lr1/error/mod.rs

//! Error reporting. For now very stupid and simplistic.

use collections::{set, Set};
use grammar::repr::*;
use itertools::Itertools;
use lr1::core::*;
use lr1::example::{Example, ExampleStyles, ExampleSymbol};
use lr1::first::FirstSets;
use lr1::lookahead::{Token, TokenSet};
use lr1::trace::Tracer;
use message::builder::{BodyCharacter, Builder, Character, MessageBuilder};
use message::Message;
use tls::Tls;

#[cfg(test)]
mod test;

pub fn report_error(grammar: &Grammar, error: &LR1TableConstructionError) -> Vec<Message> {
    let mut cx = ErrorReportingCx::new(grammar, &error.states, &error.conflicts);
    cx.report_errors()
}

struct ErrorReportingCx<'cx, 'grammar: 'cx> {
    grammar: &'grammar Grammar,
    first_sets: FirstSets,
    states: &'cx [LR1State<'grammar>],
    conflicts: &'cx [LR1Conflict<'grammar>],
}

#[derive(Debug)]
enum ConflictClassification {
    /// The grammar is ambiguous. This means we have two examples of
    /// precisely the same set of symbols which can be reduced in two
    /// distinct ways.
    Ambiguity { action: Example, reduce: Example },

    /// The grammar is ambiguous, and moreover it looks like a
    /// precedence error. This means that the reduction is to a
    /// nonterminal `T` and the shift is some symbol sandwiched
    /// between two instances of `T`.
    Precedence {
        shift: Example,
        reduce: Example,
        nonterminal: NonterminalString,
    },

    /// Suggest inlining `nonterminal`. Makes sense if there are two
    /// levels in the reduction tree in both examples, and the suffix
    /// after the inner reduction is the same in all cases.
    SuggestInline {
        shift: Example,
        reduce: Example,
        nonterminal: NonterminalString,
    },

    /// Like the previous, but suggest replacing `nonterminal` with
    /// `symbol?`. Makes sense if the thing to be inlined consists of
    /// two alternatives, `X = symbol | ()`.
    SuggestQuestion {
        shift: Example,
        reduce: Example,
        nonterminal: NonterminalString,
        symbol: Symbol,
    },

    /// Can't say much beyond that a conflict occurred.
    InsufficientLookahead { action: Example, reduce: Example },

    /// Really can't say *ANYTHING*.
    Naive,
}

type TokenConflict<'grammar> = Conflict<'grammar, Token>;

impl<'cx, 'grammar> ErrorReportingCx<'cx, 'grammar> {
    fn new(
        grammar: &'grammar Grammar,
        states: &'cx [LR1State<'grammar>],
        conflicts: &'cx [LR1Conflict<'grammar>],
    ) -> Self {
        ErrorReportingCx {
            grammar: grammar,
            first_sets: FirstSets::new(grammar),
            states: states,
            conflicts: conflicts,
        }
    }

    fn report_errors(&mut self) -> Vec<Message> {
        token_conflicts(self.conflicts)
            .iter()
            .map(|conflict| self.report_error(conflict))
            .collect()
    }

    fn report_error(&mut self, conflict: &TokenConflict<'grammar>) -> Message {
        match self.classify(conflict) {
            ConflictClassification::Ambiguity { action, reduce } => {
                self.report_error_ambiguity(conflict, action, reduce)
            }
            ConflictClassification::Precedence {
                shift,
                reduce,
                nonterminal,
            } => self.report_error_precedence(conflict, shift, reduce, nonterminal),
            ConflictClassification::SuggestInline {
                shift,
                reduce,
                nonterminal,
            } => self.report_error_suggest_inline(conflict, shift, reduce, nonterminal),
            ConflictClassification::SuggestQuestion {
                shift,
                reduce,
                nonterminal,
                symbol,
            } => self.report_error_suggest_question(conflict, shift, reduce, nonterminal, symbol),
            ConflictClassification::InsufficientLookahead { action, reduce } => {
                self.report_error_insufficient_lookahead(conflict, action, reduce)
            }
            ConflictClassification::Naive => self.report_error_naive(conflict),
        }
    }

    fn report_error_ambiguity_core(
        &self,
        conflict: &TokenConflict<'grammar>,
        shift: Example,
        reduce: Example,
    ) -> Builder<BodyCharacter> {
        let styles = ExampleStyles::ambig();
        MessageBuilder::new(conflict.production.span)
            .heading()
            .text("Ambiguous grammar detected")
            .end()
            .body()
            .begin_lines()
            .wrap_text("The following symbols can be reduced in two ways:")
            .push(reduce.to_symbol_list(reduce.symbols.len(), styles))
            .end()
            .begin_lines()
            .wrap_text("They could be reduced like so:")
            .push(reduce.into_picture(styles))
            .end()
            .begin_lines()
            .wrap_text("Alternatively, they could be reduced like so:")
            .push(shift.into_picture(styles))
            .end()
    }

    fn report_error_ambiguity(
        &self,
        conflict: &TokenConflict<'grammar>,
        shift: Example,
        reduce: Example,
    ) -> Message {
        self.report_error_ambiguity_core(conflict, shift, reduce)
            .wrap_text(
                "LALRPOP does not yet support ambiguous grammars. \
                 See the LALRPOP manual for advice on \
                 making your grammar unambiguous.",
            )
            .end()
            .end()
    }

    fn report_error_precedence(
        &self,
        conflict: &TokenConflict<'grammar>,
        shift: Example,
        reduce: Example,
        nonterminal: NonterminalString,
    ) -> Message {
        self.report_error_ambiguity_core(conflict, shift, reduce)
            .begin_wrap()
            .text("Hint:")
            .styled(Tls::session().hint_text)
            .text("This looks like a precedence error related to")
            .push(nonterminal)
            .verbatimed()
            .punctuated(".")
            .text("See the LALRPOP manual for advice on encoding precedence.")
            .end()
            .end()
            .end()
    }

    fn report_error_not_lr1_core(
        &self,
        conflict: &TokenConflict<'grammar>,
        action: Example,
        reduce: Example,
    ) -> Builder<BodyCharacter> {
        let styles = ExampleStyles::new();
        let builder = MessageBuilder::new(conflict.production.span)
            .heading()
            .text("Local ambiguity detected")
            .end()
            .body();

        let builder = builder
            .begin_lines()
            .begin_wrap()
            .text("The problem arises after having observed the following symbols")
            .text("in the input:")
            .end()
            .push(if action.cursor >= reduce.cursor {
                action.to_symbol_list(action.cursor, styles)
            } else {
                reduce.to_symbol_list(reduce.cursor, styles)
            })
            .begin_wrap();

        let builder = match conflict.lookahead {
            Token::Terminal(ref term) => builder
                .text("At that point, if the next token is a")
                .push(term.clone())
                .verbatimed()
                .styled(Tls::session().cursor_symbol)
                .punctuated(","),
            Token::Error => builder.text("If an error has been found,"),
            Token::EOF => builder.text("If the end of the input is reached,"),
        };

        let builder = builder
            .text("then the parser can proceed in two different ways.")
            .end()
            .end();

        let builder = self.describe_reduce(builder, styles, conflict.production, reduce, "First");

        match conflict.action {
            Action::Shift(ref lookahead, _) => {
                self.describe_shift(builder, styles, lookahead.clone(), action, "Alternatively")
            }
            Action::Reduce(production) => {
                self.describe_reduce(builder, styles, production, action, "Alternatively")
            }
        }
    }

    fn describe_shift<C: Character>(
        &self,
        builder: Builder<C>,
        styles: ExampleStyles,
        lookahead: TerminalString,
        example: Example,
        intro_word: &str,
    ) -> Builder<C> {
        // A shift example looks like:
        //
        // ...p1 ...p2 (*) L ...s2 ...s1
        // |     |               |     |
        // |     +-NT1-----------+     |
        // |                           |
        // |           ...             |
        // |                           |
        // +-NT2-----------------------+

        let nt1 = example.reductions[0].nonterminal.clone();

        builder
            .begin_lines()
            .begin_wrap()
            .text(intro_word)
            .punctuated(",")
            .text("the parser could shift the")
            .push(lookahead)
            .verbatimed()
            .text("token and later use it to construct a")
            .push(nt1)
            .verbatimed()
            .punctuated(".")
            .text("This might then yield a parse tree like")
            .end()
            .push(example.into_picture(styles))
            .end()
    }

    fn describe_reduce<C: Character>(
        &self,
        builder: Builder<C>,
        styles: ExampleStyles,
        production: &Production,
        example: Example,
        intro_word: &str,
    ) -> Builder<C> {
        builder
            .begin_lines()
            .begin_wrap()
            .text(intro_word)
            .punctuated(",")
            .text("the parser could execute the production at")
            .push(production.span)
            .punctuated(",")
            .text("which would consume the top")
            .text(production.symbols.len())
            .text("token(s) from the stack")
            .text("and produce a")
            .push(production.nonterminal.clone())
            .verbatimed()
            .punctuated(".")
            .text("This might then yield a parse tree like")
            .end()
            .push(example.into_picture(styles))
            .end()
    }

    fn report_error_suggest_inline(
        &self,
        conflict: &TokenConflict<'grammar>,
        shift: Example,
        reduce: Example,
        nonterminal: NonterminalString,
    ) -> Message {
        let builder = self.report_error_not_lr1_core(conflict, shift, reduce);

        builder
            .begin_wrap()
            .text("Hint:")
            .styled(Tls::session().hint_text)
            .text("It appears you could resolve this problem by adding")
            .text("the annotation `#[inline]` to the definition of")
            .push(nonterminal)
            .verbatimed()
            .punctuated(".")
            .text("For more information, see the section on inlining")
            .text("in the LALRPOP manual.")
            .end()
            .end()
            .end()
    }

    fn report_error_suggest_question(
        &self,
        conflict: &TokenConflict<'grammar>,
        shift: Example,
        reduce: Example,
        nonterminal: NonterminalString,
        symbol: Symbol,
    ) -> Message {
        let builder = self.report_error_not_lr1_core(conflict, shift, reduce);

        builder
            .begin_wrap()
            .text("Hint:")
            .styled(Tls::session().hint_text)
            .text("It appears you could resolve this problem by replacing")
            .text("uses of")
            .push(nonterminal.clone())
            .verbatimed()
            .text("with")
            .text(symbol) // intentionally disable coloring here, looks better
            .adjacent_text("`", "?`")
            .text(
                "(or, alternatively, by adding the annotation `#[inline]` \
                 to the definition of",
            )
            .push(nonterminal)
            .punctuated(").")
            .text("For more information, see the section on inlining")
            .text("in the LALROP manual.")
            .end()
            .end()
            .end()
    }

    fn report_error_insufficient_lookahead(
        &self,
        conflict: &TokenConflict<'grammar>,
        action: Example,
        reduce: Example,
    ) -> Message {
        // The reduce example will look something like:
        //
        //
        // ...p1 ...p2 (*) L ...s2 ...s1
        // |     |               |     |
        // |     +-NT1-----------+     |
        // |     |               |     |
        // |     +-...-----------+     |
        // |     |               |     |
        // |     +-NTn-----------+     |
        // |                           |
        // +-NTn+1---------------------+
        //
        // To solve the conflict, essentially, the user needs to
        // modify the grammar so that `NTn` does not appear with `L`
        // in its follow-set. How to guide them in this?

        let builder = self.report_error_not_lr1_core(conflict, action, reduce);

        builder
            .wrap_text(
                "See the LALRPOP manual for advice on \
                 making your grammar LR(1).",
            )
            .end()
            .end()
    }

    /// Naive error reporting. This is a fallback path which (I think)
    /// never actually executes.
    fn report_error_naive(&self, conflict: &TokenConflict<'grammar>) -> Message {
        let mut builder = MessageBuilder::new(conflict.production.span)
            .heading()
            .text("Conflict detected")
            .end()
            .body()
            .begin_lines()
            .wrap_text("when in this state:")
            .indented();
        for item in self.states[conflict.state.0].items.vec.iter() {
            builder = builder.text(format!("{:?}", item));
        }
        let mut builder = builder
            .end()
            .begin_wrap()
            .text(format!("and looking at a token `{:?}`", conflict.lookahead))
            .text("we can reduce to a")
            .push(conflict.production.nonterminal.clone())
            .verbatimed();
        builder = match conflict.action {
            Action::Shift(..) => builder.text("but we can also shift"),
            Action::Reduce(prod) => builder
                .text("but we can also reduce to a")
                .text(prod.nonterminal.clone())
                .verbatimed(),
        };
        builder.end().end().end()
    }

    fn classify(&mut self, conflict: &TokenConflict<'grammar>) -> ConflictClassification {
        // Find examples from the conflicting action (either a shift
        // or a reduce).
        let mut action_examples = match conflict.action {
            Action::Shift(..) => self.shift_examples(conflict),
            Action::Reduce(production) => {
                self.reduce_examples(conflict.state, production, conflict.lookahead.clone())
            }
        };

        // Find examples from the conflicting reduce.
        let mut reduce_examples = self.reduce_examples(
            conflict.state,
            conflict.production,
            conflict.lookahead.clone(),
        );

        // Prefer shorter examples to longer ones.
        action_examples.sort_by(|e, f| e.symbols.len().cmp(&f.symbols.len()));
        reduce_examples.sort_by(|e, f| e.symbols.len().cmp(&f.symbols.len()));

        // This really shouldn't happen, but if we've failed to come
        // up with examples, then report a "naive" error.
        if action_examples.is_empty() || reduce_examples.is_empty() {
            return ConflictClassification::Naive;
        }

        if let Some(classification) =
            self.try_classify_ambiguity(conflict, &action_examples, &reduce_examples)
        {
            return classification;
        }

        if let Some(classification) =
            self.try_classify_question(conflict, &action_examples, &reduce_examples)
        {
            return classification;
        }

        if let Some(classification) =
            self.try_classify_inline(conflict, &action_examples, &reduce_examples)
        {
            return classification;
        }

        // Give up. Just grab an example from each and pair them up.
        // If there aren't even two examples, something's pretty
        // bogus, but we'll just call it naive.
        action_examples
            .into_iter()
            .zip(reduce_examples)
            .next()
            .map(
                |(action, reduce)| ConflictClassification::InsufficientLookahead {
                    action: action,
                    reduce: reduce,
                },
            )
            .unwrap_or(ConflictClassification::Naive)
    }

    fn try_classify_ambiguity(
        &self,
        conflict: &TokenConflict<'grammar>,
        action_examples: &[Example],
        reduce_examples: &[Example],
    ) -> Option<ConflictClassification> {
        action_examples
            .iter()
            .cartesian_product(reduce_examples)
            .filter(|&(action, reduce)| action.symbols == reduce.symbols)
            .filter(|&(action, reduce)| action.cursor == reduce.cursor)
            .map(|(action, reduce)| {
                // Consider whether to call this a precedence
                // error. We do this if we are stuck between reducing
                // `T = T S T` and shifting `S`.
                if let Action::Shift(ref term, _) = conflict.action {
                    let nt = &conflict.production.nonterminal;
                    if conflict.production.symbols.len() == 3
                        && conflict.production.symbols[0] == Symbol::Nonterminal(nt.clone())
                        && conflict.production.symbols[1] == Symbol::Terminal(term.clone())
                        && conflict.production.symbols[2] == Symbol::Nonterminal(nt.clone())
                    {
                        return ConflictClassification::Precedence {
                            shift: action.clone(),
                            reduce: reduce.clone(),
                            nonterminal: nt.clone(),
                        };
                    }
                }
                ConflictClassification::Ambiguity {
                    action: action.clone(),
                    reduce: reduce.clone(),
                }
            })
            .next()
    }

    fn try_classify_question(
        &self,
        conflict: &TokenConflict<'grammar>,
        action_examples: &[Example],
        reduce_examples: &[Example],
    ) -> Option<ConflictClassification> {
        // If we get a shift/reduce conflict and the reduce
        // is of a nonterminal like:
        //
        //     T = { () | U }
        //
        // then suggest replacing T with U?. I'm being a bit lenient
        // here since I do not KNOW that it will help, but it often
        // does, and it's better style anyhow.

        if let Action::Reduce(_) = conflict.action {
            return None;
        }

        debug!(
            "try_classify_question: action_examples={:?}",
            action_examples
        );
        debug!(
            "try_classify_question: reduce_examples={:?}",
            reduce_examples
        );

        let nt = &conflict.production.nonterminal;
        let nt_productions = self.grammar.productions_for(nt);
        if nt_productions.len() == 2 {
            for &(i, j) in &[(0, 1), (1, 0)] {
                if nt_productions[i].symbols.is_empty() && nt_productions[j].symbols.len() == 1 {
                    return Some(ConflictClassification::SuggestQuestion {
                        shift: action_examples[0].clone(),
                        reduce: reduce_examples[0].clone(),
                        nonterminal: nt.clone(),
                        symbol: nt_productions[j].symbols[0].clone(),
                    });
                }
            }
        }

        None
    }

    fn try_classify_inline(
        &self,
        conflict: &TokenConflict<'grammar>,
        action_examples: &[Example],
        reduce_examples: &[Example],
    ) -> Option<ConflictClassification> {
        // Inlining can help resolve a shift/reduce conflict because
        // it defers the need to reduce. In particular, if we inlined
        // all the reductions up until the last one, then we would be
        // able to *shift* the lookahead instead of having to reduce.
        // This can be helpful if we can see that shifting would let
        // us delay reducing until the lookahead diverges.

        // Only applicable to shift/reduce:
        if let Action::Reduce(_) = conflict.action {
            return None;
        }

        // FIXME: The logic here finds the first example where inline
        // would help; but maybe we want to restrict it to cases
        // where inlining would help *all* the examples...?

        action_examples
            .iter()
            .cartesian_product(reduce_examples)
            .filter_map(|(shift, reduce)| {
                if self.try_classify_inline_example(shift, reduce) {
                    let nt = &reduce.reductions[0].nonterminal;
                    Some(ConflictClassification::SuggestInline {
                        shift: shift.clone(),
                        reduce: reduce.clone(),
                        nonterminal: nt.clone(),
                    })
                } else {
                    None
                }
            })
            .next()
    }

    fn try_classify_inline_example<'ex>(&self, shift: &Example, reduce: &Example) -> bool {
        debug!("try_classify_inline_example({:?}, {:?})", shift, reduce);

        // In the case of shift, the example will look like
        //
        // ```
        // ... ... (*) L ...s1 ...
        // |   |             |   |
        // |   +-R0----------+   |
        // |  ...                |
        // +-Rn------------------+
        // ```
        //
        // We want to extract the symbols ...s1: these are the
        // things we are able to shift before being forced to
        // make our next hard decision (to reduce R0 or not).
        let shift_upcoming = &shift.symbols[shift.cursor + 1..shift.reductions[0].end];
        debug!(
            "try_classify_inline_example: shift_upcoming={:?}",
            shift_upcoming
        );

        // For the reduce, the example might look like
        //
        // ```
        // ...  ...   (*) ...s ...
        // | | |    |        |
        // | | +-R0-+        |
        // | | ...  |        |
        // | +--Ri--+        |
        // |  ...            |
        // +-R(i+1)----------+
        // ```
        //
        // where Ri is the last reduction that requires
        // shifting no additional symbols. In this case, if we
        // inlined R0...Ri, then we know we can shift L.
        let r0_end = reduce.reductions[0].end;
        let i = reduce.reductions.iter().position(|r| r.end != r0_end);
        let i = match i {
            Some(v) => v,
            None => return false,
        };
        let ri = &reduce.reductions[i];
        let reduce_upcoming = &reduce.symbols[r0_end..ri.end];
        debug!(
            "try_classify_inline_example: reduce_upcoming={:?} i={:?}",
            reduce_upcoming, i
        );

        // For now, we only suggest inlining a single nonterminal,
        // mostly because I am too lazy to weak the suggestion struct
        // and error messages (but the rest of the code below doesn't
        // make this assumption for the most part).
        if i != 1 {
            return false;
        }

        // Make sure that all the things we are suggesting inlining
        // are distinct so that we are not introducing a cycle.
        let mut duplicates = set();
        if reduce.reductions[0..i + 1]
            .iter()
            .any(|r| !duplicates.insert(r.nonterminal.clone()))
        {
            return false;
        }

        // Compare the two suffixes to see whether they
        // diverge at some point.
        shift_upcoming
            .iter()
            .zip(reduce_upcoming)
            .filter_map(|(shift_sym, reduce_sym)| match (shift_sym, reduce_sym) {
                (&ExampleSymbol::Symbol(ref shift_sym), &ExampleSymbol::Symbol(ref reduce_sym)) => {
                    if shift_sym == reduce_sym {
                        // same symbol on both; we'll be able to shift them
                        None
                    } else {
                        // different symbols: for this to work, must
                        // have disjoint first sets. Note that we
                        // consider a suffix matching epsilon to be
                        // potentially overlapping, though we could
                        // supply the actual lookahead for more precision.
                        let shift_first = self.first_sets.first0(&[shift_sym.clone()]);
                        let reduce_first = self.first_sets.first0(&[reduce_sym.clone()]);
                        if shift_first.is_disjoint(&reduce_first) {
                            Some(true)
                        } else {
                            Some(false)
                        }
                    }
                }
                _ => {
                    // we don't expect to encounter any
                    // epsilons, I don't think, because those
                    // only occur with an empty reduce at the
                    // top level
                    Some(false)
                }
            })
            .next()
            .unwrap_or(false)
    }

    fn shift_examples(&self, conflict: &TokenConflict<'grammar>) -> Vec<Example> {
        log!(Tls::session(), Verbose, "Gathering shift examples");
        let state = &self.states[conflict.state.0];
        let conflicting_items = self.conflicting_shift_items(state, conflict);
        conflicting_items
            .into_iter()
            .flat_map(|item| {
                let tracer = Tracer::new(&self.first_sets, self.states);
                let shift_trace = tracer.backtrace_shift(conflict.state, item);
                let local_examples: Vec<Example> = shift_trace.lr0_examples(item).collect();
                local_examples
            })
            .collect()
    }

    fn reduce_examples(
        &self,
        state: StateIndex,
        production: &'grammar Production,
        lookahead: Token,
    ) -> Vec<Example> {
        log!(Tls::session(), Verbose, "Gathering reduce examples");
        let item = Item {
            production: production,
            index: production.symbols.len(),
            lookahead: TokenSet::from(lookahead),
        };
        let tracer = Tracer::new(&self.first_sets, self.states);
        let reduce_trace = tracer.backtrace_reduce(state, item.to_lr0());
        reduce_trace.lr1_examples(&self.first_sets, &item).collect()
    }

    fn conflicting_shift_items(
        &self,
        state: &LR1State<'grammar>,
        conflict: &TokenConflict<'grammar>,
    ) -> Set<LR0Item<'grammar>> {
        // Lookahead must be a terminal, not EOF.
        // Find an item J like `Bar = ... (*) L ...`.
        let lookahead = Symbol::Terminal(conflict.lookahead.unwrap_terminal().clone());
        state
            .items
            .vec
            .iter()
            .filter(|i| i.can_shift())
            .filter(|i| i.production.symbols[i.index] == lookahead)
            .map(|i| i.to_lr0())
            .collect()
    }
}

fn token_conflicts<'grammar>(
    conflicts: &[Conflict<'grammar, TokenSet>],
) -> Vec<TokenConflict<'grammar>> {
    conflicts
        .iter()
        .flat_map(|conflict| {
            conflict.lookahead.iter().map(move |token| Conflict {
                state: conflict.state,
                lookahead: token,
                production: conflict.production,
                action: conflict.action.clone(),
            })
        })
        .collect()
}

//fn choose_example<'grammar>(states: &[State<'grammar>],
//                            lookahead: Token,
//                            conflict: &TokenConflict<'grammar>)
//{
//    // Whenever we have a conflict in state S, there is always:
//    // - a given lookahead L that permits some reduction, due to
//    //   an item I like `Foo = ... (*) [L]`
//    // - another action that conflicts with R1.
//    //
//    // The backtrace code can give context to this item `I`, but the
//    // problem is that it often results in many different contexts,
//    // and we need to try and narrow those down to the one that will
//    // help the user understand the problem.
//    //
//    // For that, we turn to the conflicting action, which can either be
//    // a shift or reduce. Let's consider those two cases.
//    //
//    // ### Shift
//    //
//    // If the conflicting action is a shift, then there is at least
//    // one item J in the state S like `Bar = ... (*) L ...`. We can
//    // produce a backtrace from J and enumerate examples. We want to
//    // find a pair of examples from I and J that share a common
//    // prefix.
//    //
//    // ### Reduce
//    //
//    // If the conflicting action is a reduce, then there is at least
//    // one item J in S like `Bar = ... (*) [L]`. We can produce a
//    // backtrace for J and then search for an example that shares a
//    // common prefix.
//
//}
//
//fn conflicting_item<'grammar>(state: &State<'grammar>,
//                              lookahead: Token,
//                              conflict: &TokenConflict<'grammar>)
//                              -> Item<'grammar>
//{
//    match conflict.action {
//        Action::Shift(_) => {
//        }
//        Action::Reduce(production) => {
//            // Must be at least some other item J in S like `Bar = ... (*) [L]`.
//            state.items.vec.iter()
//                           .filter(|i| i.can_reduce())
//                           .filter(|i| i.lookahead == lookahead)
//                           .filter(|i| i.production == production)
//                           .cloned()
//                           .next()
//                           .unwrap()
//        }
//    }
//}