,
                    finalize,
                } => {
                    let feed = feed_fn(builder.clone());

                    let mut substate_data_ident = substate_data.clone();
                    substate_data_ident.set_span(ty.span());
                    states.push(State::new_with_builder(
                        state_name.clone(),
                        builder_data_ident,
                        &builder_data_ty,
                    ).with_field(
                        &substate_data_ident,
                        &builder,
                    ).with_mut(substate_data).with_impl(quote! {
                        match #feed(&mut #substate_data, ev, ctx)? {
                            ::core::option::Option::Some(#substate_result) => {
                                #collect
                                ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#default_state_ident {
                                    #builder_data_ident,
                                }))
                            }
                            ::core::option::Option::None => {
                                ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#state_name {
                                    #builder_data_ident,
                                    #substate_data,
                                }))
                            }
                        }
                    }));

                    builder_data_def.extend(quote! {
                        #builder_field_name: #ty,
                    });

                    builder_data_init.extend(quote! {
                        #builder_field_name: #init,
                    });

                    match matcher {
                        NestedMatcher::Selective(matcher) => {
                            child_matchers.extend(quote! {
                                let (name, attrs) = match #matcher {
                                    ::core::result::Result::Err(::xso::error::FromEventsError::Mismatch { name, attrs }) => (name, attrs),
                                    ::core::result::Result::Err(::xso::error::FromEventsError::Invalid(e)) => return ::core::result::Result::Err(e),
                                    ::core::result::Result::Ok(#substate_data) => {
                                        return ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#state_name {
                                            #builder_data_ident,
                                            #substate_data,
                                        }))
                                    }
                                };
                            });
                        }
                        NestedMatcher::Fallback(matcher) => {
                            if let Some((span, _)) = fallback_child_matcher.as_ref() {
                                let mut err = Error::new(
                                    field.span(),
                                    "more than one field is attempting to consume all unmatched child elements"
                                );
                                err.combine(Error::new(
                                    *span,
                                    "the previous field collecting all unmatched child elements is here"
                                ));
                                return Err(err);
                            }

                            let matcher = quote! {
                                ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#state_name {
                                    #builder_data_ident,
                                    #substate_data: { #matcher },
                                }))
                            };

                            fallback_child_matcher = Some((field.span(), matcher));
                        }
                    }

                    if is_tuple {
                        output_cons.extend(quote! {
                            #finalize,
                        });
                    } else {
                        output_cons.extend(quote! {
                            #member: #finalize,
                        });
                    }

                    extra_defs.extend(field_extra_defs);
                }
            }
        }

        // We always implicitly discard the `xml:lang` attribute. Its
        // processing is handled using the `#[xml(language)]` meta.
        let mut discard_attr = quote! {
            let _ = #attrs.remove(::xso::exports::rxml::Namespace::xml(), "lang");
        };
        for (xml_namespace, xml_name) in self.discard_attr.iter() {
            let xml_namespace = match xml_namespace {
                Some(v) => v.to_token_stream(),
                None => quote! {
                    ::xso::exports::rxml::Namespace::none()
                },
            };
            discard_attr.extend(quote! {
                let _ = #attrs.remove(#xml_namespace, #xml_name);
            });
        }

        let text_handler = match text_handler {
            Some(v) => v,
            None => quote! {
                // note: u8::is_ascii_whitespace includes U+000C, which is not
                // part of XML's white space definition.'
                if !::xso::is_xml_whitespace(#text.as_bytes()) {
                    ::core::result::Result::Err(::xso::error::Error::Other("Unexpected text content".into()))
                } else {
                    ::core::result::Result::Ok(::core::ops::ControlFlow::Break(
                        Self::#default_state_ident { #builder_data_ident }
                    ))
                }
            },
        };

        let unknown_attr_err = format!("Unknown attribute in {}.", output_name);
        let unknown_child_err = format!("Unknown child in {}.", output_name);
        let unknown_child_policy = &self.unknown_child_policy;

        let output_cons = match output_name {
            ParentRef::Named(ref path) => {
                quote! {
                    #path { #output_cons }
                }
            }
            ParentRef::Unnamed { .. } => {
                quote! {
                    ( #output_cons )
                }
            }
        };

        let discard_builder_ty = discard_builder_ty(Span::call_site());
        let discard_feed = feed_fn(discard_builder_ty.clone());
        let child_fallback = match fallback_child_matcher {
            Some((_, matcher)) => matcher,
            None => quote! {
                let _ = (name, attrs);
                let _: () = #unknown_child_policy.apply_policy(#unknown_child_err)?;
                ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#discard_state_ident {
                    #builder_data_ident,
                    #substate_data: #discard_builder_ty::new(),
                }))
            },
        };

        states.push(State::new_with_builder(
            discard_state_ident.clone(),
            builder_data_ident,
            &builder_data_ty,
        ).with_field(
            substate_data,
            &discard_builder_ty,
        ).with_mut(substate_data).with_impl(quote! {
            match #discard_feed(&mut #substate_data, ev, ctx)? {
                ::core::option::Option::Some(#substate_result) => {
                    ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#default_state_ident {
                        #builder_data_ident,
                    }))
                }
                ::core::option::Option::None => {
                    ::core::result::Result::Ok(::core::ops::ControlFlow::Break(Self::#discard_state_ident {
                        #builder_data_ident,
                        #substate_data,
                    }))
                }
            }
        }));

        states.push(State::new_with_builder(
            default_state_ident.clone(),
            builder_data_ident,
            &builder_data_ty,
        ).with_impl(quote! {
            match ev {
                // EndElement in Default state -> done parsing.
                ::xso::exports::rxml::Event::EndElement(_) => {
                    ::core::result::Result::Ok(::core::ops::ControlFlow::Continue(
                        #output_cons
                    ))
                }
                ::xso::exports::rxml::Event::StartElement(_, name, attrs) => {
                    #child_matchers
                    #child_fallback
                }
                ::xso::exports::rxml::Event::Text(_, #text) => {
                    #text_handler
                }
                // we ignore these: a correct parser only generates
                // them at document start, and there we want to indeed
                // not worry about them being in front of the first
                // element.
                ::xso::exports::rxml::Event::XmlDeclaration(_, ::xso::exports::rxml::XmlVersion::V1_0) => ::core::result::Result::Ok(::core::ops::ControlFlow::Break(
                    Self::#default_state_ident { #builder_data_ident }
                ))
            }
        }));

        let unknown_attribute_policy = &self.unknown_attribute_policy;

        let checks = self.assert_disjunct_attributes();

        Ok(FromEventsSubmachine {
            defs: quote! {
                #extra_defs
                #checks

                struct #builder_data_ty {
                    #builder_data_def
                }
            },
            states,
            init: quote! {
                let #builder_data_ident = #builder_data_ty {
                    #builder_data_init
                };
                #discard_attr
                if #attrs.len() > 0 {
                    let _: () = #unknown_attribute_policy.apply_policy(#unknown_attr_err)?;
                }
                ::core::result::Result::Ok(#state_ty_ident::#default_state_ident { #builder_data_ident })
            },
        })
    }

    /// Make and return a set of states which is used to destructure the
    /// target type into XML events.
    ///
    /// The states are returned as partial state machine. See the return
    /// type's documentation for details.
    ///
    /// **Important:** The returned submachine is not in functional state!
    /// It's `init` must be modified so that a variable called `name` of type
    /// `rxml::QName` is in scope.
    pub(crate) fn make_as_item_iter_statemachine(
        &self,
        input_name: &ParentRef,
        state_ty_ident: &Ident,
        state_prefix: &str,
        lifetime: &Lifetime,
    ) -> Result<AsItemsSubmachine> {
        let scope = AsItemsScope::new(lifetime, state_ty_ident.clone());

        let element_head_start_state_ident =
            quote::format_ident!("{}ElementHeadStart", state_prefix);
        let element_head_end_state_ident = quote::format_ident!("{}ElementHeadEnd", state_prefix);
        let element_foot_state_ident = quote::format_ident!("{}ElementFoot", state_prefix);
        let name_ident = quote::format_ident!("name");
        let ns_ident = quote::format_ident!("ns");
        let dummy_ident = quote::format_ident!("dummy");
        let mut header_states = Vec::new();
        let mut body_states = Vec::new();

        let is_tuple = !input_name.is_path();
        let mut destructure = TokenStream::default();
        let mut start_init = TokenStream::default();
        let mut extra_defs = TokenStream::default();

        header_states.push(
            State::new(element_head_start_state_ident.clone())
                .with_field(&dummy_ident, &phantom_lifetime_ty(lifetime.clone()))
                .with_field(&ns_ident, &namespace_ty(Span::call_site()))
                .with_field(
                    &name_ident,
                    &ncnamestr_cow_ty(Span::call_site(), lifetime.clone()),
                ),
        );

        body_states.push((
            None,
            State::new(element_head_end_state_ident.clone()).with_impl(quote! {
                ::core::option::Option::Some(::xso::Item::ElementHeadEnd)
            }),
        ));

        for (i, field) in self.fields.iter().enumerate() {
            let member = field.member();
            let bound_name = mangle_member(member);
            let part = field.make_iterator_part(&scope, input_name, &bound_name)?;
            let state_name = quote::format_ident!("{}Field{}", state_prefix, i);
            let ty = scope.borrow(field.ty().clone());

            match part {
                FieldIteratorPart::Header { generator } => {
                    // We have to make sure that we carry our data around in
                    // all the previous states.
                    // For header states, it is sufficient to do it here.
                    // For body states, we have to do it in a separate loop
                    // below to correctly handle the case when a field with a
                    // body state is placed before a field with a header
                    // state.
                    for state in header_states.iter_mut() {
                        state.add_field(&bound_name, &ty);
                    }
                    header_states.push(
                        State::new(state_name)
                            .with_field(&bound_name, &ty)
                            .with_impl(quote! {
                                #generator
                            }),
                    );

                    if is_tuple {
                        destructure.extend(quote! {
                            ref #bound_name,
                        });
                    } else {
                        destructure.extend(quote! {
                            #member: ref #bound_name,
                        });
                    }
                    start_init.extend(quote! {
                        #bound_name,
                    });
                }

                FieldIteratorPart::Text { generator } => {
                    // We have to make sure that we carry our data around in
                    // all the previous body states.
                    // We also have to make sure that our data is carried
                    // by all *header* states, but we can only do that once
                    // we have visited them all, so that happens at the bottom
                    // of the loop.
                    for (_, state) in body_states.iter_mut() {
                        state.add_field(&bound_name, &ty);
                    }
                    let state = State::new(state_name)
                        .with_field(&bound_name, &ty)
                        .with_impl(quote! {
                            #generator.map(|value| ::xso::Item::Text(
                                value,
                            ))
                        });
                    if is_tuple {
                        destructure.extend(quote! {
                            #bound_name,
                        });
                    } else {
                        destructure.extend(quote! {
                            #member: #bound_name,
                        });
                    }
                    start_init.extend(quote! {
                        #bound_name,
                    });
                    body_states.push((Some((bound_name, ty)), state));
                }

                FieldIteratorPart::Content {
                    extra_defs: field_extra_defs,
                    value: FieldTempInit { ty, init },
                    generator,
                } => {
                    // We have to make sure that we carry our data around in
                    // all the previous body states.
                    // We also have to make sure that our data is carried
                    // by all *header* states, but we can only do that once
                    // we have visited them all, so that happens at the bottom
                    // of the loop.
                    for (_, state) in body_states.iter_mut() {
                        state.add_field(&bound_name, &ty);
                    }

                    let state = State::new(state_name.clone())
                        .with_field(&bound_name, &ty)
                        .with_mut(&bound_name)
                        .with_impl(quote! {
                            #generator?
                        });
                    if is_tuple {
                        destructure.extend(quote! {
                            #bound_name,
                        });
                    } else {
                        destructure.extend(quote! {
                            #member: #bound_name,
                        });
                    }
                    start_init.extend(quote! {
                        #bound_name: #init,
                    });

                    extra_defs.extend(field_extra_defs);
                    body_states.push((Some((bound_name, ty)), state));
                }
            }
        }

        header_states[0].set_impl(quote! {
            {
                ::core::option::Option::Some(::xso::Item::ElementHeadStart(
                    #ns_ident,
                    #name_ident,
                ))
            }
        });

        for (data, _) in body_states.iter() {
            if let Some((bound_name, ty)) = data.as_ref() {
                for state in header_states.iter_mut() {
                    state.add_field(bound_name, ty);
                }
            }
        }

        header_states.extend(body_states.into_iter().map(|(_, state)| state));
        let mut states = header_states;

        states.push(
            State::new(element_foot_state_ident.clone()).with_impl(quote! {
                ::core::option::Option::Some(::xso::Item::ElementFoot)
            }),
        );

        let destructure = match input_name {
            ParentRef::Named(ref input_path) => quote! {
                #input_path { #destructure }
            },
            ParentRef::Unnamed { .. } => quote! {
                ( #destructure )
            },
        };

        let checks = self.assert_disjunct_attributes();
        extra_defs.extend(checks);

        Ok(AsItemsSubmachine {
            defs: extra_defs,
            states,
            destructure,
            init: quote! {
                Self::#element_head_start_state_ident { #dummy_ident: ::core::marker::PhantomData, #name_ident: name.1, #ns_ident: name.0, #start_init }
            },
        })
    }

    /// Return a reference to this compound's only field's type.
    ///
    /// If the compound does not have exactly one field, this function returns
    /// None.
    pub(crate) fn single_ty(&self) -> Option<&Type> {
        if self.fields.len() > 1 {
            return None;
        }
        self.fields.first().map(|x| x.ty())
    }

    /// Construct a tuple type with this compound's field's types in the same
    /// order as they appear in the compound.
    pub(crate) fn to_tuple_ty(&self) -> TypeTuple {
        TypeTuple {
            paren_token: token::Paren::default(),
            elems: self.fields.iter().map(|x| x.ty().clone()).collect(),
        }
    }

    /// Construct a tuple type with this compound's field's types in the same
    /// order as they appear in the compound.
    pub(crate) fn to_single_or_tuple_ty(&self) -> Type {
        match self.single_ty() {
            None => self.to_tuple_ty().into(),
            Some(v) => v.clone(),
        }
    }

    /// Construct a tuple type with references to this compound's field's
    /// types in the same order as they appear in the compound, with the given
    /// lifetime.
    pub(crate) fn to_ref_tuple_ty(&self, lifetime: &Lifetime) -> TypeTuple {
        TypeTuple {
            paren_token: token::Paren::default(),
            elems: self
                .fields
                .iter()
                .map(|x| ref_ty(x.ty().clone(), lifetime.clone()))
                .collect(),
        }
    }

    /// Return the number of fields in this compound.
    pub(crate) fn field_count(&self) -> usize {
        self.fields.len()
    }
}