tame/tamer/src/xir/parse/ele.rs

// XIR element parser generator
//
//  Copyright (C) 2014-2023 Ryan Specialty, LLC.
//
//  This file is part of TAME.
//
//  This program is free software: you can redistribute it and/or modify
//  it under the terms of the GNU General Public License as published by
//  the Free Software Foundation, either version 3 of the License, or
//  (at your option) any later version.
//
//  This program is distributed in the hope that it will be useful,
//  but WITHOUT ANY WARRANTY; without even the implied warranty of
//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//  GNU General Public License for more details.
//
//  You should have received a copy of the GNU General Public License
//  along with this program.  If not, see <http://www.gnu.org/licenses/>.

//! Element parser generator for parsing of [XIRF](super::super::flat).
//!
//! _TODO:_ This needs significantly more documentation;
//!   this is one of the most confusing and complex components of TAMER.

use super::AttrParseState;
use crate::{
    diagnostic_panic,
    fmt::{DisplayWrapper, TtQuote},
    parse::{
        ClosedParseState, Context, ParseState, Transition, TransitionResult,
    },
    span::Span,
    xir::{flat::Depth, CloseSpan, OpenSpan, Prefix, QName},
};
use arrayvec::ArrayVec;
use std::{
    fmt::{Debug, Display, Formatter},
    marker::PhantomData,
};

#[cfg(doc)]
use crate::{ele_parse, parse::Parser};

/// A parser accepting a single element.
pub trait EleParseState: ParseState {}

/// Maximum level of parser nesting.
///
/// Unfortunately,
///   this limit _does not_ correspond to the level of XML nesting;
///     parsers composed of Sum NTs,
///       in particular,
///       push multiple parsers onto the stack for a single element.
///
/// Note that this is assuming that this parser is used only for TAME
///   sources.
/// If that's not the case,
///   this can be made to be configurable like XIRF.
pub const MAX_DEPTH: usize = 64;

/// Parser stack for trampoline.
///
/// This can be used as a call stack for parsers while avoiding creating
///   otherwise-recursive data structures with composition-based delegation.
/// However,
///   it is more similar to CPS,
///   in that the parser popped off the stack need not be the parser that
///     initiated the request and merely represents the next step in
///     a delayed computation.
/// If such a return context is unneeded,
///   a [`ParseState`] may implement tail calls by simply not pushing itself
///   onto the stack before requesting transfer to another [`ParseState`].
#[derive(Debug, Default)]
pub struct StateStack<S: SuperState>(ArrayVec<S, MAX_DEPTH>);

/// [`SuperState`] [`Context`] that gets propagated to each child parser.
pub type SuperStateContext<S> = Context<StateStack<S>>;

// Note that public visibility is needed because `ele_parse` expands outside
//   of this module.
impl<S: SuperState> StateStack<S> {
    /// Request a transfer to another [`ParseState`],
    ///   expecting that control be returned to `ret` after it has
    ///   completed.
    ///
    /// This can be reasoned about like calling a thunk:
    ///   the return [`ParseState`] is put onto the stack,
    ///   the target [`ParseState`] is used for the state transition to
    ///     cause [`Parser`] to perform the call to it,
    ///   and when it is done
    ///     (e.g. a dead state),
    ///     `ret` will be pop'd from the stack and we'll transition back to
    ///     it.
    /// Note that this method is not responsible for returning;
    ///   see [`Self::ret_or_dead`] to perform a return.
    ///
    /// However,
    ///   the calling [`ParseState`] is not responsible for its return,
    ///   unlike a typical function call.
    /// Instead,
    ///   this _actually_ more closely resembles CPS
    ///     (continuation passing style),
    ///     and so [`ele_parse!`] must be careful to ensure that stack
    ///     operations are properly paired.
    /// On the upside,
    ///   if something is erroneously `ret`'d,
    ///   the parser is guaranteed to be in a consistent state since the
    ///   entire state has been reified
    ///     (but the input would then be parsed incorrectly).
    ///
    /// Note that tail calls can be implemented by transferring control
    ///   without pushing an entry on the stack to return to,
    ///     but that hasn't been formalized \[yet\] and requires extra care.
    pub fn transfer_with_ret<SA, ST>(
        &mut self,
        Transition(ret): Transition<SA>,
        target: TransitionResult<ST>,
    ) -> TransitionResult<ST>
    where
        SA: ParseState<Super = S::Super>,
        ST: ParseState,
    {
        let Self(stack) = self;

        // TODO: Global configuration to (hopefully) ensure that XIRF will
        //   actually catch this.
        if stack.is_full() {
            // TODO: We need some spans here and ideally convert the
            //   parenthetical error message into a diagnostic footnote.
            // TODO: Or should we have a special error type that tells the
            //   parent `Parser` to panic with context?
            diagnostic_panic!(
                vec![],
                "maximum parsing depth of {} exceeded while attempting \
                   to push return state {} \
                   (try reducing XML nesting as a workaround)",
                MAX_DEPTH,
                TtQuote::wrap(ret),
            );
        }

        stack.push(ret.into());
        target
    }

    /// Attempt to return to a previous [`ParseState`] that transferred
    ///   control away from itself,
    ///     otherwise yield a dead state transition to `deadst`.
    ///
    /// Conceptually,
    ///   this is like returning from a function call,
    ///   where the function was invoked using [`Self::transfer_with_ret`].
    /// However,
    ///   this system is more akin to CPS
    ///     (continuation passing style);
    ///       see [`Self::transfer_with_ret`] for important information.
    ///
    /// If there is no state to return to on the stack,
    ///   then it is assumed that we have received more input than expected
    ///   after having completed a full parse.
    pub fn ret_or_dead(
        &mut self,
        lookahead: S::Token,
        deadst: S,
    ) -> TransitionResult<S> {
        let Self(stack) = self;

        // This should certainly never happen unless there is a bug in the
        //   `ele_parse!` parser-generator,
        //     since it means that we're trying to return to a caller that
        //     does not exist.
        match stack.pop() {
            Some(st) => Transition(st).incomplete().with_lookahead(lookahead),
            None => Transition(deadst).dead(lookahead),
        }
    }

    /// Test every [`ParseState`] on the stack against the predicate `f`.
    pub fn all(&self, f: impl Fn(&S) -> bool) -> bool {
        let Self(stack) = self;
        stack[..].iter().all(f)
    }
}

/// Match some type of node.
#[derive(Debug, PartialEq, Eq)]
pub enum NodeMatcher {
    /// Static [`QName`] with a simple equality check.
    QName(QName),
    /// Any element with a matching [`Prefix`].
    Prefix(Prefix),
}

impl NodeMatcher {
    /// Match against the provided [`QName`].
    pub fn matches(&self, qname: QName) -> bool {
        match self {
            Self::QName(qn_match) if qn_match == &qname => true,
            Self::Prefix(prefix) if Some(*prefix) == qname.prefix() => true,
            _ => false,
        }
    }
}

impl From<QName> for NodeMatcher {
    fn from(qname: QName) -> Self {
        Self::QName(qname)
    }
}

impl From<Prefix> for NodeMatcher {
    fn from(prefix: Prefix) -> Self {
        Self::Prefix(prefix)
    }
}

impl Display for NodeMatcher {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        use crate::xir::fmt::XmlPrefixAnyLocal;

        match self {
            Self::QName(qname) => Display::fmt(qname, f),
            Self::Prefix(prefix) => XmlPrefixAnyLocal::fmt(prefix, f),
        }
    }
}

#[macro_export]
macro_rules! ele_parse {
    (
        $(#[$super_attr:meta])*
        $vis:vis enum $super:ident;

        // Attr has to be first to avoid ambiguity with `$rest`.
        type AttrValueError = $evty:ty;
        type Object = $objty:ty;

        $(
            [super] {
                $($super_body:tt)*
            };
        )?

        // Combination of square brackets above and the prefix here are
        //   needed for disambiguation.
        $(#[$nt_first_attr:meta])*
        $nt_first:ident := $($nt_defs:tt)*
    ) => {
        ele_parse! {@!next $vis $super
            type AttrValueError = $evty;
            type Object = $objty;
            $(#[$nt_first_attr])*
            $nt_first := $($nt_defs)*
        }

        ele_parse!(@!super_sum <$objty> $(#[$super_attr])* $vis $super
            $([super] { $($super_body)* })?
            $nt_first := $($nt_defs)*
        );
    };

    (@!next $vis:vis $super:ident
        // Attr has to be first to avoid ambiguity with `$rest`.
        type AttrValueError = $evty:ty;
        type Object = $objty:ty;

        $($rest:tt)*
    ) => {
        ele_parse!(@!nonterm_decl <$objty, $evty> $vis $super $($rest)*);
    };

    (@!nonterm_decl <$objty:ty, $evty:ty>
        $vis:vis $super:ident $(#[$nt_attr:meta])* $nt:ident := $($rest:tt)*
    ) => {
        ele_parse!(@!nonterm_def <$objty, $evty>
            $vis $super $(#[$nt_attr])* $nt $($rest)*
        );
    };

    (@!nonterm_def <$objty:ty, $evty:ty>
        $vis:vis $super:ident $(#[$nt_attr:meta])* $nt:ident $qname:ident $(($($ntp:tt)*))?
        { $($matches:tt)* }; $($rest:tt)*
    ) => {
        ele_parse!(@!ele_expand_body <$objty, $evty>
            $vis $super $(#[$nt_attr])* $nt $qname ($($($ntp)*)?) $($matches)*
        );

        ele_parse! {@!next $vis $super
            type AttrValueError = $evty;
            type Object = $objty;
            $($rest)*
        }
    };

    (@!nonterm_def <$objty:ty, $evty:ty>
        $vis:vis $super:ident $(#[$nt_attr:meta])* $nt:ident
        ($ntref_first:ident $(| $ntref:ident)+); $($rest:tt)*
    ) => {
        ele_parse!(@!ele_dfn_sum <$objty>
            $vis $super $(#[$nt_attr])* $nt [$ntref_first $($ntref)*]
        );

        ele_parse! {@!next $vis $super
            type AttrValueError = $evty;
            type Object = $objty;
            $($rest)*
        }
    };

    (@!nonterm_decl <$objty:ty, $evty:ty> $vis:vis $super:ident) => {};

    // Expand the provided data to a more verbose form that provides the
    //   context necessary for state transitions.
    (@!ele_expand_body <$objty:ty, $evty:ty>
        $vis:vis $super:ident
        $(#[$nt_attr:meta])* $nt:ident $qname:ident ($($ntp:tt)*)

        @ { $($attrbody:tt)* } => $attrmap:expr,
        $(/$(($close_span:ident))? => $closemap:expr,)?

        // Special forms (`[sp](args) => expr`).
        $(
            [$special:ident]$(($($special_arg:tt)*))?
                => $special_map:expr,
        )?

        // Nonterminal references are provided as a list.
        // A configuration specifier can be provided,
        //   currently intended to support the Kleene star.
        $(
            $ntref:ident,
        )*
    ) => { paste::paste! {
        ele_parse! {
            @!ele_dfn_body <$objty, $evty>
            $vis $super $(#[$nt_attr])*$nt $qname ($($ntp)*)

            @ { $($attrbody)* } => $attrmap,
            /$($($close_span)?)? => ele_parse!(@!ele_close $($closemap)?),

            $([$special]$(($($special_arg)*))? => $special_map,)?

            <> {
                $(
                    $ntref,
                )*
            }

            // Generate state transitions of the form `(S) -> (S')`.
            -> {
                @ ->
                $(
                    ([<$nt ChildNt_>]::$ntref, $ntref),
                    ([<$nt ChildNt_>]::$ntref, $ntref) ->
                )* ([<$nt ChildNt_>]::ExpectClose_, ()),
            }
        }
    } };

    // No explicit Close mapping defaults to doing nothing at all
    //   (so yield Incomplete).
    (@!ele_close) => {
        $crate::parse::ParseStatus::Incomplete
    };

    (@!ele_close $close:expr) => {
        $crate::parse::ParseStatus::Object($close)
    };

    // Delegation when the destination type is `()`,
    //   indicating that the next state is not a child NT
    //   (it is likely the state expecting a closing tag).
    (@!ntref_delegate
        $stack:ident, $ret:expr, (), $_target:expr, $done:expr
    ) => {
        $done
    };

    // Delegate to a child parser by pushing self onto the stack and
    //   yielding to one of the child's states.
    // This uses a trampoline,
    //   which avoids recursive data structures
    //     (due to `ParseState` composition/stitching)
    //   and does not grow the call stack.
    (@!ntref_delegate
        $stack:ident, $ret:expr, $ntnext_st:ty, $target:expr, $_done:expr
    ) => {
        $stack.transfer_with_ret(
            Transition($ret),
            $target,
        )
    };

    // Same as above,
    //   but in situations where we will never transition to a done state.
    (@!ntref_delegate_nodone
        $stack:ident, $ret:expr, $ntnext_st:ty, $target:expr
    ) => {
        $stack.transfer_with_ret(
            Transition($ret),
            $target,
        )
    };

    (@!ele_dfn_body <$objty:ty, $evty:ty>
        $vis:vis $super:ident $(#[$nt_attr:meta])* $nt:ident $qname:ident
        ($($qname_matched:pat, $open_span:pat)?)

        // Attribute definition special form.
        @ {
            // We must lightly parse attributes here so that we can retrieve
            //   the field identifiers that may be later used as bindings in
            //   `$attrmap`.
            $(
                $(#[$fattr:meta])*
                $fmatch:tt => $fexpr:expr,
            )*
        } => $attrmap:expr,

        // Close expression
        //   (defaulting to Incomplete via @!ele_expand_body).
        /$($close_span:ident)? => $closemap:expr,

        // Streaming (as opposed to aggregate) attribute parsing.
        $([attr]($attr_stream_binding:pat) => $attr_stream_map:expr,)?

        // Nonterminal references.
        <> {
            $(
                $ntref:ident,
            )*
        }

        -> {
            @ -> ($ntfirst:path, $ntfirst_st:ty),
            $(
                ($ntprev:path, $ntprev_st:ty) -> ($ntnext:path, $ntnext_st:ty),
            )*
        }
    ) => { paste::paste! {
        $crate::attr_parse_stream! {
            /// Attribute parser for
            #[doc=concat!("[`", stringify!($nt), "`].")]
            type Object = $objty;
            type ValueError = $evty;

            #[doc(hidden)]
            $vis [<$nt AttrState_>] {
                $(
                    $(#[$fattr])*
                    $fmatch => $fexpr,
                )*
            }
        }

        #[doc(hidden)]
        #[derive(Debug, PartialEq, Eq)]
        $vis enum [<$nt ChildNt_>] {
            $(
                $ntref(
                    (
                        $crate::xir::QName,
                        $crate::xir::OpenSpan,
                        $crate::xir::flat::Depth
                    ),
                ),
            )*

            ExpectClose_(
                (
                    $crate::xir::QName,
                    $crate::xir::OpenSpan,
                    $crate::xir::flat::Depth
                ),
            ),
        }

        $(#[$nt_attr])*
        ///
        #[doc=concat!("Parser for element [`", stringify!($qname), "`].")]
        #[derive(Debug, PartialEq, Eq, Default)]
        $vis struct $nt($crate::xir::parse::NtState<$nt>);

        impl $nt {
            /// A default state that cannot be preempted by the superstate.
            #[allow(dead_code)] // not utilized for every NT
            fn non_preemptable() -> Self {
                Self($crate::xir::parse::NtState::NonPreemptableExpecting)
            }

            /// Whether the given QName would be matched by any of the
            ///   parsers associated with this type.
            #[inline]
            fn matches(qname: $crate::xir::QName) -> bool {
                <Self as $crate::xir::parse::Nt>::matcher().matches(qname)
            }

            /// Number of
            ///   [`NodeMatcher`](crate::xir::parse::NodeMatcher)s
            ///   considered by this parser.
            ///
            /// This is always `1` for this parser.
            #[allow(dead_code)] // used by Sum NTs
            const fn matches_n() -> usize {
                1
            }

            /// Format matcher for display.
            ///
            /// This value may be rendered singularly or as part of a list of
            ///   values joined together by Sum NTs.
            /// This function receives the number of values to be formatted
            ///   as `n` and the current 0-indexed offset within that list
            ///   as `i`.
            /// This allows for zero-copy rendering of composable NTs.
            ///
            /// `i` must be incremented after the operation.
            #[allow(dead_code)] // used by Sum NTs
            fn fmt_matches(
                n: usize,
                i: &mut usize,
                f: &mut std::fmt::Formatter
            ) -> std::fmt::Result {
                use $crate::{
                    fmt::ListDisplayWrapper,
                    xir::{fmt::EleSumList, parse::Nt},
                };

                let matcher = &<Self as Nt>::matcher();
                EleSumList::fmt_nth(n, *i, matcher, f)?;
                *i += 1;

                Ok(())
            }

            /// Whether the parser is in a state that can tolerate superstate
            ///   node preemption.
            ///
            /// For more information,
            ///   see the superstate
            #[doc=concat!(
                " [`", stringify!($super), "::can_preempt_node`]."
            )]
            fn can_preempt_node(&self) -> bool {
                match self {
                    Self(st) => st.can_preempt_node(),
                }
            }

            #[allow(dead_code)] // used only when there are child NTs
            /// Whether the current state represents the last child NT.
            fn is_last_nt(&self) -> bool {
                use $crate::xir::parse::NtState::*;

                let Self(st) = self;

                // This results in `Self::$ntref(..) => true,` for the
                //   _last_ NT,
                //     and `=> false` for all others.
                // If there are no NTs,
                //   it results in `Self::Attrs(..) => true,`,
                //     which is technically true but will never be called in
                //     that context.
                match st {
                    Attrs(..) => $(
                        false,
                        Jmp([<$nt ChildNt_>]::$ntref(..)) =>
                    )* true,

                    _ => false,
                }
            }
        }

        impl $crate::xir::parse::Nt for $nt {
            type AttrState = [<$nt AttrState_>];
            type ChildNt = [<$nt ChildNt_>];

            #[inline]
            fn matcher() -> $crate::xir::parse::NodeMatcher {
                $crate::xir::parse::NodeMatcher::from($qname)
            }
        }

        impl std::fmt::Display for $nt {
            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                match self {
                    Self(st) => std::fmt::Display::fmt(st, f),
                }
            }
        }

        impl $crate::parse::ParseState for $nt {
            type Token = $crate::xir::flat::XirfToken<
                $crate::xir::flat::RefinedText
            >;
            type Object = $objty;
            type Error = $crate::xir::parse::NtError<$nt>;
            type Context = $crate::xir::parse::SuperStateContext<Self::Super>;
            type Super = $super;

            fn parse_token(
                self,
                tok: Self::Token,
                #[allow(unused_variables)] // used only if child NTs
                stack: &mut Self::Context,
            ) -> $crate::parse::TransitionResult<Self::Super> {
                use $crate::{
                    parse::{Transition, Transitionable},
                    xir::{
                        EleSpan,
                        flat::XirfToken,
                        parse::{parse_attrs, NtState},
                    },
                };

                use NtState::{
                    Attrs, Expecting, NonPreemptableExpecting,
                    RecoverEleIgnore, CloseRecoverIgnore,
                    RecoverEleIgnoreClosed, Closed, Jmp,
                };

                let Self(selfst) = self;

                match (selfst, tok) {
                    (
                        Expecting | NonPreemptableExpecting | Closed(..),
                        XirfToken::Open(qname, span, depth)
                    ) if $nt::matches(qname) => {
                        $(
                            let $qname_matched = qname;
                            let $open_span = span;
                        )?

                        Transition(Self(Attrs(
                            (qname, span, depth),
                            parse_attrs(qname, span)
                        ))).ok(<$objty>::from($attrmap))
                    },

                    // We only attempt recovery when encountering an
                    //   unknown token if we're forced to accept that token.
                    (
                        NonPreemptableExpecting,
                        XirfToken::Open(qname, span, depth)
                    ) => {
                        Transition(Self(
                            RecoverEleIgnore(qname, span, depth)
                        )).err(
                            Self::Error::UnexpectedEle(
                                qname, span.name_span()
                            )
                        )
                    },

                    (
                        RecoverEleIgnore(qname, _, depth_open),
                        XirfToken::Close(_, span, depth_close)
                    ) if depth_open == depth_close => {
                        Transition(Self(
                            RecoverEleIgnoreClosed(qname, span)
                        )).incomplete()
                    },

                    // Streaming attribute matching takes precedence over
                    //   aggregate.
                    // This is primarily me being lazy,
                    //   because it's not worth a robust syntax for something
                    //   that's rarely used
                    //     (macro-wise, I mean;
                    //       it's heavily utilized as a percentage of
                    //         source file parsed since short-hand template
                    //         applications are heavily used).
                    $(
                        (
                            st @ Attrs(..),
                            XirfToken::Attr($attr_stream_binding),
                        ) => {
                            Transition(Self(st))
                                .ok(<$objty>::from($attr_stream_map))
                        },

                        // Override the aggregate attribute parser
                        //   delegation by forcing the below match to become
                        //   unreachable
                        //     (xref anchor <<SATTR>>).
                        // Since we have already emitted the `$attrmap`
                        //   object on `Open`,
                        //     this yields an incomplete parse.
                        (Attrs(meta, _), tok) => {
                            ele_parse!(@!ntref_delegate
                                stack,
                                Self(Jmp($ntfirst(meta))),
                                $ntfirst_st,
                                Transition($ntfirst_st::default())
                                       .incomplete()
                                       .with_lookahead(tok),
                                Transition(Self(Jmp($ntfirst(meta))))
                                    .incomplete()
                                    .with_lookahead(tok)
                            )
                        }
                    )?

                    // This becomes unreachable when the `[attr]` special
                    //   form is provided,
                    //     which overrides this match directly above
                    //       (xref <<SATTR>>).
                    #[allow(unreachable_patterns)]
                    (Attrs(meta, sa), tok) => {
                        sa.delegate::<Self, _>(
                            tok,
                            $crate::parse::EmptyContext,
                            |sa| Transition(Self(Attrs(meta, sa))),
                            || Transition(Self(Jmp($ntfirst(meta)))),
                        )
                    },

                    $(
                        // We're transitioning from `(ntprev) -> (ntnext)`.
                        // If we have a token that matches `ntprev`,
                        //   we can transition _back_ to that state rather
                        //   than transitioning forward.
                        // We can _only_ do this when we know we are
                        //   transitioning away from this state,
                        //     otherwise we could return to a previous state,
                        //     which violates the semantics of the implied
                        //     DFA.
                        (
                            Jmp($ntprev(meta)),
                            XirfToken::Open(qname, span, depth)
                        ) if $ntprev_st::matches(qname) => {
                            let tok = XirfToken::Open(qname, span, depth);

                            ele_parse!(@!ntref_delegate
                                stack,
                                Self(Jmp($ntprev(meta))),
                                $ntprev_st,
                                // This NT said it could process this token,
                                //   so force it to either do so or error,
                                //   to ensure that bugs don't cause infinite
                                //     processing of lookahead.
                                Transition(<$ntprev_st>::non_preemptable())
                                    .incomplete()
                                    .with_lookahead(tok),
                                Transition(Self(Jmp($ntprev(meta))))
                                    .incomplete()
                                    .with_lookahead(tok)
                            )
                        },

                        (Jmp($ntprev(meta)), tok) => {
                            ele_parse!(@!ntref_delegate
                                stack,
                                Self(Jmp($ntnext(meta))),
                                $ntnext_st,
                                Transition(<$ntnext_st>::default())
                                    .incomplete()
                                    .with_lookahead(tok),
                                Transition(Self(Jmp($ntnext(meta))))
                                    .incomplete()
                                    .with_lookahead(tok)
                            )
                        },

                        // Since `ExpectClose_` does not have an `$ntprev`
                        //   match,
                        //     we have to handle transitioning back to the
                        //     previous state as a special case.
                        // Further,
                        //   we choose to transition back to this state
                        //   _no matter what the element_,
                        //     to force error recovery and diagnostics
                        //     in that context,
                        //       which will tell the user what elements were
                        //       expected in the last NT rather than just
                        //       telling them a closing tag was expected.
                        //
                        // To avoid a bunch of rework of this macro
                        //   (which can hopefully be done in the future),
                        //   this match is output for _every_ NT,
                        //     but takes effect only for the final NT because
                        //     of the `is_last_nt` predicate.
                        // _It is important that it only affect the
                        //   final NT_,
                        //     otherwise we'll transition back to _any_
                        //     previous state at the close,
                        //       which completely defeats the purpose of
                        //       having ordered states.
                        (
                            Jmp([<$nt ChildNt_>]::ExpectClose_(meta)),
                            XirfToken::Open(qname, span, depth)
                        ) if Self(Jmp($ntprev(meta))).is_last_nt() => {
                            let tok = XirfToken::Open(qname, span, depth);
                            ele_parse!(@!ntref_delegate_nodone
                                stack,
                                Self(Jmp($ntprev(meta))),
                                $ntprev_st,
                                // If this NT cannot handle this element,
                                //   it should error and enter recovery to
                                //   ignore it.
                                Transition(<$ntprev_st>::non_preemptable())
                                    .incomplete()
                                    .with_lookahead(tok)
                            )
                        },
                    )*

                    // XIRF ensures proper nesting,
                    //   so we do not need to check the element name.
                    (
                        Jmp([<$nt ChildNt_>]::ExpectClose_((qname, _, depth)))
                        | CloseRecoverIgnore((qname, _, depth), _),
                        XirfToken::Close(_, span, tok_depth)
                    ) if tok_depth == depth => {
                        $(
                            let $close_span = span;
                        )?
                        $closemap.transition(Self(Closed(Some(qname), span.tag_span())))
                    },

                    (
                        Jmp([<$nt ChildNt_>]::ExpectClose_(meta @ (qname, otspan, _))),
                        unexpected_tok
                    ) => {
                        use $crate::parse::Token;
                        Transition(Self(
                            CloseRecoverIgnore(meta, unexpected_tok.span())
                        )).err(
                            Self::Error::CloseExpected(qname, otspan, unexpected_tok)
                        )
                    }

                    // We're still in recovery,
                    //   so this token gets thrown out.
                    (st @ (RecoverEleIgnore(..) | CloseRecoverIgnore(..)), _) => {
                        Transition(Self(st)).incomplete()
                    },

                    // Note that this does not necessarily represent an
                    //   accepting state
                    //     (see `is_accepting`).
                    (
                        st @ (
                            Expecting
                            | NonPreemptableExpecting
                            | Closed(..)
                            | RecoverEleIgnoreClosed(..)
                        ),
                        tok
                    ) => {
                        Transition(Self(st)).dead(tok)
                    }
                }
            }

            fn is_accepting(&self, _: &Self::Context) -> bool {
                use $crate::xir::parse::NtState::*;
                matches!(*self, Self(Closed(..) | RecoverEleIgnoreClosed(..)))
            }
        }
    }};

    (@!ele_dfn_sum <$objty:ty> $vis:vis $super:ident
        $(#[$nt_attr:meta])* $nt:ident [$($ntref:ident)*]
    ) => {paste::paste! {
        $(#[$nt_attr])*
        ///
        #[doc=concat!(
            "Parser expecting one of ",
            $("[`", stringify!($ntref), "`], ",)*
            "."
        )]
        #[derive(Debug, PartialEq, Eq, Default)]
        $vis struct $nt($crate::xir::parse::SumNtState<$nt>);

        impl $nt {
            fn non_preemptable() -> Self {
                Self($crate::xir::parse::SumNtState::NonPreemptableExpecting)
            }

            // Whether the given QName would be matched by any of the
            //   parsers associated with this type.
            //
            // This is short-circuiting and will return as soon as one
            //   parser is found,
            //     so it may be a good idea to order the sum type according
            //     to the most likely value to be encountered.
            // At its worst,
            //   this may be equivalent to a linear search of the parsers.
            // With that said,
            //   Rust/LLVM may optimize this in any number of ways,
            //   especially if each inner parser matches on a QName
            //     constant.
            // Let a profiler and disassembly guide you.
            #[allow(dead_code)] // used by superstate
            fn matches(qname: $crate::xir::QName) -> bool {
                // If we used an array or a trait,
                //   then we'd need everything to be a similar type;
                //     this allows for _any_ type provided that it expands
                //     into something that contains a `matches` associated
                //     function of a compatible type.
                false $(|| $ntref::matches(qname))*
            }

            // Number of
            //   [`NodeMatcher`](crate::xir::parse::NodeMatcher)s
            //   considered by this parser.
            //
            // This is the sum of the number of matches of each
            //   constituent NT.
            const fn matches_n() -> usize {
                // Count the number of NTs by adding the number of
                //   matches in each.
                0 $(+ $ntref::matches_n())*
            }

            /// Format constituent NTs for display.
            ///
            /// This function receives the number of values to be
            ///   formatted as `n` and the current 0-indexed offset within
            ///   that list as `i`.
            /// This allows for zero-copy rendering of composable NTs.
            ///
            /// See also [`SumNt::fmt_matches_top`] to initialize the
            ///   formatting process with the correct values.
            ///
            /// [`SumNt::fmt_matches_top`]: crate::xir::parse::SumNt
            fn fmt_matches(
                n: usize,
                i: &mut usize,
                f: &mut std::fmt::Formatter
            ) -> std::fmt::Result {
                $(
                    $ntref::fmt_matches(n, i, f)?;
                )*

                Ok(())
            }

            /// Whether the parser is in a state that can tolerate
            ///   superstate node preemption.
            ///
            /// For more information,
            ///   see the superstate
            #[doc=concat!(
                " [`", stringify!($super), "::can_preempt_node`]."
            )]
            fn can_preempt_node(&self) -> bool {
                match self {
                    Self(st) => st.can_preempt_node(),
                }
            }
        }

        impl $crate::xir::parse::SumNt for $nt {
            /// Begin formatting using [`Self::fmt_matches`].
            ///
            /// This provides the initial values for the function.
            fn fmt_matches_top(f: &mut std::fmt::Formatter) -> std::fmt::Result {
                Self::fmt_matches(Self::matches_n().saturating_sub(1), &mut 0, f)
            }
        }

        impl std::fmt::Display for $nt {
            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                match self {
                    Self(st) => std::fmt::Display::fmt(st, f),
                }
            }
        }

        impl $crate::parse::ParseState for $nt {
            type Token = $crate::xir::flat::XirfToken<
                $crate::xir::flat::RefinedText
            >;
            type Object = $objty;
            type Error = $crate::xir::parse::SumNtError<$nt>;
            type Context = $crate::xir::parse::SuperStateContext<Self::Super>;
            type Super = $super;

            fn parse_token(
                self,
                tok: Self::Token,
                stack: &mut Self::Context,
            ) -> $crate::parse::TransitionResult<Self::Super> {
                use $crate::{
                    parse::Transition,
                    xir::{
                        flat::XirfToken,
                        EleSpan,
                        parse::SumNtState::{
                            Expecting,
                            NonPreemptableExpecting,
                            RecoverEleIgnore,
                        },
                    },
                };

                match (self.0, tok) {
                    $(
                        (
                            st @ (Expecting | NonPreemptableExpecting),
                            XirfToken::Open(qname, span, depth)
                        ) if $ntref::matches(qname) => {
                            ele_parse!(@!ntref_delegate_nodone
                                stack,
                                Self(Expecting),
                                $ntref,
                                Transition(
                                    // Propagate non-preemption status,
                                    //   otherwise we'll provide a lookback
                                    //   of the original token and end up
                                    //   recursing until we hit the `stack`
                                    //   limit.
                                    match st {
                                        NonPreemptableExpecting => {
                                            $ntref::non_preemptable()
                                        }
                                        _ => {
                                            $ntref::default()
                                        }
                                    }
                                ).incomplete().with_lookahead(
                                    XirfToken::Open(qname, span, depth)
                                )
                            )
                        },

                        (
                            NonPreemptableExpecting,
                            XirfToken::Open(qname, span, depth)
                        ) if $ntref::matches(qname) => {
                            ele_parse!(@!ntref_delegate_nodone
                                stack,
                                Self(Expecting),
                                $ntref,
                                Transition(
                                    $ntref::non_preemptable()
                                ).incomplete().with_lookahead(
                                    XirfToken::Open(qname, span, depth)
                                )
                            )
                        },
                    )*

                    // If we're non-preemptable,
                    //   then we're expected to be able to process this
                    //   token or fail trying.
                    (
                        NonPreemptableExpecting,
                        XirfToken::Open(qname, span, depth)
                    ) => {
                        Transition(Self(
                            RecoverEleIgnore(qname, span, depth, Default::default())
                        )).err(
                            // Use name span rather than full `OpenSpan`
                            //   since it's specifically the name that was
                            //   unexpected,
                            //     not the fact that it's an element.
                            Self::Error::UnexpectedEle(
                                qname,
                                span.name_span(),
                                Default::default(),
                            )
                        )
                    },

                    // An unexpected token when repeating ends repetition
                    //   and should not result in an error.
                    (
                        Expecting | NonPreemptableExpecting,
                        tok
                    ) => Transition(Self(Expecting)).dead(tok),

                    // XIRF ensures that the closing tag matches the opening,
                    //   so we need only check depth.
                    (
                        RecoverEleIgnore(_, _, depth_open, _),
                        XirfToken::Close(_, _, depth_close)
                    ) if depth_open == depth_close => {
                        Transition(Self(Expecting)).incomplete()
                    },

                    (st @ RecoverEleIgnore(..), _) => {
                        Transition(Self(st)).incomplete()
                    },
                }
            }

            fn is_accepting(&self, _: &Self::Context) -> bool {
                use $crate::xir::parse::SumNtState;
                matches!(self, Self(SumNtState::Expecting))
            }
        }
    }};

    // Generate superstate sum type.
    //
    // This is really annoying because we cannot read the output of another
    //   macro,
    //     and so we have to do our best to re-parse the body of the
    //     original `ele_parse!` invocation without duplicating too much
    //     logic,
    //       and we have to do so in a way that we can aggregate all of
    //       those data.
    (@!super_sum <$objty:ty> $(#[$super_attr:meta])* $vis:vis $super:ident
        $(
            [super] {
                // Non-whitespace text nodes can be mapped into elements
                //   with the given QName as a preprocessing step,
                //     allowing them to reuse the existing element NT system.
                $([text]($text:ident, $text_span:ident) => $text_map:expr,)?

                // Optional _single_ NT to preempt arbitrary elements.
                // Sum NTs can be used to preempt multiple elements.
                $($pre_nt:ident)?
            }
        )?
        $(
            // NT definition is always followed by `:=`.
            $(#[$_ident_attr:meta])*
            $nt:ident :=
                // Identifier if an element NT.
                $($_i:ident)?
                // Parenthesis for a sum NT,
                //   or possibly the span match for an element NT.
                // So: `:= QN_IDENT(span)` or `:= (A | B | C)`.
                $( ($($_p:tt)*) )?
                // Braces for an element NT body.
                $( {$($_b:tt)*} )?
            // Element and sum NT both conclude with a semicolon,
            //   which we need to disambiguate the next `$nt`.
            ;
        )*
    ) => { paste::paste! {
        $(#[$super_attr])*
        ///
        /// Superstate representing the union of all related parsers.
        ///
        /// This [`ParseState`] allows sub-parsers to independently the
        ///   states associated with their own subgraph,
        ///     and then yield a state transition directly to a state of
        ///     another parser.
        /// This is conceptually like CPS (continuation passing style),
        ///   where this [`ParseState`] acts as a trampoline.
        ///
        /// This [`ParseState`] is required for use with [`Parser`];
        ///   see [`ClosedParseState`] for more information.
        ///
        /// [`Parser`]: crate::parse::Parser
        /// [`ParseState`]: crate::parse::ParseState
        /// [`ClosedParseState`]: crate::parse::ClosedParseState
        #[derive(Debug, PartialEq, Eq)]
        $vis enum $super {
            $(
                $nt($nt),
            )*
        }

        // Default parser is the first NT,
        //   and is non-preemptable to force error handling if the root node
        //   is unexpected.
        // Note that this also prevents preemption at the root,
        //   which is necessary for now anyway since we need to be able
        //   to statically resolve imports without template expansion in
        //     NIR
        //     (otherwise we have a chicken-and-egg problem).
        impl Default for $super {
            fn default() -> Self {
                ele_parse!(@!ntfirst_init $super, $($nt)*)
            }
        }

        $(
            impl From<$nt> for $super {
                fn from(st: $nt) -> Self {
                    $super::$nt(st)
                }
            }
        )*

        impl std::fmt::Display for $super {
            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                match self {
                    $(
                        Self::$nt(e) => std::fmt::Display::fmt(e, f),
                    )*
                }
            }
        }

        /// Superstate error object representing the union of all related
        ///   parsers' errors.
        #[derive(Debug, PartialEq)]
        $vis enum [<$super Error_>] {
            $(
                $nt(<$nt as $crate::parse::ParseState>::Error),
            )*
        }

        $(
            impl From<<$nt as $crate::parse::ParseState>::Error>
                for [<$super Error_>]
            {
                fn from(e: <$nt as $crate::parse::ParseState>::Error) -> Self {
                    [<$super Error_>]::$nt(e)
                }
            }
        )*

        impl std::error::Error for [<$super Error_>] {
            fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
                // TODO
                None
            }
        }

        impl std::fmt::Display for [<$super Error_>] {
            fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
                match self {
                    $(
                        Self::$nt(e) => std::fmt::Display::fmt(e, f),
                    )*
                }
            }
        }

        impl $crate::diagnose::Diagnostic for [<$super Error_>] {
            fn describe(&self) -> Vec<$crate::diagnose::AnnotatedSpan> {
                match self {
                    $(
                        Self::$nt(e) => e.describe(),
                    )*
                }
            }
        }

        impl $crate::parse::ParseState for $super {
            type Token = $crate::xir::flat::XirfToken<
                $crate::xir::flat::RefinedText
            >;
            type Object = $objty;
            type Error = [<$super Error_>];
            type Context = $crate::xir::parse::SuperStateContext<Self>;

            fn parse_token(
                self,
                tok: Self::Token,
                stack: &mut Self::Context,
            ) -> $crate::parse::TransitionResult<Self> {
                use $crate::{
                    parse::Transition,
                    xir::flat::{XirfToken, RefinedText},
                };

                // Used only by _some_ expansions.
                #[allow(unused_imports)]
                use $crate::xir::flat::Text;

                match (self, tok) {
                    // [super] {
                    $(
                        // [text] preemption;
                        //   see `Self::can_preempt_node`.
                        $(
                            (
                                st,
                                XirfToken::Text(
                                    RefinedText::Unrefined(
                                        Text($text, $text_span)
                                    ),
                                    _,
                                )
                            ) if st.can_preempt_node() => {
                                Transition(st).ok(<$objty>::from($text_map))
                            },
                        )?

                        // Preemption NT
                        $(
                            (
                                st,
                                XirfToken::Open(
                                    qname,
                                    ospan,
                                    depth,
                                ),
                            ) if st.can_preempt_node() && $pre_nt::matches(qname) => {
                                stack.transfer_with_ret(
                                    Transition(st),
                                    Transition(
                                        // Prevent recursing on this token.
                                        $pre_nt::non_preemptable()
                                    )
                                    .incomplete()
                                    .with_lookahead(XirfToken::Open(
                                        qname,
                                        ospan,
                                        depth,
                                    )),
                                )
                            },
                        )?
                    )?
                    // }

                    // Depth check is unnecessary since _all_ xir::parse
                    //   parsers
                    //     (at least at the time of writing)
                    //     ignore whitespace and comments,
                    //       so may as well return early.
                    // TODO: I'm ignoring _all_ text for now to
                    //   proceed with development; fix.
                    (
                        st,
                        XirfToken::Text(RefinedText::Whitespace(..), _)
                        | XirfToken::Comment(..)
                    ) => {
                        Transition(st).incomplete()
                    }

                    $(
                        // Pass token directly to child until it reports
                        //   a dead state,
                        //     after which we return to the `ParseState`
                        //     atop of the stack.
                        (Self::$nt(st), tok) => st.delegate_child(
                            tok,
                            stack,
                            |deadst, tok, stack| {
                                stack.ret_or_dead(tok, deadst)
                            },
                        ),
                    )*
                }
            }

            fn is_accepting(&self, stack: &Self::Context) -> bool {
                // This is short-circuiting,
                //   starting at the _bottom_ of the stack and moving
                //   upward.
                // The idea is that,
                //   is we're still in the middle of parsing,
                //   then it's almost certain that the [`ParseState`] on the
                //     bottom of the stack will not be in an accepting
                //     state,
                //       and so we can stop checking early.
                // In most cases,
                //   if we haven't hit EOF early,
                //   the stack should be either empty or consist of only the
                //     root state.
                //
                // After having considered the stack,
                //   we can then consider the active `ParseState`.
                stack.all(|st| st.is_inner_accepting(stack))
                    && self.is_inner_accepting(stack)
            }
        }

        impl $super {
            /// Whether the inner (active child) [`ParseState`] is in an
            ///   accepting state.
            ///
            /// [`ParseState`]: crate::parse::ParseState
            fn is_inner_accepting(
                &self,
                ctx: &<Self as $crate::parse::ParseState>::Context
            ) -> bool {
                use $crate::parse::ParseState;

                match self {
                    $(
                        Self::$nt(st) => st.is_accepting(ctx),
                    )*
                }
            }

            /// Whether the inner parser is in a state that can tolerate
            ///   superstate node preemption.
            ///
            /// Node preemption allows us (the superstate) to ask for
            ///   permission from the inner parser to parse some token
            ///   ourselves,
            ///     by asking whether the parser is in a state that would
            ///     cause semantic issues if we were to do so.
            ///
            /// For example,
            ///   if we were to preempt text nodes while an inner parser was
            ///   still parsing attributes,
            ///     then we would emit an object associated with that text
            ///     before the inner parser had a chance to conclude that
            ///     attribute parsing has completed and emit the opening
            ///     object for that node;
            ///       the result would otherwise be an incorrect
            ///       `Text, Open` instead of the correct `Open, Text`,
            ///         which would effectively unparent the text.
            /// Similarly,
            ///   if we were to parse our own tokens while an inner parser
            ///   was performing error recovery in such a way as to ignore
            ///   all child tokens,
            ///     then we would emit an object in an incorrect context.
            #[allow(dead_code)] // TODO: Remove when using for tpl apply
            fn can_preempt_node(&self) -> bool {
                match self {
                    $(
                        Self::$nt(st) => st.can_preempt_node(),
                    )*
                }
            }
        }

        impl $crate::xir::parse::SuperState for $super {}
    }};

    (@!ntfirst_init $super:ident, $ntfirst:ident $($nt:ident)*) => {
        $super::$ntfirst($ntfirst::non_preemptable())
    }
}

/// Superstate.
///
/// A superstate is responsible for aggregating all nonterminals and serving
///   as a trampoline to delegate parsing operations.
///
/// Conceptually,
///   a superstate acts as a runtime for the state machine defined by NT
///   interdependencies.
/// It represents the reification of such a state machine and all of its
///   transitions.
pub trait SuperState: ClosedParseState {}

/// Nonterminal.
///
/// This trait is used internally by the [`ele_parse!`] parser-generator.
pub trait Nt: Debug {
    /// Attribute parser for this element.
    type AttrState: AttrParseState;
    /// [`NtState::Jmp`] states for child NTs.
    type ChildNt: Debug + PartialEq + Eq;

    /// Matcher describing the node recognized by this parser.
    fn matcher() -> NodeMatcher;
}

/// States for nonterminals (NTs).
#[derive(Debug, PartialEq, Eq)]
pub enum NtState<NT: Nt> {
    /// Expecting opening tag for element.
    Expecting,

    /// Non-preemptable [`Self::Expecting`].
    NonPreemptableExpecting,

    /// Recovery state ignoring all remaining tokens for this
    ///   element.
    RecoverEleIgnore(QName, OpenSpan, Depth),

    // Recovery completed because end tag corresponding to the
    //   invalid element has been found.
    RecoverEleIgnoreClosed(QName, CloseSpan),

    /// Recovery state ignoring all tokens when a `Close` is
    ///   expected.
    ///
    /// This is token-agnostic---it
    ///   may be a child element,
    ///     but it may be text,
    ///     for example.
    CloseRecoverIgnore((QName, OpenSpan, Depth), Span),

    /// Parsing element attributes.
    Attrs((QName, OpenSpan, Depth), NT::AttrState),

    /// Preparing to pass control (jump) to a child NT's parser.
    Jmp(NT::ChildNt),

    /// Closing tag found and parsing of the element is
    ///   complete.
    Closed(Option<QName>, Span),
}

impl<NT: Nt> Default for NtState<NT> {
    fn default() -> Self {
        Self::Expecting
    }
}

impl<NT: Nt> NtState<NT> {
    pub fn can_preempt_node(&self) -> bool {
        use NtState::*;

        match self {
            // Preemption before the opening tag is safe,
            //   since we haven't started processing yet.
            Expecting => true,

            // The name says it all.
            // Instantiated by the superstate.
            NonPreemptableExpecting => false,

            // Preemption during recovery would cause tokens to be parsed
            //   when they ought to be ignored,
            //     so we must process all tokens during recovery.
            RecoverEleIgnore(..) | CloseRecoverIgnore(..) => false,

            // It is _not_ safe to preempt attribute parsing since attribute
            //   parsers aggregate until a non-attribute token is
            //   encountered;
            //     we must allow attribute parsing to finish its job
            //     _before_ any preempted nodes are emitted since the
            //     attributes came _before_ that node.
            Attrs(..) => false,

            // These states represent jump states where we're about to
            //   transition to the next child parser.
            // It's safe to preempt here,
            //   since we're not in the middle of parsing.
            //
            // Note that this includes `ExpectClose_` because of the macro
            //   preprocessing,
            //     and Rust's exhaustiveness check will ensure that it is
            //     accounted for if that changes.
            // If we're expecting that the next token is a `Close`,
            //     then it must be safe to preempt other nodes that may
            //     appear in this context as children.
            Jmp(..) => true,

            // If we're done,
            //   we want to be able to yield a dead state so that we can
            //   transition away from this parser.
            RecoverEleIgnoreClosed(..) | Closed(..) => false,
        }
    }
}

impl<NT: Nt> Display for NtState<NT> {
    fn fmt(&self, f: &mut Formatter) -> std::fmt::Result {
        use crate::xir::fmt::{TtCloseXmlEle, TtOpenXmlEle};
        use NtState::*;

        match self {
            Expecting | NonPreemptableExpecting => write!(
                f,
                "expecting opening tag {}",
                TtOpenXmlEle::wrap(NT::matcher()),
            ),
            RecoverEleIgnore(name, _, _) | RecoverEleIgnoreClosed(name, _) => {
                write!(
                    f,
                    "attempting to recover by ignoring element \
                    with unexpected name {given} \
                    (expected {expected})",
                    given = TtQuote::wrap(name),
                    expected = TtQuote::wrap(NT::matcher()),
                )
            }
            CloseRecoverIgnore((qname, _, depth), _) => write!(
                f,
                "attempting to recover by ignoring input \
                    until the expected end tag {expected} \
                    at depth {depth}",
                expected = TtCloseXmlEle::wrap(qname),
            ),

            Attrs(_, sa) => Display::fmt(sa, f),
            Closed(Some(qname), _) => {
                write!(f, "done parsing element {}", TtQuote::wrap(qname),)
            }
            // Should only happen on an unexpected `Close`.
            Closed(None, _) => write!(
                f,
                "skipped parsing element {}",
                TtQuote::wrap(NT::matcher()),
            ),
            // TODO: A better description.
            Jmp(_) => {
                write!(
                    f,
                    "preparing to transition to \
                        parser for next child element(s)"
                )
            }
        }
    }
}

/// Sum nonterminal.
///
/// This trait is used internally by the [`ele_parse!`] parser-generator.
pub trait SumNt: Debug {
    fn fmt_matches_top(f: &mut std::fmt::Formatter) -> std::fmt::Result;
}

/// States for sum nonterminals.
///
/// Sum NTs act like a sum type,
///   transitioning to the appropriate inner NT based on the next token of
///   input.
/// Sum NTs have order-based precedence when faced with ambiguity,
///   like a PEG.
///
/// This is expected to be wrapped by a newtype for each Sum NT,
///   and does not implement [`ParseState`] itself.
#[derive(Debug, PartialEq, Eq, Default)]
pub enum SumNtState<NT: SumNt> {
    /// Expecting an opening tag for an element.
    #[default]
    Expecting,

    /// Non-preemptable [`Self::Expecting`].
    NonPreemptableExpecting,

    /// Recovery state ignoring all remaining tokens for this
    ///   element.
    RecoverEleIgnore(QName, OpenSpan, Depth, PhantomData<NT>),
}

impl<NT: SumNt> SumNtState<NT> {
    /// Whether the parser is in a state that can tolerate
    ///   superstate node preemption.
    pub fn can_preempt_node(&self) -> bool {
        use SumNtState::*;

        match self {
            // Preemption before the opening tag is safe,
            //   since we haven't started processing yet.
            Expecting => true,

            // The name says it all.
            // Instantiated by the superstate.
            NonPreemptableExpecting => false,

            // Preemption during recovery would cause tokens to
            //   be parsed when they ought to be ignored,
            //     so we must process all tokens during recovery.
            RecoverEleIgnore(..) => false,
        }
    }
}

impl<NT: SumNt> Display for SumNtState<NT> {
    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
        use SumNtState::*;

        match self {
            Expecting | NonPreemptableExpecting => {
                write!(f, "expecting ")?;
                NT::fmt_matches_top(f)
            }

            RecoverEleIgnore(name, _, _, _) => {
                write!(
                    f,
                    "attempting to recover by ignoring element \
                    with unexpected name {given} \
                    (expected",
                    given = TtQuote::wrap(name),
                )?;

                NT::fmt_matches_top(f)?;
                f.write_str(")")
            }
        }
    }
}

#[cfg(test)]
mod test;