Merge #7513

7513: NFA parser for mbe matcher r=matklad a=edwin0cheng

Almost straight porting from rustc one, but a little bit slow :(

```
rust-analyzer analysis-stats -q . 
```

From:
```log
Database loaded:     636.11ms, 277minstr
  crates: 36, mods: 594, decls: 11527, fns: 9017
Item Collection:     10.99s, 60ginstr
  exprs: 249618, ??ty: 2699 (1%), ?ty: 2101 (0%), !ty: 932
Inference:           28.94s, 123ginstr
Total:               39.93s, 184ginstr
```

To:
```log
Database loaded:     630.90ms, 277minstr
  crates: 36, mods: 594, decls: 11528, fns: 9018
Item Collection:     13.70s, 77ginstr
  exprs: 249482, ??ty: 2699 (1%), ?ty: 2101 (0%), !ty: 932
Inference:           30.27s, 133ginstr
Total:               43.97s, 211ginstr
```

Fixes #4777

Co-authored-by: Edwin Cheng <[email protected]>

8 files changed, 579 insertions, 163 deletions

diff --git a/crates/mbe/Cargo.toml b/crates/mbe/Cargo.toml
index 8b5cca22c..bb2656a80 100644
--- a/crates/mbe/Cargo.toml
+++ b/crates/mbe/Cargo.toml
@@ -18,6 +18,7 @@ syntax = { path = "../syntax", version = "0.0.0" }
 parser = { path = "../parser", version = "0.0.0" }
 tt = { path = "../tt", version = "0.0.0" }
 test_utils = { path = "../test_utils", version = "0.0.0" }
+stdx = { path = "../stdx", version = "0.0.0" }
 
 [dev-dependencies]
 profile = { path = "../profile" }
diff --git a/crates/mbe/src/benchmark.rs b/crates/mbe/src/benchmark.rs
index 6d81be880..503ad1355 100644
--- a/crates/mbe/src/benchmark.rs
+++ b/crates/mbe/src/benchmark.rs
@@ -40,18 +40,12 @@ fn benchmark_expand_macro_rules() {
             .into_iter()
             .map(|(id, tt)| {
                 let res = rules[&id].expand(&tt);
-                if res.err.is_some() {
-                    // FIXME:
-                    // Currently `invocation_fixtures` will generate some correct invocations but
-                    // cannot be expanded by mbe. We ignore errors here.
-                    // See: https://github.com/rust-analyzer/rust-analyzer/issues/4777
-                    eprintln!("err from {} {:?}", id, res.err);
-                }
+                assert!(res.err.is_none());
                 res.value.token_trees.len()
             })
             .sum()
     };
-    assert_eq!(hash, 66995);
+    assert_eq!(hash, 69413);
 }
 
 fn macro_rules_fixtures() -> FxHashMap<String, MacroRules> {
@@ -77,7 +71,7 @@ fn macro_rules_fixtures_tt() -> FxHashMap<String, tt::Subtree> {
         .collect()
 }
 
-// Generate random invocation fixtures from rules
+/// Generate random invocation fixtures from rules
 fn invocation_fixtures(rules: &FxHashMap<String, MacroRules>) -> Vec<(String, tt::Subtree)> {
     let mut seed = 123456789;
     let mut res = Vec::new();
@@ -86,11 +80,31 @@ fn invocation_fixtures(rules: &FxHashMap<String, MacroRules>) -> Vec<(String, tt
         for rule in &it.rules {
             // Generate twice
             for _ in 0..2 {
-                let mut subtree = tt::Subtree::default();
-                for op in rule.lhs.iter() {
-                    collect_from_op(op, &mut subtree, &mut seed);
+                // The input are generated by filling the `Op` randomly.
+                // However, there are some cases generated are ambiguous for expanding, for example:
+                // ```rust
+                // macro_rules! m {
+                //    ($($t:ident),* as $ty:ident) => {}
+                // }
+                // m!(as u32);  // error: local ambiguity: multiple parsing options: built-in NTs ident ('t') or 1 other option.
+                // ```
+                //
+                // So we just skip any error cases and try again
+                let mut try_cnt = 0;
+                loop {
+                    let mut subtree = tt::Subtree::default();
+                    for op in rule.lhs.iter() {
+                        collect_from_op(op, &mut subtree, &mut seed);
+                    }
+                    if it.expand(&subtree).err.is_none() {
+                        res.push((name.clone(), subtree));
+                        break;
+                    }
+                    try_cnt += 1;
+                    if try_cnt > 100 {
+                        panic!("invocaton fixture {} cannot be generated.\n", name);
+                    }
                 }
-                res.push((name.clone(), subtree));
             }
         }
     }
diff --git a/crates/mbe/src/expander.rs b/crates/mbe/src/expander.rs
index e7e14b3cc..2efff8f52 100644
--- a/crates/mbe/src/expander.rs
+++ b/crates/mbe/src/expander.rs
@@ -5,7 +5,7 @@
 mod matcher;
 mod transcriber;
 
-use rustc_hash::FxHashMap;
+use smallvec::SmallVec;
 use syntax::SmolStr;
 
 use crate::{ExpandError, ExpandResult};
@@ -28,10 +28,10 @@ pub(crate) fn expand_rules(
                 return ExpandResult::ok(value);
             }
         }
-        // Use the rule if we matched more tokens, or had fewer errors
+        // Use the rule if we matched more tokens, or bound variables count
         if let Some((prev_match, _)) = &match_ {
-            if (new_match.unmatched_tts, new_match.err_count)
-                < (prev_match.unmatched_tts, prev_match.err_count)
+            if (new_match.unmatched_tts, -(new_match.bound_count as i32))
+                < (prev_match.unmatched_tts, -(prev_match.bound_count as i32))
             {
                 match_ = Some((new_match, rule));
             }
@@ -94,19 +94,19 @@ pub(crate) fn expand_rules(
 /// In other words, `Bindings` is a *multi* mapping from `SmolStr` to
 /// `tt::TokenTree`, where the index to select a particular `TokenTree` among
 /// many is not a plain `usize`, but an `&[usize]`.
-#[derive(Debug, Default)]
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
 struct Bindings {
-    inner: FxHashMap<SmolStr, Binding>,
+    inner: SmallVec<[(SmolStr, Binding); 4]>,
 }
 
-#[derive(Debug)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 enum Binding {
     Fragment(Fragment),
     Nested(Vec<Binding>),
     Empty,
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 enum Fragment {
     /// token fragments are just copy-pasted into the output
     Tokens(tt::TokenTree),
diff --git a/crates/mbe/src/expander/matcher.rs b/crates/mbe/src/expander/matcher.rs
index e3bd4c09a..9d3d28055 100644
--- a/crates/mbe/src/expander/matcher.rs
+++ b/crates/mbe/src/expander/matcher.rs
@@ -1,14 +1,74 @@
-//! FIXME: write short doc here
+//! An NFA-based parser, which is porting from rustc mbe parsing code
+//!
+//! See https://github.com/rust-lang/rust/blob/70b18bc2cbac4712020019f5bf57c00905373205/compiler/rustc_expand/src/mbe/macro_parser.rs
+//! Here is a quick intro to how the parser works, copied from rustc:
+//!
+//! A 'position' is a dot in the middle of a matcher, usually represented as a
+//! dot. For example `· a $( a )* a b` is a position, as is `a $( · a )* a b`.
+//!
+//! The parser walks through the input a character at a time, maintaining a list
+//! of threads consistent with the current position in the input string: `cur_items`.
+//!
+//! As it processes them, it fills up `eof_items` with threads that would be valid if
+//! the macro invocation is now over, `bb_items` with threads that are waiting on
+//! a Rust non-terminal like `$e:expr`, and `next_items` with threads that are waiting
+//! on a particular token. Most of the logic concerns moving the · through the
+//! repetitions indicated by Kleene stars. The rules for moving the · without
+//! consuming any input are called epsilon transitions. It only advances or calls
+//! out to the real Rust parser when no `cur_items` threads remain.
+//!
+//! Example:
+//!
+//! ```text, ignore
+//! Start parsing a a a a b against [· a $( a )* a b].
+//!
+//! Remaining input: a a a a b
+//! next: [· a $( a )* a b]
+//!
+//! - - - Advance over an a. - - -
+//!
+//! Remaining input: a a a b
+//! cur: [a · $( a )* a b]
+//! Descend/Skip (first item).
+//! next: [a $( · a )* a b]  [a $( a )* · a b].
+//!
+//! - - - Advance over an a. - - -
+//!
+//! Remaining input: a a b
+//! cur: [a $( a · )* a b]  [a $( a )* a · b]
+//! Follow epsilon transition: Finish/Repeat (first item)
+//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
+//!
+//! - - - Advance over an a. - - - (this looks exactly like the last step)
+//!
+//! Remaining input: a b
+//! cur: [a $( a · )* a b]  [a $( a )* a · b]
+//! Follow epsilon transition: Finish/Repeat (first item)
+//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
+//!
+//! - - - Advance over an a. - - - (this looks exactly like the last step)
+//!
+//! Remaining input: b
+//! cur: [a $( a · )* a b]  [a $( a )* a · b]
+//! Follow epsilon transition: Finish/Repeat (first item)
+//! next: [a $( a )* · a b]  [a $( · a )* a b]  [a $( a )* a · b]
+//!
+//! - - - Advance over a b. - - -
+//!
+//! Remaining input: ''
+//! eof: [a $( a )* a b ·]
+//! ```
 
 use crate::{
     expander::{Binding, Bindings, Fragment},
-    parser::{Op, RepeatKind, Separator},
+    parser::{Op, OpDelimited, OpDelimitedIter, RepeatKind, Separator},
     tt_iter::TtIter,
     ExpandError, MetaTemplate,
 };
 
 use super::ExpandResult;
 use parser::FragmentKind::*;
+use smallvec::{smallvec, SmallVec};
 use syntax::SmolStr;
 
 impl Bindings {
@@ -16,19 +76,19 @@ impl Bindings {
         // FIXME: Do we have a better way to represent an empty token ?
         // Insert an empty subtree for empty token
         let tt = tt::Subtree::default().into();
-        self.inner.insert(name.clone(), Binding::Fragment(Fragment::Tokens(tt)));
+        self.inner.push((name.clone(), Binding::Fragment(Fragment::Tokens(tt))));
     }
 
     fn push_empty(&mut self, name: &SmolStr) {
-        self.inner.insert(name.clone(), Binding::Empty);
+        self.inner.push((name.clone(), Binding::Empty));
     }
 
     fn push_nested(&mut self, idx: usize, nested: Bindings) -> Result<(), ExpandError> {
         for (key, value) in nested.inner {
-            if !self.inner.contains_key(&key) {
-                self.inner.insert(key.clone(), Binding::Nested(Vec::new()));
+            if self.get_mut(&key).is_none() {
+                self.inner.push((key.clone(), Binding::Nested(Vec::new())));
             }
-            match self.inner.get_mut(&key) {
+            match self.get_mut(&key) {
                 Some(Binding::Nested(it)) => {
                     // insert empty nested bindings before this one
                     while it.len() < idx {
@@ -46,6 +106,14 @@ impl Bindings {
         }
         Ok(())
     }
+
+    fn get_mut(&mut self, name: &str) -> Option<&mut Binding> {
+        self.inner.iter_mut().find_map(|(n, b)| if n == name { Some(b) } else { None })
+    }
+
+    fn bindings(&self) -> impl Iterator<Item = &Binding> {
+        self.inner.iter().map(|(_, b)| b)
+    }
 }
 
 macro_rules! err {
@@ -57,7 +125,7 @@ macro_rules! err {
     };
 }
 
-#[derive(Debug, Default)]
+#[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub(super) struct Match {
     pub(super) bindings: Bindings,
     /// We currently just keep the first error and count the rest to compare matches.
@@ -65,6 +133,8 @@ pub(super) struct Match {
     pub(super) err_count: usize,
     /// How many top-level token trees were left to match.
     pub(super) unmatched_tts: usize,
+    /// The number of bound variables
+    pub(super) bound_count: usize,
 }
 
 impl Match {
@@ -76,72 +146,373 @@ impl Match {
 }
 
 /// Matching errors are added to the `Match`.
-pub(super) fn match_(pattern: &MetaTemplate, src: &tt::Subtree) -> Match {
-    let mut res = Match::default();
-    let mut src = TtIter::new(src);
+pub(super) fn match_(pattern: &MetaTemplate, input: &tt::Subtree) -> Match {
+    let mut res = match_loop(pattern, &input);
+    res.bound_count = count(res.bindings.bindings());
+    return res;
+
+    fn count<'a>(bindings: impl Iterator<Item = &'a Binding>) -> usize {
+        bindings
+            .map(|it| match it {
+                Binding::Fragment(_) => 1,
+                Binding::Empty => 1,
+                Binding::Nested(it) => count(it.iter()),
+            })
+            .sum()
+    }
+}
 
-    match_tokens(&mut res, pattern, &mut src);
+#[derive(Debug, Clone)]
+struct MatchState<'t> {
+    /// The position of the "dot" in this matcher
+    dot: OpDelimitedIter<'t>,
 
-    if src.len() > 0 {
-        res.unmatched_tts += src.len();
-        res.add_err(err!("leftover tokens"));
-    }
+    /// Token subtree stack
+    /// When matching against matchers with nested delimited submatchers (e.g., `pat ( pat ( .. )
+    /// pat ) pat`), we need to keep track of the matchers we are descending into. This stack does
+    /// that where the bottom of the stack is the outermost matcher.
+    stack: SmallVec<[OpDelimitedIter<'t>; 4]>,
+
+    /// The "parent" matcher position if we are in a repetition. That is, the matcher position just
+    /// before we enter the repetition.
+    up: Option<Box<MatchState<'t>>>,
+
+    /// The separator if we are in a repetition.
+    sep: Option<Separator>,
+
+    /// The KleeneOp of this sequence if we are in a repetition.
+    sep_kind: Option<RepeatKind>,
 
-    res
+    /// Number of tokens of seperator parsed
+    sep_parsed: Option<usize>,
+
+    /// Matched meta variables bindings
+    bindings: SmallVec<[Bindings; 4]>,
+
+    /// Cached result of meta variable parsing
+    meta_result: Option<(TtIter<'t>, ExpandResult<Option<Fragment>>)>,
+
+    /// Is error occuried in this state, will `poised` to "parent"
+    is_error: bool,
 }
 
-fn match_tokens(res: &mut Match, pattern: &MetaTemplate, src: &mut TtIter) {
-    for op in pattern.iter() {
-        match op {
-            Op::Leaf(lhs) => {
-                if let Err(err) = match_leaf(lhs, src) {
-                    res.add_err(err);
-                    continue;
+/// Process the matcher positions of `cur_items` until it is empty. In the process, this will
+/// produce more items in `next_items`, `eof_items`, and `bb_items`.
+///
+/// For more info about the how this happens, see the module-level doc comments and the inline
+/// comments of this function.
+///
+/// # Parameters
+///
+/// - `src`: the current token of the parser.
+/// - `stack`: the "parent" frames of the token tree
+/// - `res`: the match result to store errors
+/// - `cur_items`: the set of current items to be processed. This should be empty by the end of a
+///   successful execution of this function.
+/// - `next_items`: the set of newly generated items. These are used to replenish `cur_items` in
+///   the function `parse`.
+/// - `eof_items`: the set of items that would be valid if this was the EOF.
+/// - `bb_items`: the set of items that are waiting for the black-box parser.
+/// - `error_items`: the set of items in errors, used for error-resilient parsing
+fn match_loop_inner<'t>(
+    src: TtIter<'t>,
+    stack: &[TtIter<'t>],
+    res: &mut Match,
+    cur_items: &mut SmallVec<[MatchState<'t>; 1]>,
+    bb_items: &mut SmallVec<[MatchState<'t>; 1]>,
+    next_items: &mut Vec<MatchState<'t>>,
+    eof_items: &mut SmallVec<[MatchState<'t>; 1]>,
+    error_items: &mut SmallVec<[MatchState<'t>; 1]>,
+) {
+    macro_rules! try_push {
+        ($items: expr, $it:expr) => {
+            if $it.is_error {
+                error_items.push($it);
+            } else {
+                $items.push($it);
+            }
+        };
+    }
+
+    while let Some(mut item) = cur_items.pop() {
+        while item.dot.is_eof() {
+            match item.stack.pop() {
+                Some(frame) => {
+                    item.dot = frame;
+                    item.dot.next();
                 }
+                None => break,
             }
-            Op::Subtree { tokens, delimiter: delim } => {
-                let rhs = match src.expect_subtree() {
-                    Ok(s) => s,
-                    Err(()) => {
-                        res.add_err(err!("expected subtree"));
-                        continue;
+        }
+        let op = match item.dot.peek() {
+            None => {
+                // We are at or past the end of the matcher of `item`.
+                if item.up.is_some() {
+                    if item.sep_parsed.is_none() {
+                        // Get the `up` matcher
+                        let mut new_pos = *item.up.clone().unwrap();
+                        // Add matches from this repetition to the `matches` of `up`
+                        if let Some(bindings) = new_pos.bindings.last_mut() {
+                            for (i, b) in item.bindings.iter_mut().enumerate() {
+                                bindings.push_nested(i, b.clone()).unwrap();
+                            }
+                        }
+                        // Move the "dot" past the repetition in `up`
+                        new_pos.dot.next();
+                        new_pos.is_error = new_pos.is_error || item.is_error;
+                        cur_items.push(new_pos);
+                    }
+
+                    // Check if we need a separator.
+                    // We check the separator one by one
+                    let sep_idx = *item.sep_parsed.as_ref().unwrap_or(&0);
+                    let sep_len = item.sep.as_ref().map_or(0, Separator::tt_count);
+                    if item.sep.is_some() && sep_idx != sep_len {
+                        let sep = item.sep.as_ref().unwrap();
+                        if src.clone().expect_separator(&sep, sep_idx) {
+                            item.dot.next();
+                            item.sep_parsed = Some(sep_idx + 1);
+                            try_push!(next_items, item);
+                        }
+                    }
+                    // We don't need a separator. Move the "dot" back to the beginning of the matcher
+                    // and try to match again UNLESS we are only allowed to have _one_ repetition.
+                    else if item.sep_kind != Some(RepeatKind::ZeroOrOne) {
+                        item.dot = item.dot.reset();
+                        item.sep_parsed = None;
+                        item.bindings.push(Bindings::default());
+                        cur_items.push(item);
+                    }
+                } else {
+                    // If we are not in a repetition, then being at the end of a matcher means that we have
+                    // reached the potential end of the input.
+                    try_push!(eof_items, item);
+                }
+                continue;
+            }
+            Some(it) => it,
+        };
+
+        // We are in the middle of a matcher.
+        match op {
+            OpDelimited::Op(Op::Repeat { tokens, kind, separator }) => {
+                if matches!(kind, RepeatKind::ZeroOrMore | RepeatKind::ZeroOrOne) {
+                    let mut new_item = item.clone();
+                    new_item.dot.next();
+                    let mut vars = Vec::new();
+                    let bindings = new_item.bindings.last_mut().unwrap();
+                    collect_vars(&mut vars, tokens);
+                    for var in vars {
+                        bindings.push_empty(&var);
                     }
-                };
-                if delim.map(|it| it.kind) != rhs.delimiter_kind() {
-                    res.add_err(err!("mismatched delimiter"));
-                    continue;
+                    cur_items.push(new_item);
                 }
-                let mut src = TtIter::new(rhs);
-                match_tokens(res, tokens, &mut src);
-                if src.len() > 0 {
-                    res.add_err(err!("leftover tokens"));
+                cur_items.push(MatchState {
+                    dot: tokens.iter_delimited(None),
+                    stack: Default::default(),
+                    up: Some(Box::new(item)),
+                    sep: separator.clone(),
+                    sep_kind: Some(*kind),
+                    sep_parsed: None,
+                    bindings: smallvec![Bindings::default()],
+                    meta_result: None,
+                    is_error: false,
+                })
+            }
+            OpDelimited::Op(Op::Subtree { tokens, delimiter }) => {
+                if let Ok(subtree) = src.clone().expect_subtree() {
+                    if subtree.delimiter_kind() == delimiter.map(|it| it.kind) {
+                        item.stack.push(item.dot);
+                        item.dot = tokens.iter_delimited(delimiter.as_ref());
+                        cur_items.push(item);
+                    }
                 }
             }
-            Op::Var { name, kind, .. } => {
-                let kind = match kind {
-                    Some(k) => k,
-                    None => {
-                        res.add_err(ExpandError::UnexpectedToken);
-                        continue;
+            OpDelimited::Op(Op::Var { kind, name, .. }) => {
+                if let Some(kind) = kind {
+                    let mut fork = src.clone();
+                    let match_res = match_meta_var(kind.as_str(), &mut fork);
+                    match match_res.err {
+                        None => {
+                            // Some meta variables are optional (e.g. vis)
+                            if match_res.value.is_some() {
+                                item.meta_result = Some((fork, match_res));
+                                try_push!(bb_items, item);
+                            } else {
+                                item.bindings.last_mut().unwrap().push_optional(name);
+                                item.dot.next();
+                                cur_items.push(item);
+                            }
+                        }
+                        Some(err) => {
+                            res.add_err(err);
+                            match match_res.value {
+                                Some(fragment) => {
+                                    item.bindings
+                                        .last_mut()
+                                        .unwrap()
+                                        .inner
+                                        .push((name.clone(), Binding::Fragment(fragment)));
+                                }
+                                _ => {}
+                            }
+                            item.is_error = true;
+                            error_items.push(item);
+                        }
                     }
-                };
-                let ExpandResult { value: matched, err: match_err } =
-                    match_meta_var(kind.as_str(), src);
-                match matched {
+                }
+            }
+            OpDelimited::Op(Op::Leaf(leaf)) => {
+                if let Err(err) = match_leaf(&leaf, &mut src.clone()) {
+                    res.add_err(err);
+                    item.is_error = true;
+                } else {
+                    item.dot.next();
+                }
+                try_push!(next_items, item);
+            }
+            OpDelimited::Open => {
+                if matches!(src.clone().next(), Some(tt::TokenTree::Subtree(..))) {
+                    item.dot.next();
+                    try_push!(next_items, item);
+                }
+            }
+            OpDelimited::Close => {
+                let is_delim_closed = src.peek_n(0).is_none() && !stack.is_empty();
+                if is_delim_closed {
+                    item.dot.next();
+                    try_push!(next_items, item);
+                }
+            }
+        }
+    }
+}
+
+fn match_loop(pattern: &MetaTemplate, src: &tt::Subtree) -> Match {
+    let mut src = TtIter::new(src);
+    let mut stack: SmallVec<[TtIter; 1]> = SmallVec::new();
+    let mut res = Match::default();
+    let mut error_reover_item = None;
+
+    let mut cur_items = smallvec![MatchState {
+        dot: pattern.iter_delimited(None),
+        stack: Default::default(),
+        up: None,
+        sep: None,
+        sep_kind: None,
+        sep_parsed: None,
+        bindings: smallvec![Bindings::default()],
+        is_error: false,
+        meta_result: None,
+    }];
+
+    let mut next_items = vec![];
+
+    loop {
+        let mut bb_items = SmallVec::new();
+        let mut eof_items = SmallVec::new();
+        let mut error_items = SmallVec::new();
+
+        stdx::always!(next_items.is_empty());
+
+        match_loop_inner(
+            src.clone(),
+            &stack,
+            &mut res,
+            &mut cur_items,
+            &mut bb_items,
+            &mut next_items,
+            &mut eof_items,
+            &mut error_items,
+        );
+        stdx::always!(cur_items.is_empty());
+
+        if error_items.len() > 0 {
+            error_reover_item = error_items.pop();
+        } else if eof_items.len() > 0 {
+            error_reover_item = Some(eof_items[0].clone());
+        }
+
+        // We need to do some post processing after the `match_loop_inner`.
+        // If we reached the EOF, check that there is EXACTLY ONE possible matcher. Otherwise,
+        // either the parse is ambiguous (which should never happen) or there is a syntax error.
+        if src.peek_n(0).is_none() && stack.is_empty() {
+            if eof_items.len() == 1 {
+                // remove all errors, because it is the correct answer !
+                res = Match::default();
+                res.bindings = eof_items[0].bindings[0].clone();
+            } else {
+                // Error recovery
+                if error_reover_item.is_some() {
+                    res.bindings = error_reover_item.unwrap().bindings[0].clone();
+                }
+                res.add_err(ExpandError::UnexpectedToken);
+            }
+            return res;
+        }
+
+        // If there are no possible next positions AND we aren't waiting for the black-box parser,
+        // then there is a syntax error.
+        //
+        // Another possibility is that we need to call out to parse some rust nonterminal
+        // (black-box) parser. However, if there is not EXACTLY ONE of these, something is wrong.
+        if (bb_items.is_empty() && next_items.is_empty())
+            || (!bb_items.is_empty() && !next_items.is_empty())
+            || bb_items.len() > 1
+        {
+            res.unmatched_tts += src.len();
+            while let Some(it) = stack.pop() {
+                src = it;
+                res.unmatched_tts += src.len();
+            }
+            res.add_err(err!("leftover tokens"));
+
+            if let Some(mut error_reover_item) = error_reover_item {
+                res.bindings = error_reover_item.bindings.remove(0);
+            }
+            return res;
+        }
+        // Dump all possible `next_items` into `cur_items` for the next iteration.
+        else if !next_items.is_empty() {
+            // Now process the next token
+            cur_items.extend(next_items.drain(..));
+
+            match src.next() {
+                Some(tt::TokenTree::Subtree(subtree)) => {
+                    stack.push(src.clone());
+                    src = TtIter::new(subtree);
+                }
+                None if !stack.is_empty() => src = stack.pop().unwrap(),
+                _ => (),
+            }
+        }
+        // Finally, we have the case where we need to call the black-box parser to get some
+        // nonterminal.
+        else {
+            stdx::always!(bb_items.len() == 1);
+            let mut item = bb_items.pop().unwrap();
+
+            if let Some(OpDelimited::Op(Op::Var { name, .. })) = item.dot.peek() {
+                let (iter, match_res) = item.meta_result.take().unwrap();
+                let bindings = item.bindings.last_mut().unwrap();
+                match match_res.value {
                     Some(fragment) => {
-                        res.bindings.inner.insert(name.clone(), Binding::Fragment(fragment));
+                        bindings.inner.push((name.clone(), Binding::Fragment(fragment)));
                     }
-                    None if match_err.is_none() => res.bindings.push_optional(name),
+                    None if match_res.err.is_none() => bindings.push_optional(name),
                     _ => {}
                 }
-                if let Some(err) = match_err {
+                if let Some(err) = match_res.err {
                     res.add_err(err);
                 }
+                src = iter.clone();
+                item.dot.next();
+            } else {
+                unreachable!()
             }
-            Op::Repeat { tokens: subtree, kind, separator } => {
-                match_repeat(res, subtree, *kind, separator, src);
-            }
+            cur_items.push(item);
         }
+        stdx::always!(!cur_items.is_empty());
     }
 }
 
@@ -173,73 +544,6 @@ fn match_leaf(lhs: &tt::Leaf, src: &mut TtIter) -> Result<(), ExpandError> {
     Ok(())
 }
 
-fn match_repeat(
-    res: &mut Match,
-    pattern: &MetaTemplate,
-    kind: RepeatKind,
-    separator: &Option<Separator>,
-    src: &mut TtIter,
-) {
-    // Dirty hack to make macro-expansion terminate.
-    // This should be replaced by a proper macro-by-example implementation
-    let mut limit = 65536;
-    let mut counter = 0;
-
-    for i in 0.. {
-        let mut fork = src.clone();
-
-        if let Some(separator) = &separator {
-            if i != 0 && !fork.eat_separator(separator) {
-                break;
-            }
-        }
-
-        let mut nested = Match::default();
-        match_tokens(&mut nested, pattern, &mut fork);
-        if nested.err.is_none() {
-            limit -= 1;
-            if limit == 0 {
-                log::warn!(
-                    "match_lhs exceeded repeat pattern limit => {:#?}\n{:#?}\n{:#?}\n{:#?}",
-                    pattern,
-                    src,
-                    kind,
-                    separator
-                );
-                break;
-            }
-            *src = fork;
-
-            if let Err(err) = res.bindings.push_nested(counter, nested.bindings) {
-                res.add_err(err);
-            }
-            counter += 1;
-            if counter == 1 {
-                if let RepeatKind::ZeroOrOne = kind {
-                    break;
-                }
-            }
-        } else {
-            break;
-        }
-    }
-
-    match (kind, counter) {
-        (RepeatKind::OneOrMore, 0) => {
-            res.add_err(ExpandError::UnexpectedToken);
-        }
-        (_, 0) => {
-            // Collect all empty variables in subtrees
-            let mut vars = Vec::new();
-            collect_vars(&mut vars, pattern);
-            for var in vars {
-                res.bindings.push_empty(&var)
-            }
-        }
-        _ => (),
-    }
-}
-
 fn match_meta_var(kind: &str, input: &mut TtIter) -> ExpandResult<Option<Fragment>> {
     let fragment = match kind {
         "path" => Path,
@@ -303,14 +607,14 @@ fn collect_vars(buf: &mut Vec<SmolStr>, pattern: &MetaTemplate) {
 }
 
 impl<'a> TtIter<'a> {
-    fn eat_separator(&mut self, separator: &Separator) -> bool {
+    fn expect_separator(&mut self, separator: &Separator, idx: usize) -> bool {
         let mut fork = self.clone();
         let ok = match separator {
-            Separator::Ident(lhs) => match fork.expect_ident() {
+            Separator::Ident(lhs) if idx == 0 => match fork.expect_ident() {
                 Ok(rhs) => rhs.text == lhs.text,
                 _ => false,
             },
-            Separator::Literal(lhs) => match fork.expect_literal() {
+            Separator::Literal(lhs) if idx == 0 => match fork.expect_literal() {
                 Ok(rhs) => match rhs {
                     tt::Leaf::Literal(rhs) => rhs.text == lhs.text,
                     tt::Leaf::Ident(rhs) => rhs.text == lhs.text,
@@ -318,10 +622,11 @@ impl<'a> TtIter<'a> {
                 },
                 _ => false,
             },
-            Separator::Puncts(lhss) => lhss.iter().all(|lhs| match fork.expect_punct() {
-                Ok(rhs) => rhs.char == lhs.char,
+            Separator::Puncts(lhss) if idx < lhss.len() => match fork.expect_punct() {
+                Ok(rhs) => rhs.char == lhss[idx].char,
                 _ => false,
-            }),
+            },
+            _ => false,
         };
         if ok {
             *self = fork;
diff --git a/crates/mbe/src/expander/transcriber.rs b/crates/mbe/src/expander/transcriber.rs
index 78368a33e..ad9953a7d 100644
--- a/crates/mbe/src/expander/transcriber.rs
+++ b/crates/mbe/src/expander/transcriber.rs
@@ -13,13 +13,17 @@ use crate::{
 
 impl Bindings {
     fn contains(&self, name: &str) -> bool {
-        self.inner.contains_key(name)
+        self.inner.iter().any(|(n, _)| n == name)
     }
 
     fn get(&self, name: &str, nesting: &mut [NestingState]) -> Result<&Fragment, ExpandError> {
-        let mut b = self.inner.get(name).ok_or_else(|| {
-            ExpandError::BindingError(format!("could not find binding `{}`", name))
-        })?;
+        let mut b: &Binding = self
+            .inner
+            .iter()
+            .find_map(|(n, b)| if n == name { Some(b) } else { None })
+            .ok_or_else(|| {
+                ExpandError::BindingError(format!("could not find binding `{}`", name))
+            })?;
         for nesting_state in nesting.iter_mut() {
             nesting_state.hit = true;
             b = match b {
diff --git a/crates/mbe/src/lib.rs b/crates/mbe/src/lib.rs
index 4c298f85f..f3d2da55a 100644
--- a/crates/mbe/src/lib.rs
+++ b/crates/mbe/src/lib.rs
@@ -21,7 +21,7 @@ use test_utils::mark;
 pub use tt::{Delimiter, DelimiterKind, Punct};
 
 use crate::{
-    parser::{parse_pattern, parse_template, Op},
+    parser::{parse_pattern, parse_template, MetaTemplate, Op},
     tt_iter::TtIter,
 };
 
@@ -94,15 +94,6 @@ struct Rule {
     rhs: MetaTemplate,
 }
 
-#[derive(Clone, Debug, PartialEq, Eq)]
-struct MetaTemplate(Vec<Op>);
-
-impl<'a> MetaTemplate {
-    fn iter(&self) -> impl Iterator<Item = &Op> {
-        self.0.iter()
-    }
-}
-
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 struct Shift(u32);
 
diff --git a/crates/mbe/src/parser.rs b/crates/mbe/src/parser.rs
index f891ec29c..8671322e1 100644
--- a/crates/mbe/src/parser.rs
+++ b/crates/mbe/src/parser.rs
@@ -5,7 +5,75 @@ use smallvec::SmallVec;
 use syntax::SmolStr;
 use tt::Delimiter;
 
-use crate::{tt_iter::TtIter, MetaTemplate, ParseError};
+use crate::{tt_iter::TtIter, ParseError};
+
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub(crate) struct MetaTemplate(pub(crate) Vec<Op>);
+
+#[derive(Debug, Clone, Copy)]
+pub(crate) enum OpDelimited<'a> {
+    Op(&'a Op),
+    Open,
+    Close,
+}
+
+#[derive(Debug, Clone, Copy)]
+pub(crate) struct OpDelimitedIter<'a> {
+    inner: &'a Vec<Op>,
+    delimited: Option<&'a Delimiter>,
+    idx: usize,
+}
+
+impl<'a> OpDelimitedIter<'a> {
+    pub(crate) fn is_eof(&self) -> bool {
+        let len = self.inner.len() + if self.delimited.is_some() { 2 } else { 0 };
+        self.idx >= len
+    }
+
+    pub(crate) fn peek(&self) -> Option<OpDelimited<'a>> {
+        match self.delimited {
+            None => self.inner.get(self.idx).map(OpDelimited::Op),
+            Some(_) => match self.idx {
+                0 => Some(OpDelimited::Open),
+                i if i == self.inner.len() + 1 => Some(OpDelimited::Close),
+                i => self.inner.get(i - 1).map(OpDelimited::Op),
+            },
+        }
+    }
+
+    pub(crate) fn reset(&self) -> Self {
+        Self { inner: &self.inner, idx: 0, delimited: self.delimited }
+    }
+}
+
+impl<'a> Iterator for OpDelimitedIter<'a> {
+    type Item = OpDelimited<'a>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let res = self.peek();
+        self.idx += 1;
+        res
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let len = self.inner.len() + if self.delimited.is_some() { 2 } else { 0 };
+        let remain = len.checked_sub(self.idx).unwrap_or(0);
+        (remain, Some(remain))
+    }
+}
+
+impl<'a> MetaTemplate {
+    pub(crate) fn iter(&self) -> impl Iterator<Item = &Op> {
+        self.0.iter()
+    }
+
+    pub(crate) fn iter_delimited(
+        &'a self,
+        delimited: Option<&'a Delimiter>,
+    ) -> OpDelimitedIter<'a> {
+        OpDelimitedIter { inner: &self.0, idx: 0, delimited }
+    }
+}
 
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub(crate) enum Op {
@@ -47,6 +115,16 @@ impl PartialEq for Separator {
     }
 }
 
+impl Separator {
+    pub(crate) fn tt_count(&self) -> usize {
+        match self {
+            Separator::Literal(_) => 1,
+            Separator::Ident(_) => 1,
+            Separator::Puncts(it) => it.len(),
+        }
+    }
+}
+
 pub(crate) fn parse_template(template: &tt::Subtree) -> Result<Vec<Op>, ParseError> {
     parse_inner(&template, Mode::Template).into_iter().collect()
 }
diff --git a/crates/mbe/src/tests.rs b/crates/mbe/src/tests.rs
index f1eadcd1e..5c641ebf2 100644
--- a/crates/mbe/src/tests.rs
+++ b/crates/mbe/src/tests.rs
@@ -457,6 +457,17 @@ fn test_match_group_with_multichar_sep() {
 }
 
 #[test]
+fn test_match_group_with_multichar_sep2() {
+    parse_macro(
+        r#"
+        macro_rules! foo {
+            (fn $name:ident {$($i:literal)&&*} ) => ( fn $name() -> bool { $($i)&&*} );
+        }"#,
+    )
+    .assert_expand_items("foo! (fn baz {true && true} );", "fn baz () -> bool {true &&true}");
+}
+
+#[test]
 fn test_match_group_zero_match() {
     parse_macro(
         r#"
@@ -1267,6 +1278,18 @@ macro_rules! m {
     .is_some());
 }
 
+#[test]
+fn test_match_is_not_greedy() {
+    parse_macro(
+        r#"
+macro_rules! foo {
+    ($($i:ident $(,)*),*) => {};
+}
+"#,
+    )
+    .assert_expand_items(r#"foo!(a,b);"#, r#""#);
+}
+
 // The following tests are based on real world situations
 #[test]
 fn test_vec() {