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use crate::ast::Ident;
use crate::ext::base::ExtCtxt;
use crate::ext::expand::Marker;
use crate::ext::tt::macro_parser::{MatchedNonterminal, MatchedSeq, NamedMatch};
use crate::ext::tt::quoted;
use crate::mut_visit::noop_visit_tt;
use crate::parse::token::{self, NtTT, Token};
use crate::tokenstream::{DelimSpan, TokenStream, TokenTree, TreeAndJoint};
use smallvec::{smallvec, SmallVec};
use rustc_data_structures::fx::FxHashMap;
use rustc_data_structures::sync::Lrc;
use std::mem;
use std::rc::Rc;
/// An iterator over the token trees in a delimited token tree (`{ ... }`) or a sequence (`$(...)`).
enum Frame {
Delimited { forest: Lrc<quoted::Delimited>, idx: usize, span: DelimSpan },
Sequence { forest: Lrc<quoted::SequenceRepetition>, idx: usize, sep: Option<Token> },
impl Frame {
/// Construct a new frame around the delimited set of tokens.
fn new(tts: Vec<quoted::TokenTree>) -> Frame {
let forest = Lrc::new(quoted::Delimited { delim: token::NoDelim, tts });
Frame::Delimited { forest, idx: 0, span: DelimSpan::dummy() }
impl Iterator for Frame {
type Item = quoted::TokenTree;
fn next(&mut self) -> Option<quoted::TokenTree> {
match *self {
Frame::Delimited { ref forest, ref mut idx, .. } => {
*idx += 1;
forest.tts.get(*idx - 1).cloned()
Frame::Sequence { ref forest, ref mut idx, .. } => {
*idx += 1;
forest.tts.get(*idx - 1).cloned()
/// This can do Macro-By-Example transcription.
/// - `interp` is a map of meta-variables to the tokens (non-terminals) they matched in the
/// invocation. We are assuming we already know there is a match.
/// - `src` is the RHS of the MBE, that is, the "example" we are filling in.
/// For example,
/// ```rust
/// macro_rules! foo {
/// ($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 fn transcribe(
cx: &ExtCtxt<'_>,
interp: &FxHashMap<Ident, Rc<NamedMatch>>,
src: Vec<quoted::TokenTree>,
) -> TokenStream {
// Nothing for us to transcribe...
if src.is_empty() {
return TokenStream::empty();
// 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.
let mut stack: SmallVec<[Frame; 1]> = smallvec![Frame::new(src)];
// 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<TreeAndJoint> = Vec::new();
let mut result_stack = Vec::new();
loop {
// Look at the last frame on the stack.
let tree = if let Some(tree) = stack.last_mut().unwrap().next() {
// If it still has a TokenTree we have not looked at yet, use that tree.
// The else-case never produces a value for `tree` (it `continue`s or `return`s).
else {
// 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 {
// 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 { .. } => {
// 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 { forest, span, .. } => {
if result_stack.is_empty() {
// No results left to compute! We are back at the top-level.
return TokenStream::new(result);
// Step back into the parent Delimited.
let tree =
TokenTree::Delimited(span, forest.delim, TokenStream::new(result).into());
result = result_stack.pop().unwrap();
// 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 @ quoted::TokenTree::Sequence(..) => {
match lockstep_iter_size(&seq, interp, &repeats) {
LockstepIterSize::Unconstrained => {
seq.span(), /* blame macro writer */
"attempted to repeat an expression containing no syntax variables \
matched as repeating at this depth",
LockstepIterSize::Contradiction(ref 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.
cx.span_fatal(seq.span(), &msg[..]);
LockstepIterSize::Constraint(len, _) => {
// We do this to avoid an extra clone above. We know that this is a
// sequence already.
let (sp, seq) = if let quoted::TokenTree::Sequence(sp, seq) = seq {
(sp, seq)
} else {
// Is the repetition empty?
if len == 0 {
if seq.kleene.op == quoted::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.
cx.span_fatal(sp.entire(), "this must repeat at least once");
} else {
// 0 is the initial counter (we have done 0 repretitions so far). `len`
// is the total number of reptitions 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(),
forest: seq,
// Replace the meta-var with the matched token tree from the invocation.
quoted::TokenTree::MetaVar(mut sp, ident) => {
// Find the matched nonterminal from the macro invocation, and use it to replace
// the meta-var.
if let Some(cur_matched) = lookup_cur_matched(ident, interp, &repeats) {
if let MatchedNonterminal(ref nt) = *cur_matched {
// FIXME #2887: why do we apply a mark when matching a token tree meta-var
// (e.g. `$x:tt`), but not when we are matching any other type of token
// tree?
if let NtTT(ref tt) = **nt {
} else {
sp = sp.apply_mark(;
let token = TokenTree::token(token::Interpolated(nt.clone()), sp);
} else {
// We were unable to descend far enough. This is an error.
sp, /* blame the macro writer */
&format!("variable '{}' is still repeating at this depth", ident),
} 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).
let ident =
Ident::new(, ident.span.apply_mark(;
sp = sp.apply_mark(;
result.push(TokenTree::token(token::Dollar, sp).into());
// 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).
quoted::TokenTree::Delimited(mut span, delimited) => {
span = span.apply_mark(;
stack.push(Frame::Delimited { forest: delimited, idx: 0, span });
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.
quoted::TokenTree::Token(token) => {
let mut marker = Marker(;
let mut tt = TokenTree::Token(token);
noop_visit_tt(&mut tt, &mut marker);
// There should be no meta-var declarations in the invocation of a macro.
quoted::TokenTree::MetaVarDecl(..) => panic!("unexpected `TokenTree::MetaVarDecl"),
/// 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(
ident: Ident,
interpolations: &FxHashMap<Ident, Rc<NamedMatch>>,
repeats: &[(usize, usize)],
) -> Option<Rc<NamedMatch>> {
interpolations.get(&ident).map(|matched| {
let mut matched = matched.clone();
for &(idx, _) in repeats {
let m = matched.clone();
match *m {
MatchedNonterminal(_) => break,
MatchedSeq(ref ads, _) => matched = Rc::new(ads[idx].clone()),
/// 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).
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`).
/// A `MetaVar` with an actual `MatchedSeq`. The length of the match and the name of the
/// meta-var are returned.
Constraint(usize, Ident),
/// Two `Constraint`s on the same sequence had different lengths. This is an error.
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, ref 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 {} times, but `{}` repeats {} times",
l_id, l_len, r_id, r_len
/// 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 presnece of
/// multiple nested matcher sequences.
fn lockstep_iter_size(
tree: &quoted::TokenTree,
interpolations: &FxHashMap<Ident, Rc<NamedMatch>>,
repeats: &[(usize, usize)],
) -> LockstepIterSize {
use quoted::TokenTree;
match *tree {
TokenTree::Delimited(_, ref delimed) => {
delimed.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
size.with(lockstep_iter_size(tt, interpolations, repeats))
TokenTree::Sequence(_, ref seq) => {
seq.tts.iter().fold(LockstepIterSize::Unconstrained, |size, tt| {
size.with(lockstep_iter_size(tt, interpolations, repeats))
TokenTree::MetaVar(_, name) | TokenTree::MetaVarDecl(_, name, _) => {
match lookup_cur_matched(name, interpolations, repeats) {
Some(matched) => match *matched {
MatchedNonterminal(_) => LockstepIterSize::Unconstrained,
MatchedSeq(ref ads, _) => LockstepIterSize::Constraint(ads.len(), name),
_ => LockstepIterSize::Unconstrained,
TokenTree::Token(..) => LockstepIterSize::Unconstrained,
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