transcribe.rs

use crate::base::ExtCtxt;
use crate::errors::{
    CountRepetitionMisplaced, MetaVarExprUnrecognizedVar, MetaVarsDifSeqMatchers, MustRepeatOnce,
    NoSyntaxVarsExprRepeat, VarStillRepeating,
};
use crate::mbe::macro_parser::{NamedMatch, NamedMatch::*};
use crate::mbe::{self, KleeneOp, MetaVarExpr};
use rustc_ast::mut_visit::{self, MutVisitor};
use rustc_ast::token::{self, Delimiter, Token, TokenKind};
use rustc_ast::tokenstream::{DelimSpacing, DelimSpan, Spacing, TokenStream, TokenTree};
use rustc_data_structures::fx::FxHashMap;
use rustc_errors::{pluralize, Diag, PResult};
use rustc_parse::parser::ParseNtResult;
use rustc_span::hygiene::{LocalExpnId, Transparency};
use rustc_span::symbol::{sym, Ident, MacroRulesNormalizedIdent};
use rustc_span::{with_metavar_spans, Span, SyntaxContext};

use smallvec::{smallvec, SmallVec};
use std::mem;

// A Marker adds the given mark to the syntax context.
struct Marker(LocalExpnId, Transparency, FxHashMap<SyntaxContext, SyntaxContext>);

impl MutVisitor for Marker {
    const VISIT_TOKENS: bool = true;

    fn visit_span(&mut self, span: &mut Span) {
        // `apply_mark` is a relatively expensive operation, both due to taking hygiene lock, and
        // by itself. All tokens in a macro body typically have the same syntactic context, unless
        // it's some advanced case with macro-generated macros. So if we cache the marked version
        // of that context once, we'll typically have a 100% cache hit rate after that.
        let Marker(expn_id, transparency, ref mut cache) = *self;
        let data = span.data();
        let marked_ctxt = *cache
            .entry(data.ctxt)
            .or_insert_with(|| data.ctxt.apply_mark(expn_id.to_expn_id(), transparency));
        *span = data.with_ctxt(marked_ctxt);
    }
}

/// An iterator over the token trees in a delimited token tree (`{ ... }`) or a sequence (`$(...)`).
enum Frame<'a> {
    Delimited {
        tts: &'a [mbe::TokenTree],
        idx: usize,
        delim: Delimiter,
        span: DelimSpan,
        spacing: DelimSpacing,
    },
    Sequence {
        tts: &'a [mbe::TokenTree],
        idx: usize,
        sep: Option<Token>,
        kleene_op: KleeneOp,
    },
}

impl<'a> Frame<'a> {
    /// Construct a new frame around the delimited set of tokens.
    fn new(src: &'a mbe::Delimited, span: DelimSpan, spacing: DelimSpacing) -> Frame<'a> {
        Frame::Delimited { tts: &src.tts, idx: 0, delim: src.delim, span, spacing }
    }
}

impl<'a> Iterator for Frame<'a> {
    type Item = &'a mbe::TokenTree;

    fn next(&mut self) -> Option<&'a mbe::TokenTree> {
        match self {
            Frame::Delimited { tts, idx, .. } | Frame::Sequence { tts, idx, .. } => {
                let res = tts.get(*idx);
                *idx += 1;
                res
            }
        }
    }
}

/// This can do Macro-By-Example transcription.
/// - `interp` is a map of meta-variables to the tokens (non-terminals) they matched in the
///   invocation. We are assuming we already know there is a match.
/// - `src` is the RHS of the MBE, that is, the "example" we are filling in.
///
/// For example,
///
/// ```rust
/// macro_rules! foo {
///     ($id:ident) => { println!("{}", stringify!($id)); }
/// }
///
/// foo!(bar);
/// ```
///
/// `interp` would contain `$id => bar` and `src` would contain `println!("{}", stringify!($id));`.
///
/// `transcribe` would return a `TokenStream` containing `println!("{}", stringify!(bar));`.
///
/// Along the way, we do some additional error checking.
pub(super) fn transcribe<'a>(
    cx: &ExtCtxt<'a>,
    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    src: &mbe::Delimited,
    src_span: DelimSpan,
    transparency: Transparency,
) -> PResult<'a, TokenStream> {
    // Nothing for us to transcribe...
    if src.tts.is_empty() {
        return Ok(TokenStream::default());
    }

    // We descend into the RHS (`src`), expanding things as we go. This stack contains the things
    // we have yet to expand/are still expanding. We start the stack off with the whole RHS. The
    // choice of spacing values doesn't matter.
    let mut stack: SmallVec<[Frame<'_>; 1]> =
        smallvec![Frame::new(src, src_span, DelimSpacing::new(Spacing::Alone, Spacing::Alone))];

    // As we descend in the RHS, we will need to be able to match nested sequences of matchers.
    // `repeats` keeps track of where we are in matching at each level, with the last element being
    // the most deeply nested sequence. This is used as a stack.
    let mut repeats = Vec::new();

    // `result` contains resulting token stream from the TokenTree we just finished processing. At
    // the end, this will contain the full result of transcription, but at arbitrary points during
    // `transcribe`, `result` will contain subsets of the final result.
    //
    // Specifically, as we descend into each TokenTree, we will push the existing results onto the
    // `result_stack` and clear `results`. We will then produce the results of transcribing the
    // TokenTree into `results`. Then, as we unwind back out of the `TokenTree`, we will pop the
    // `result_stack` and append `results` too it to produce the new `results` up to that point.
    //
    // Thus, if we try to pop the `result_stack` and it is empty, we have reached the top-level
    // again, and we are done transcribing.
    let mut result: Vec<TokenTree> = Vec::new();
    let mut result_stack = Vec::new();
    let mut marker = Marker(cx.current_expansion.id, transparency, Default::default());

    loop {
        // Look at the last frame on the stack.
        // If it still has a TokenTree we have not looked at yet, use that tree.
        let Some(tree) = stack.last_mut().unwrap().next() else {
            // This else-case never produces a value for `tree` (it `continue`s or `return`s).

            // Otherwise, if we have just reached the end of a sequence and we can keep repeating,
            // go back to the beginning of the sequence.
            if let Frame::Sequence { idx, sep, .. } = stack.last_mut().unwrap() {
                let (repeat_idx, repeat_len) = repeats.last_mut().unwrap();
                *repeat_idx += 1;
                if repeat_idx < repeat_len {
                    *idx = 0;
                    if let Some(sep) = sep {
                        result.push(TokenTree::Token(sep.clone(), Spacing::Alone));
                    }
                    continue;
                }
            }

            // We are done with the top of the stack. Pop it. Depending on what it was, we do
            // different things. Note that the outermost item must be the delimited, wrapped RHS
            // that was passed in originally to `transcribe`.
            match stack.pop().unwrap() {
                // Done with a sequence. Pop from repeats.
                Frame::Sequence { .. } => {
                    repeats.pop();
                }

                // We are done processing a Delimited. If this is the top-level delimited, we are
                // done. Otherwise, we unwind the result_stack to append what we have produced to
                // any previous results.
                Frame::Delimited { delim, span, mut spacing, .. } => {
                    // Hack to force-insert a space after `]` in certain case.
                    // See discussion of the `hex-literal` crate in #114571.
                    if delim == Delimiter::Bracket {
                        spacing.close = Spacing::Alone;
                    }
                    if result_stack.is_empty() {
                        // No results left to compute! We are back at the top-level.
                        return Ok(TokenStream::new(result));
                    }

                    // Step back into the parent Delimited.
                    let tree = TokenTree::Delimited(span, spacing, delim, TokenStream::new(result));
                    result = result_stack.pop().unwrap();
                    result.push(tree);
                }
            }
            continue;
        };

        // At this point, we know we are in the middle of a TokenTree (the last one on `stack`).
        // `tree` contains the next `TokenTree` to be processed.
        match tree {
            // We are descending into a sequence. We first make sure that the matchers in the RHS
            // and the matches in `interp` have the same shape. Otherwise, either the caller or the
            // macro writer has made a mistake.
            seq @ mbe::TokenTree::Sequence(_, delimited) => {
                match lockstep_iter_size(seq, interp, &repeats) {
                    LockstepIterSize::Unconstrained => {
                        return Err(cx
                            .dcx()
                            .create_err(NoSyntaxVarsExprRepeat { span: seq.span() }));
                    }

                    LockstepIterSize::Contradiction(msg) => {
                        // FIXME: this really ought to be caught at macro definition time... It
                        // happens when two meta-variables are used in the same repetition in a
                        // sequence, but they come from different sequence matchers and repeat
                        // different amounts.
                        return Err(cx
                            .dcx()
                            .create_err(MetaVarsDifSeqMatchers { span: seq.span(), msg }));
                    }

                    LockstepIterSize::Constraint(len, _) => {
                        // We do this to avoid an extra clone above. We know that this is a
                        // sequence already.
                        let mbe::TokenTree::Sequence(sp, seq) = seq else { unreachable!() };

                        // Is the repetition empty?
                        if len == 0 {
                            if seq.kleene.op == KleeneOp::OneOrMore {
                                // FIXME: this really ought to be caught at macro definition
                                // time... It happens when the Kleene operator in the matcher and
                                // the body for the same meta-variable do not match.
                                return Err(cx
                                    .dcx()
                                    .create_err(MustRepeatOnce { span: sp.entire() }));
                            }
                        } else {
                            // 0 is the initial counter (we have done 0 repetitions so far). `len`
                            // is the total number of repetitions we should generate.
                            repeats.push((0, len));

                            // The first time we encounter the sequence we push it to the stack. It
                            // then gets reused (see the beginning of the loop) until we are done
                            // repeating.
                            stack.push(Frame::Sequence {
                                idx: 0,
                                sep: seq.separator.clone(),
                                tts: &delimited.tts,
                                kleene_op: seq.kleene.op,
                            });
                        }
                    }
                }
            }

            // Replace the meta-var with the matched token tree from the invocation.
            mbe::TokenTree::MetaVar(mut sp, mut original_ident) => {
                // Find the matched nonterminal from the macro invocation, and use it to replace
                // the meta-var.
                let ident = MacroRulesNormalizedIdent::new(original_ident);
                if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
                    let tt = match cur_matched {
                        MatchedSingle(ParseNtResult::Tt(tt)) => {
                            // `tt`s are emitted into the output stream directly as "raw tokens",
                            // without wrapping them into groups.
                            maybe_use_metavar_location(cx, &stack, sp, tt, &mut marker)
                        }
                        MatchedSingle(ParseNtResult::Nt(nt)) => {
                            // Other variables are emitted into the output stream as groups with
                            // `Delimiter::Invisible` to maintain parsing priorities.
                            // `Interpolated` is currently used for such groups in rustc parser.
                            marker.visit_span(&mut sp);
                            TokenTree::token_alone(token::Interpolated(nt.clone()), sp)
                        }
                        MatchedSeq(..) => {
                            // We were unable to descend far enough. This is an error.
                            return Err(cx.dcx().create_err(VarStillRepeating { span: sp, ident }));
                        }
                    };
                    result.push(tt)
                } else {
                    // If we aren't able to match the meta-var, we push it back into the result but
                    // with modified syntax context. (I believe this supports nested macros).
                    marker.visit_span(&mut sp);
                    marker.visit_ident(&mut original_ident);
                    result.push(TokenTree::token_joint_hidden(token::Dollar, sp));
                    result.push(TokenTree::Token(
                        Token::from_ast_ident(original_ident),
                        Spacing::Alone,
                    ));
                }
            }

            // Replace meta-variable expressions with the result of their expansion.
            mbe::TokenTree::MetaVarExpr(sp, expr) => {
                transcribe_metavar_expr(cx, expr, interp, &mut marker, &repeats, &mut result, sp)?;
            }

            // If we are entering a new delimiter, we push its contents to the `stack` to be
            // processed, and we push all of the currently produced results to the `result_stack`.
            // We will produce all of the results of the inside of the `Delimited` and then we will
            // jump back out of the Delimited, pop the result_stack and add the new results back to
            // the previous results (from outside the Delimited).
            mbe::TokenTree::Delimited(mut span, spacing, delimited) => {
                mut_visit::visit_delim_span(&mut span, &mut marker);
                stack.push(Frame::Delimited {
                    tts: &delimited.tts,
                    delim: delimited.delim,
                    idx: 0,
                    span,
                    spacing: *spacing,
                });
                result_stack.push(mem::take(&mut result));
            }

            // Nothing much to do here. Just push the token to the result, being careful to
            // preserve syntax context.
            mbe::TokenTree::Token(token) => {
                let mut token = token.clone();
                mut_visit::visit_token(&mut token, &mut marker);
                let tt = TokenTree::Token(token, Spacing::Alone);
                result.push(tt);
            }

            // There should be no meta-var declarations in the invocation of a macro.
            mbe::TokenTree::MetaVarDecl(..) => panic!("unexpected `TokenTree::MetaVarDecl`"),
        }
    }
}

/// Store the metavariable span for this original span into a side table.
/// FIXME: Try to put the metavariable span into `SpanData` instead of a side table (#118517).
/// An optimal encoding for inlined spans will need to be selected to minimize regressions.
/// The side table approach is relatively good, but not perfect due to collisions.
/// In particular, collisions happen when token is passed as an argument through several macro
/// calls, like in recursive macros.
/// The old heuristic below is used to improve spans in case of collisions, but diagnostics are
/// still degraded sometimes in those cases.
///
/// The old heuristic:
///
/// Usually metavariables `$var` produce interpolated tokens, which have an additional place for
/// keeping both the original span and the metavariable span. For `tt` metavariables that's not the
/// case however, and there's no place for keeping a second span. So we try to give the single
/// produced span a location that would be most useful in practice (the hygiene part of the span
/// must not be changed).
///
/// Different locations are useful for different purposes:
/// - The original location is useful when we need to report a diagnostic for the original token in
///   isolation, without combining it with any surrounding tokens. This case occurs, but it is not
///   very common in practice.
/// - The metavariable location is useful when we need to somehow combine the token span with spans
///   of its surrounding tokens. This is the most common way to use token spans.
///
/// So this function replaces the original location with the metavariable location in all cases
/// except these two:
/// - The metavariable is an element of undelimited sequence `$($tt)*`.
///   These are typically used for passing larger amounts of code, and tokens in that code usually
///   combine with each other and not with tokens outside of the sequence.
/// - The metavariable span comes from a different crate, then we prefer the more local span.
fn maybe_use_metavar_location(
    cx: &ExtCtxt<'_>,
    stack: &[Frame<'_>],
    mut metavar_span: Span,
    orig_tt: &TokenTree,
    marker: &mut Marker,
) -> TokenTree {
    let undelimited_seq = matches!(
        stack.last(),
        Some(Frame::Sequence {
            tts: [_],
            sep: None,
            kleene_op: KleeneOp::ZeroOrMore | KleeneOp::OneOrMore,
            ..
        })
    );
    if undelimited_seq {
        // Do not record metavar spans for tokens from undelimited sequences, for perf reasons.
        return orig_tt.clone();
    }

    let insert = |mspans: &mut FxHashMap<_, _>, s, ms| match mspans.try_insert(s, ms) {
        Ok(_) => true,
        Err(err) => *err.entry.get() == ms, // Tried to insert the same span, still success
    };
    marker.visit_span(&mut metavar_span);
    let no_collision = match orig_tt {
        TokenTree::Token(token, ..) => {
            with_metavar_spans(|mspans| insert(mspans, token.span, metavar_span))
        }
        TokenTree::Delimited(dspan, ..) => with_metavar_spans(|mspans| {
            insert(mspans, dspan.open, metavar_span)
                && insert(mspans, dspan.close, metavar_span)
                && insert(mspans, dspan.entire(), metavar_span)
        }),
    };
    if no_collision || cx.source_map().is_imported(metavar_span) {
        return orig_tt.clone();
    }

    // Setting metavar spans for the heuristic spans gives better opportunities for combining them
    // with neighboring spans even despite their different syntactic contexts.
    match orig_tt {
        TokenTree::Token(Token { kind, span }, spacing) => {
            let span = metavar_span.with_ctxt(span.ctxt());
            with_metavar_spans(|mspans| insert(mspans, span, metavar_span));
            TokenTree::Token(Token { kind: kind.clone(), span }, *spacing)
        }
        TokenTree::Delimited(dspan, dspacing, delimiter, tts) => {
            let open = metavar_span.with_ctxt(dspan.open.ctxt());
            let close = metavar_span.with_ctxt(dspan.close.ctxt());
            with_metavar_spans(|mspans| {
                insert(mspans, open, metavar_span) && insert(mspans, close, metavar_span)
            });
            let dspan = DelimSpan::from_pair(open, close);
            TokenTree::Delimited(dspan, *dspacing, *delimiter, tts.clone())
        }
    }
}

/// Lookup the meta-var named `ident` and return the matched token tree from the invocation using
/// the set of matches `interpolations`.
///
/// See the definition of `repeats` in the `transcribe` function. `repeats` is used to descend
/// into the right place in nested matchers. If we attempt to descend too far, the macro writer has
/// made a mistake, and we return `None`.
fn lookup_cur_matched<'a>(
    ident: MacroRulesNormalizedIdent,
    interpolations: &'a FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    repeats: &[(usize, usize)],
) -> Option<&'a NamedMatch> {
    interpolations.get(&ident).map(|mut matched| {
        for &(idx, _) in repeats {
            match matched {
                MatchedSingle(_) => break,
                MatchedSeq(ads) => matched = ads.get(idx).unwrap(),
            }
        }

        matched
    })
}

/// An accumulator over a TokenTree to be used with `fold`. During transcription, we need to make
/// sure that the size of each sequence and all of its nested sequences are the same as the sizes
/// of all the matched (nested) sequences in the macro invocation. If they don't match, somebody
/// has made a mistake (either the macro writer or caller).
#[derive(Clone)]
enum LockstepIterSize {
    /// No constraints on length of matcher. This is true for any TokenTree variants except a
    /// `MetaVar` with an actual `MatchedSeq` (as opposed to a `MatchedNonterminal`).
    Unconstrained,

    /// A `MetaVar` with an actual `MatchedSeq`. The length of the match and the name of the
    /// meta-var are returned.
    Constraint(usize, MacroRulesNormalizedIdent),

    /// Two `Constraint`s on the same sequence had different lengths. This is an error.
    Contradiction(String),
}

impl LockstepIterSize {
    /// Find incompatibilities in matcher/invocation sizes.
    /// - `Unconstrained` is compatible with everything.
    /// - `Contradiction` is incompatible with everything.
    /// - `Constraint(len)` is only compatible with other constraints of the same length.
    fn with(self, other: LockstepIterSize) -> LockstepIterSize {
        match self {
            LockstepIterSize::Unconstrained => other,
            LockstepIterSize::Contradiction(_) => self,
            LockstepIterSize::Constraint(l_len, l_id) => match other {
                LockstepIterSize::Unconstrained => self,
                LockstepIterSize::Contradiction(_) => other,
                LockstepIterSize::Constraint(r_len, _) if l_len == r_len => self,
                LockstepIterSize::Constraint(r_len, r_id) => {
                    let msg = format!(
                        "meta-variable `{}` repeats {} time{}, but `{}` repeats {} time{}",
                        l_id,
                        l_len,
                        pluralize!(l_len),
                        r_id,
                        r_len,
                        pluralize!(r_len),
                    );
                    LockstepIterSize::Contradiction(msg)
                }
            },
        }
    }
}

/// Given a `tree`, make sure that all sequences have the same length as the matches for the
/// appropriate meta-vars in `interpolations`.
///
/// Note that if `repeats` does not match the exact correct depth of a meta-var,
/// `lookup_cur_matched` will return `None`, which is why this still works even in the presence of
/// multiple nested matcher sequences.
///
/// Example: `$($($x $y)+*);+` -- we need to make sure that `x` and `y` repeat the same amount as
/// each other at the given depth when the macro was invoked. If they don't it might mean they were
/// declared at depths which weren't equal or there was a compiler bug. For example, if we have 3 repetitions of
/// the outer sequence and 4 repetitions of the inner sequence for `x`, we should have the same for
/// `y`; otherwise, we can't transcribe them both at the given depth.
fn lockstep_iter_size(
    tree: &mbe::TokenTree,
    interpolations: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    repeats: &[(usize, usize)],
) -> LockstepIterSize {
    use mbe::TokenTree;
    match tree {
        TokenTree::Delimited(.., delimited) => {
            delimited.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
                size.with(lockstep_iter_size(tt, interpolations, repeats))
            })
        }
        TokenTree::Sequence(_, seq) => {
            seq.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
                size.with(lockstep_iter_size(tt, interpolations, repeats))
            })
        }
        TokenTree::MetaVar(_, name) | TokenTree::MetaVarDecl(_, name, _) => {
            let name = MacroRulesNormalizedIdent::new(*name);
            match lookup_cur_matched(name, interpolations, repeats) {
                Some(matched) => match matched {
                    MatchedSingle(_) => LockstepIterSize::Unconstrained,
                    MatchedSeq(ads) => LockstepIterSize::Constraint(ads.len(), name),
                },
                _ => LockstepIterSize::Unconstrained,
            }
        }
        TokenTree::MetaVarExpr(_, expr) => {
            let default_rslt = LockstepIterSize::Unconstrained;
            let Some(ident) = expr.ident() else {
                return default_rslt;
            };
            let name = MacroRulesNormalizedIdent::new(ident);
            match lookup_cur_matched(name, interpolations, repeats) {
                Some(MatchedSeq(ads)) => {
                    default_rslt.with(LockstepIterSize::Constraint(ads.len(), name))
                }
                _ => default_rslt,
            }
        }
        TokenTree::Token(..) => LockstepIterSize::Unconstrained,
    }
}

/// Used solely by the `count` meta-variable expression, counts the outer-most repetitions at a
/// given optional nested depth.
///
/// For example, a macro parameter of `$( { $( $foo:ident ),* } )*` called with `{ a, b } { c }`:
///
/// * `[ $( ${count(foo)} ),* ]` will return [2, 1] with a, b = 2 and c = 1
/// * `[ $( ${count(foo, 0)} ),* ]` will be the same as `[ $( ${count(foo)} ),* ]`
/// * `[ $( ${count(foo, 1)} ),* ]` will return an error because `${count(foo, 1)}` is
///   declared inside a single repetition and the index `1` implies two nested repetitions.
fn count_repetitions<'a>(
    cx: &ExtCtxt<'a>,
    depth_user: usize,
    mut matched: &NamedMatch,
    repeats: &[(usize, usize)],
    sp: &DelimSpan,
) -> PResult<'a, usize> {
    // Recursively count the number of matches in `matched` at given depth
    // (or at the top-level of `matched` if no depth is given).
    fn count<'a>(depth_curr: usize, depth_max: usize, matched: &NamedMatch) -> PResult<'a, usize> {
        match matched {
            MatchedSingle(_) => Ok(1),
            MatchedSeq(named_matches) => {
                if depth_curr == depth_max {
                    Ok(named_matches.len())
                } else {
                    named_matches.iter().map(|elem| count(depth_curr + 1, depth_max, elem)).sum()
                }
            }
        }
    }

    /// Maximum depth
    fn depth(counter: usize, matched: &NamedMatch) -> usize {
        match matched {
            MatchedSingle(_) => counter,
            MatchedSeq(named_matches) => {
                let rslt = counter + 1;
                if let Some(elem) = named_matches.first() { depth(rslt, elem) } else { rslt }
            }
        }
    }

    let depth_max = depth(0, matched)
        .checked_sub(1)
        .and_then(|el| el.checked_sub(repeats.len()))
        .unwrap_or_default();
    if depth_user > depth_max {
        return Err(out_of_bounds_err(cx, depth_max + 1, sp.entire(), "count"));
    }

    // `repeats` records all of the nested levels at which we are currently
    // matching meta-variables. The meta-var-expr `count($x)` only counts
    // matches that occur in this "subtree" of the `NamedMatch` where we
    // are currently transcribing, so we need to descend to that subtree
    // before we start counting. `matched` contains the various levels of the
    // tree as we descend, and its final value is the subtree we are currently at.
    for &(idx, _) in repeats {
        if let MatchedSeq(ads) = matched {
            matched = &ads[idx];
        }
    }

    if let MatchedSingle(_) = matched {
        return Err(cx.dcx().create_err(CountRepetitionMisplaced { span: sp.entire() }));
    }

    count(depth_user, depth_max, matched)
}

/// Returns a `NamedMatch` item declared on the LHS given an arbitrary [Ident]
fn matched_from_ident<'ctx, 'interp, 'rslt>(
    cx: &ExtCtxt<'ctx>,
    ident: Ident,
    interp: &'interp FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
) -> PResult<'ctx, &'rslt NamedMatch>
where
    'interp: 'rslt,
{
    let span = ident.span;
    let key = MacroRulesNormalizedIdent::new(ident);
    interp.get(&key).ok_or_else(|| cx.dcx().create_err(MetaVarExprUnrecognizedVar { span, key }))
}

/// Used by meta-variable expressions when an user input is out of the actual declared bounds. For
/// example, index(999999) in an repetition of only three elements.
fn out_of_bounds_err<'a>(cx: &ExtCtxt<'a>, max: usize, span: Span, ty: &str) -> Diag<'a> {
    let msg = if max == 0 {
        format!(
            "meta-variable expression `{ty}` with depth parameter \
             must be called inside of a macro repetition"
        )
    } else {
        format!(
            "depth parameter of meta-variable expression `{ty}` \
             must be less than {max}"
        )
    };
    cx.dcx().struct_span_err(span, msg)
}

fn transcribe_metavar_expr<'a>(
    cx: &ExtCtxt<'a>,
    expr: &MetaVarExpr,
    interp: &FxHashMap<MacroRulesNormalizedIdent, NamedMatch>,
    marker: &mut Marker,
    repeats: &[(usize, usize)],
    result: &mut Vec<TokenTree>,
    sp: &DelimSpan,
) -> PResult<'a, ()> {
    let mut visited_span = || {
        let mut span = sp.entire();
        marker.visit_span(&mut span);
        span
    };
    match *expr {
        MetaVarExpr::Count(original_ident, depth) => {
            let matched = matched_from_ident(cx, original_ident, interp)?;
            let count = count_repetitions(cx, depth, matched, repeats, sp)?;
            let tt = TokenTree::token_alone(
                TokenKind::lit(token::Integer, sym::integer(count), None),
                visited_span(),
            );
            result.push(tt);
        }
        MetaVarExpr::Ignore(original_ident) => {
            // Used to ensure that `original_ident` is present in the LHS
            let _ = matched_from_ident(cx, original_ident, interp)?;
        }
        MetaVarExpr::Index(depth) => match repeats.iter().nth_back(depth) {
            Some((index, _)) => {
                result.push(TokenTree::token_alone(
                    TokenKind::lit(token::Integer, sym::integer(*index), None),
                    visited_span(),
                ));
            }
            None => return Err(out_of_bounds_err(cx, repeats.len(), sp.entire(), "index")),
        },
        MetaVarExpr::Length(depth) => match repeats.iter().nth_back(depth) {
            Some((_, length)) => {
                result.push(TokenTree::token_alone(
                    TokenKind::lit(token::Integer, sym::integer(*length), None),
                    visited_span(),
                ));
            }
            None => return Err(out_of_bounds_err(cx, repeats.len(), sp.entire(), "length")),
        },
    }
    Ok(())
}