1 files changed, 832 insertions, 0 deletions

diff --git a/crates/mbe/src/syntax_bridge.rs b/crates/mbe/src/syntax_bridge.rs
new file mode 100644
index 000000000..a8ad917fb
--- /dev/null
+++ b/crates/mbe/src/syntax_bridge.rs
@@ -0,0 +1,832 @@
+//! FIXME: write short doc here
+
+use parser::{FragmentKind, ParseError, TreeSink};
+use rustc_hash::FxHashMap;
+use syntax::{
+    ast::{self, make::tokens::doc_comment},
+    tokenize, AstToken, Parse, SmolStr, SyntaxKind,
+    SyntaxKind::*,
+    SyntaxNode, SyntaxToken, SyntaxTreeBuilder, TextRange, TextSize, Token as RawToken, T,
+};
+use tt::buffer::{Cursor, TokenBuffer};
+
+use crate::subtree_source::SubtreeTokenSource;
+use crate::ExpandError;
+
+#[derive(Debug, PartialEq, Eq, Clone, Copy)]
+pub enum TokenTextRange {
+    Token(TextRange),
+    Delimiter(TextRange, TextRange),
+}
+
+impl TokenTextRange {
+    pub fn by_kind(self, kind: SyntaxKind) -> Option<TextRange> {
+        match self {
+            TokenTextRange::Token(it) => Some(it),
+            TokenTextRange::Delimiter(open, close) => match kind {
+                T!['{'] | T!['('] | T!['['] => Some(open),
+                T!['}'] | T![')'] | T![']'] => Some(close),
+                _ => None,
+            },
+        }
+    }
+}
+
+/// Maps `tt::TokenId` to the relative range of the original token.
+#[derive(Debug, PartialEq, Eq, Clone, Default)]
+pub struct TokenMap {
+    /// Maps `tt::TokenId` to the *relative* source range.
+    entries: Vec<(tt::TokenId, TokenTextRange)>,
+}
+
+/// Convert the syntax tree (what user has written) to a `TokenTree` (what macro
+/// will consume).
+pub fn ast_to_token_tree(ast: &impl ast::AstNode) -> Option<(tt::Subtree, TokenMap)> {
+    syntax_node_to_token_tree(ast.syntax())
+}
+
+/// Convert the syntax node to a `TokenTree` (what macro
+/// will consume).
+pub fn syntax_node_to_token_tree(node: &SyntaxNode) -> Option<(tt::Subtree, TokenMap)> {
+    let global_offset = node.text_range().start();
+    let mut c = Convertor::new(node, global_offset);
+    let subtree = c.go()?;
+    Some((subtree, c.id_alloc.map))
+}
+
+// The following items are what `rustc` macro can be parsed into :
+// link: https://github.com/rust-lang/rust/blob/9ebf47851a357faa4cd97f4b1dc7835f6376e639/src/libsyntax/ext/expand.rs#L141
+// * Expr(P<ast::Expr>)                     -> token_tree_to_expr
+// * Pat(P<ast::Pat>)                       -> token_tree_to_pat
+// * Ty(P<ast::Ty>)                         -> token_tree_to_ty
+// * Stmts(SmallVec<[ast::Stmt; 1]>)        -> token_tree_to_stmts
+// * Items(SmallVec<[P<ast::Item>; 1]>)     -> token_tree_to_items
+//
+// * TraitItems(SmallVec<[ast::TraitItem; 1]>)
+// * AssocItems(SmallVec<[ast::AssocItem; 1]>)
+// * ForeignItems(SmallVec<[ast::ForeignItem; 1]>
+
+pub fn token_tree_to_syntax_node(
+    tt: &tt::Subtree,
+    fragment_kind: FragmentKind,
+) -> Result<(Parse<SyntaxNode>, TokenMap), ExpandError> {
+    let tmp;
+    let tokens = match tt {
+        tt::Subtree { delimiter: None, token_trees } => token_trees.as_slice(),
+        _ => {
+            tmp = [tt.clone().into()];
+            &tmp[..]
+        }
+    };
+    let buffer = TokenBuffer::new(&tokens);
+    let mut token_source = SubtreeTokenSource::new(&buffer);
+    let mut tree_sink = TtTreeSink::new(buffer.begin());
+    parser::parse_fragment(&mut token_source, &mut tree_sink, fragment_kind);
+    if tree_sink.roots.len() != 1 {
+        return Err(ExpandError::ConversionError);
+    }
+    //FIXME: would be cool to report errors
+    let (parse, range_map) = tree_sink.finish();
+    Ok((parse, range_map))
+}
+
+/// Convert a string to a `TokenTree`
+pub fn parse_to_token_tree(text: &str) -> Option<(tt::Subtree, TokenMap)> {
+    let (tokens, errors) = tokenize(text);
+    if !errors.is_empty() {
+        return None;
+    }
+
+    let mut conv = RawConvertor {
+        text,
+        offset: TextSize::default(),
+        inner: tokens.iter(),
+        id_alloc: TokenIdAlloc {
+            map: Default::default(),
+            global_offset: TextSize::default(),
+            next_id: 0,
+        },
+    };
+
+    let subtree = conv.go()?;
+    Some((subtree, conv.id_alloc.map))
+}
+
+impl TokenMap {
+    pub fn token_by_range(&self, relative_range: TextRange) -> Option<tt::TokenId> {
+        let &(token_id, _) = self.entries.iter().find(|(_, range)| match range {
+            TokenTextRange::Token(it) => *it == relative_range,
+            TokenTextRange::Delimiter(open, close) => {
+                *open == relative_range || *close == relative_range
+            }
+        })?;
+        Some(token_id)
+    }
+
+    pub fn range_by_token(&self, token_id: tt::TokenId) -> Option<TokenTextRange> {
+        let &(_, range) = self.entries.iter().find(|(tid, _)| *tid == token_id)?;
+        Some(range)
+    }
+
+    fn insert(&mut self, token_id: tt::TokenId, relative_range: TextRange) {
+        self.entries.push((token_id, TokenTextRange::Token(relative_range)));
+    }
+
+    fn insert_delim(
+        &mut self,
+        token_id: tt::TokenId,
+        open_relative_range: TextRange,
+        close_relative_range: TextRange,
+    ) -> usize {
+        let res = self.entries.len();
+        self.entries
+            .push((token_id, TokenTextRange::Delimiter(open_relative_range, close_relative_range)));
+        res
+    }
+
+    fn update_close_delim(&mut self, idx: usize, close_relative_range: TextRange) {
+        let (_, token_text_range) = &mut self.entries[idx];
+        if let TokenTextRange::Delimiter(dim, _) = token_text_range {
+            *token_text_range = TokenTextRange::Delimiter(*dim, close_relative_range);
+        }
+    }
+
+    fn remove_delim(&mut self, idx: usize) {
+        // FIXME: This could be accidently quadratic
+        self.entries.remove(idx);
+    }
+}
+
+/// Returns the textual content of a doc comment block as a quoted string
+/// That is, strips leading `///` (or `/**`, etc)
+/// and strips the ending `*/`
+/// And then quote the string, which is needed to convert to `tt::Literal`
+fn doc_comment_text(comment: &ast::Comment) -> SmolStr {
+    let prefix_len = comment.prefix().len();
+    let mut text = &comment.text()[prefix_len..];
+
+    // Remove ending "*/"
+    if comment.kind().shape == ast::CommentShape::Block {
+        text = &text[0..text.len() - 2];
+    }
+
+    // Quote the string
+    // Note that `tt::Literal` expect an escaped string
+    let text = format!("{:?}", text.escape_default().to_string());
+    text.into()
+}
+
+fn convert_doc_comment(token: &syntax::SyntaxToken) -> Option<Vec<tt::TokenTree>> {
+    let comment = ast::Comment::cast(token.clone())?;
+    let doc = comment.kind().doc?;
+
+    // Make `doc="\" Comments\""
+    let mut meta_tkns = Vec::new();
+    meta_tkns.push(mk_ident("doc"));
+    meta_tkns.push(mk_punct('='));
+    meta_tkns.push(mk_doc_literal(&comment));
+
+    // Make `#![]`
+    let mut token_trees = Vec::new();
+    token_trees.push(mk_punct('#'));
+    if let ast::CommentPlacement::Inner = doc {
+        token_trees.push(mk_punct('!'));
+    }
+    token_trees.push(tt::TokenTree::from(tt::Subtree {
+        delimiter: Some(tt::Delimiter {
+            kind: tt::DelimiterKind::Bracket,
+            id: tt::TokenId::unspecified(),
+        }),
+        token_trees: meta_tkns,
+    }));
+
+    return Some(token_trees);
+
+    // Helper functions
+    fn mk_ident(s: &str) -> tt::TokenTree {
+        tt::TokenTree::from(tt::Leaf::from(tt::Ident {
+            text: s.into(),
+            id: tt::TokenId::unspecified(),
+        }))
+    }
+
+    fn mk_punct(c: char) -> tt::TokenTree {
+        tt::TokenTree::from(tt::Leaf::from(tt::Punct {
+            char: c,
+            spacing: tt::Spacing::Alone,
+            id: tt::TokenId::unspecified(),
+        }))
+    }
+
+    fn mk_doc_literal(comment: &ast::Comment) -> tt::TokenTree {
+        let lit = tt::Literal { text: doc_comment_text(comment), id: tt::TokenId::unspecified() };
+
+        tt::TokenTree::from(tt::Leaf::from(lit))
+    }
+}
+
+struct TokenIdAlloc {
+    map: TokenMap,
+    global_offset: TextSize,
+    next_id: u32,
+}
+
+impl TokenIdAlloc {
+    fn alloc(&mut self, absolute_range: TextRange) -> tt::TokenId {
+        let relative_range = absolute_range - self.global_offset;
+        let token_id = tt::TokenId(self.next_id);
+        self.next_id += 1;
+        self.map.insert(token_id, relative_range);
+        token_id
+    }
+
+    fn open_delim(&mut self, open_abs_range: TextRange) -> (tt::TokenId, usize) {
+        let token_id = tt::TokenId(self.next_id);
+        self.next_id += 1;
+        let idx = self.map.insert_delim(
+            token_id,
+            open_abs_range - self.global_offset,
+            open_abs_range - self.global_offset,
+        );
+        (token_id, idx)
+    }
+
+    fn close_delim(&mut self, idx: usize, close_abs_range: Option<TextRange>) {
+        match close_abs_range {
+            None => {
+                self.map.remove_delim(idx);
+            }
+            Some(close) => {
+                self.map.update_close_delim(idx, close - self.global_offset);
+            }
+        }
+    }
+}
+
+/// A Raw Token (straightly from lexer) convertor
+struct RawConvertor<'a> {
+    text: &'a str,
+    offset: TextSize,
+    id_alloc: TokenIdAlloc,
+    inner: std::slice::Iter<'a, RawToken>,
+}
+
+trait SrcToken: std::fmt::Debug {
+    fn kind(&self) -> SyntaxKind;
+
+    fn to_char(&self) -> Option<char>;
+
+    fn to_text(&self) -> SmolStr;
+}
+
+trait TokenConvertor {
+    type Token: SrcToken;
+
+    fn go(&mut self) -> Option<tt::Subtree> {
+        let mut subtree = tt::Subtree::default();
+        subtree.delimiter = None;
+        while self.peek().is_some() {
+            self.collect_leaf(&mut subtree.token_trees);
+        }
+        if subtree.token_trees.is_empty() {
+            return None;
+        }
+        if subtree.token_trees.len() == 1 {
+            if let tt::TokenTree::Subtree(first) = &subtree.token_trees[0] {
+                return Some(first.clone());
+            }
+        }
+        Some(subtree)
+    }
+
+    fn collect_leaf(&mut self, result: &mut Vec<tt::TokenTree>) {
+        let (token, range) = match self.bump() {
+            None => return,
+            Some(it) => it,
+        };
+
+        let k: SyntaxKind = token.kind();
+        if k == COMMENT {
+            if let Some(tokens) = self.convert_doc_comment(&token) {
+                result.extend(tokens);
+            }
+            return;
+        }
+
+        result.push(if k.is_punct() {
+            assert_eq!(range.len(), TextSize::of('.'));
+            let delim = match k {
+                T!['('] => Some((tt::DelimiterKind::Parenthesis, T![')'])),
+                T!['{'] => Some((tt::DelimiterKind::Brace, T!['}'])),
+                T!['['] => Some((tt::DelimiterKind::Bracket, T![']'])),
+                _ => None,
+            };
+
+            if let Some((kind, closed)) = delim {
+                let mut subtree = tt::Subtree::default();
+                let (id, idx) = self.id_alloc().open_delim(range);
+                subtree.delimiter = Some(tt::Delimiter { kind, id });
+
+                while self.peek().map(|it| it.kind() != closed).unwrap_or(false) {
+                    self.collect_leaf(&mut subtree.token_trees);
+                }
+                let last_range = match self.bump() {
+                    None => {
+                        // For error resilience, we insert an char punct for the opening delim here
+                        self.id_alloc().close_delim(idx, None);
+                        let leaf: tt::Leaf = tt::Punct {
+                            id: self.id_alloc().alloc(range),
+                            char: token.to_char().unwrap(),
+                            spacing: tt::Spacing::Alone,
+                        }
+                        .into();
+                        result.push(leaf.into());
+                        result.extend(subtree.token_trees);
+                        return;
+                    }
+                    Some(it) => it.1,
+                };
+                self.id_alloc().close_delim(idx, Some(last_range));
+                subtree.into()
+            } else {
+                let spacing = match self.peek() {
+                    Some(next)
+                        if next.kind().is_trivia()
+                            || next.kind() == T!['[']
+                            || next.kind() == T!['{']
+                            || next.kind() == T!['('] =>
+                    {
+                        tt::Spacing::Alone
+                    }
+                    Some(next) if next.kind().is_punct() => tt::Spacing::Joint,
+                    _ => tt::Spacing::Alone,
+                };
+                let char = match token.to_char() {
+                    Some(c) => c,
+                    None => {
+                        panic!("Token from lexer must be single char: token = {:#?}", token);
+                    }
+                };
+                tt::Leaf::from(tt::Punct { char, spacing, id: self.id_alloc().alloc(range) }).into()
+            }
+        } else {
+            macro_rules! make_leaf {
+                ($i:ident) => {
+                    tt::$i { id: self.id_alloc().alloc(range), text: token.to_text() }.into()
+                };
+            }
+            let leaf: tt::Leaf = match k {
+                T![true] | T![false] => make_leaf!(Ident),
+                IDENT => make_leaf!(Ident),
+                k if k.is_keyword() => make_leaf!(Ident),
+                k if k.is_literal() => make_leaf!(Literal),
+                LIFETIME => {
+                    let char_unit = TextSize::of('\'');
+                    let r = TextRange::at(range.start(), char_unit);
+                    let apostrophe = tt::Leaf::from(tt::Punct {
+                        char: '\'',
+                        spacing: tt::Spacing::Joint,
+                        id: self.id_alloc().alloc(r),
+                    });
+                    result.push(apostrophe.into());
+
+                    let r = TextRange::at(range.start() + char_unit, range.len() - char_unit);
+                    let ident = tt::Leaf::from(tt::Ident {
+                        text: SmolStr::new(&token.to_text()[1..]),
+                        id: self.id_alloc().alloc(r),
+                    });
+                    result.push(ident.into());
+                    return;
+                }
+                _ => return,
+            };
+
+            leaf.into()
+        });
+    }
+
+    fn convert_doc_comment(&self, token: &Self::Token) -> Option<Vec<tt::TokenTree>>;
+
+    fn bump(&mut self) -> Option<(Self::Token, TextRange)>;
+
+    fn peek(&self) -> Option<Self::Token>;
+
+    fn id_alloc(&mut self) -> &mut TokenIdAlloc;
+}
+
+impl<'a> SrcToken for (RawToken, &'a str) {
+    fn kind(&self) -> SyntaxKind {
+        self.0.kind
+    }
+
+    fn to_char(&self) -> Option<char> {
+        self.1.chars().next()
+    }
+
+    fn to_text(&self) -> SmolStr {
+        self.1.into()
+    }
+}
+
+impl RawConvertor<'_> {}
+
+impl<'a> TokenConvertor for RawConvertor<'a> {
+    type Token = (RawToken, &'a str);
+
+    fn convert_doc_comment(&self, token: &Self::Token) -> Option<Vec<tt::TokenTree>> {
+        convert_doc_comment(&doc_comment(token.1))
+    }
+
+    fn bump(&mut self) -> Option<(Self::Token, TextRange)> {
+        let token = self.inner.next()?;
+        let range = TextRange::at(self.offset, token.len);
+        self.offset += token.len;
+
+        Some(((*token, &self.text[range]), range))
+    }
+
+    fn peek(&self) -> Option<Self::Token> {
+        let token = self.inner.as_slice().get(0).cloned();
+
+        token.map(|it| {
+            let range = TextRange::at(self.offset, it.len);
+            (it, &self.text[range])
+        })
+    }
+
+    fn id_alloc(&mut self) -> &mut TokenIdAlloc {
+        &mut self.id_alloc
+    }
+}
+
+struct Convertor {
+    id_alloc: TokenIdAlloc,
+    current: Option<SyntaxToken>,
+    range: TextRange,
+    punct_offset: Option<(SyntaxToken, TextSize)>,
+}
+
+impl Convertor {
+    fn new(node: &SyntaxNode, global_offset: TextSize) -> Convertor {
+        Convertor {
+            id_alloc: { TokenIdAlloc { map: TokenMap::default(), global_offset, next_id: 0 } },
+            current: node.first_token(),
+            range: node.text_range(),
+            punct_offset: None,
+        }
+    }
+}
+
+#[derive(Debug)]
+enum SynToken {
+    Ordiniary(SyntaxToken),
+    Punch(SyntaxToken, TextSize),
+}
+
+impl SynToken {
+    fn token(&self) -> &SyntaxToken {
+        match self {
+            SynToken::Ordiniary(it) => it,
+            SynToken::Punch(it, _) => it,
+        }
+    }
+}
+
+impl SrcToken for SynToken {
+    fn kind(&self) -> SyntaxKind {
+        self.token().kind()
+    }
+    fn to_char(&self) -> Option<char> {
+        match self {
+            SynToken::Ordiniary(_) => None,
+            SynToken::Punch(it, i) => it.text().chars().nth((*i).into()),
+        }
+    }
+    fn to_text(&self) -> SmolStr {
+        self.token().text().clone()
+    }
+}
+
+impl TokenConvertor for Convertor {
+    type Token = SynToken;
+    fn convert_doc_comment(&self, token: &Self::Token) -> Option<Vec<tt::TokenTree>> {
+        convert_doc_comment(token.token())
+    }
+
+    fn bump(&mut self) -> Option<(Self::Token, TextRange)> {
+        if let Some((punct, offset)) = self.punct_offset.clone() {
+            if usize::from(offset) + 1 < punct.text().len() {
+                let offset = offset + TextSize::of('.');
+                let range = punct.text_range();
+                self.punct_offset = Some((punct.clone(), offset));
+                let range = TextRange::at(range.start() + offset, TextSize::of('.'));
+                return Some((SynToken::Punch(punct, offset), range));
+            }
+        }
+
+        let curr = self.current.clone()?;
+        if !&self.range.contains_range(curr.text_range()) {
+            return None;
+        }
+        self.current = curr.next_token();
+
+        let token = if curr.kind().is_punct() {
+            let range = curr.text_range();
+            let range = TextRange::at(range.start(), TextSize::of('.'));
+            self.punct_offset = Some((curr.clone(), 0.into()));
+            (SynToken::Punch(curr, 0.into()), range)
+        } else {
+            self.punct_offset = None;
+            let range = curr.text_range();
+            (SynToken::Ordiniary(curr), range)
+        };
+
+        Some(token)
+    }
+
+    fn peek(&self) -> Option<Self::Token> {
+        if let Some((punct, mut offset)) = self.punct_offset.clone() {
+            offset = offset + TextSize::of('.');
+            if usize::from(offset) < punct.text().len() {
+                return Some(SynToken::Punch(punct, offset));
+            }
+        }
+
+        let curr = self.current.clone()?;
+        if !self.range.contains_range(curr.text_range()) {
+            return None;
+        }
+
+        let token = if curr.kind().is_punct() {
+            SynToken::Punch(curr, 0.into())
+        } else {
+            SynToken::Ordiniary(curr)
+        };
+        Some(token)
+    }
+
+    fn id_alloc(&mut self) -> &mut TokenIdAlloc {
+        &mut self.id_alloc
+    }
+}
+
+struct TtTreeSink<'a> {
+    buf: String,
+    cursor: Cursor<'a>,
+    open_delims: FxHashMap<tt::TokenId, TextSize>,
+    text_pos: TextSize,
+    inner: SyntaxTreeBuilder,
+    token_map: TokenMap,
+
+    // Number of roots
+    // Use for detect ill-form tree which is not single root
+    roots: smallvec::SmallVec<[usize; 1]>,
+}
+
+impl<'a> TtTreeSink<'a> {
+    fn new(cursor: Cursor<'a>) -> Self {
+        TtTreeSink {
+            buf: String::new(),
+            cursor,
+            open_delims: FxHashMap::default(),
+            text_pos: 0.into(),
+            inner: SyntaxTreeBuilder::default(),
+            roots: smallvec::SmallVec::new(),
+            token_map: TokenMap::default(),
+        }
+    }
+
+    fn finish(self) -> (Parse<SyntaxNode>, TokenMap) {
+        (self.inner.finish(), self.token_map)
+    }
+}
+
+fn delim_to_str(d: Option<tt::DelimiterKind>, closing: bool) -> SmolStr {
+    let texts = match d {
+        Some(tt::DelimiterKind::Parenthesis) => "()",
+        Some(tt::DelimiterKind::Brace) => "{}",
+        Some(tt::DelimiterKind::Bracket) => "[]",
+        None => return "".into(),
+    };
+
+    let idx = closing as usize;
+    let text = &texts[idx..texts.len() - (1 - idx)];
+    text.into()
+}
+
+impl<'a> TreeSink for TtTreeSink<'a> {
+    fn token(&mut self, kind: SyntaxKind, mut n_tokens: u8) {
+        if kind == L_DOLLAR || kind == R_DOLLAR {
+            self.cursor = self.cursor.bump_subtree();
+            return;
+        }
+        if kind == LIFETIME {
+            n_tokens = 2;
+        }
+
+        let mut last = self.cursor;
+        for _ in 0..n_tokens {
+            if self.cursor.eof() {
+                break;
+            }
+            last = self.cursor;
+            let text: SmolStr = match self.cursor.token_tree() {
+                Some(tt::TokenTree::Leaf(leaf)) => {
+                    // Mark the range if needed
+                    let (text, id) = match leaf {
+                        tt::Leaf::Ident(ident) => (ident.text.clone(), ident.id),
+                        tt::Leaf::Punct(punct) => {
+                            (SmolStr::new_inline_from_ascii(1, &[punct.char as u8]), punct.id)
+                        }
+                        tt::Leaf::Literal(lit) => (lit.text.clone(), lit.id),
+                    };
+                    let range = TextRange::at(self.text_pos, TextSize::of(text.as_str()));
+                    self.token_map.insert(id, range);
+                    self.cursor = self.cursor.bump();
+                    text
+                }
+                Some(tt::TokenTree::Subtree(subtree)) => {
+                    self.cursor = self.cursor.subtree().unwrap();
+                    if let Some(id) = subtree.delimiter.map(|it| it.id) {
+                        self.open_delims.insert(id, self.text_pos);
+                    }
+                    delim_to_str(subtree.delimiter_kind(), false)
+                }
+                None => {
+                    if let Some(parent) = self.cursor.end() {
+                        self.cursor = self.cursor.bump();
+                        if let Some(id) = parent.delimiter.map(|it| it.id) {
+                            if let Some(open_delim) = self.open_delims.get(&id) {
+                                let open_range = TextRange::at(*open_delim, TextSize::of('('));
+                                let close_range = TextRange::at(self.text_pos, TextSize::of('('));
+                                self.token_map.insert_delim(id, open_range, close_range);
+                            }
+                        }
+                        delim_to_str(parent.delimiter_kind(), true)
+                    } else {
+                        continue;
+                    }
+                }
+            };
+            self.buf += &text;
+            self.text_pos += TextSize::of(text.as_str());
+        }
+
+        let text = SmolStr::new(self.buf.as_str());
+        self.buf.clear();
+        self.inner.token(kind, text);
+
+        // Add whitespace between adjoint puncts
+        let next = last.bump();
+        if let (
+            Some(tt::TokenTree::Leaf(tt::Leaf::Punct(curr))),
+            Some(tt::TokenTree::Leaf(tt::Leaf::Punct(_))),
+        ) = (last.token_tree(), next.token_tree())
+        {
+            // Note: We always assume the semi-colon would be the last token in
+            // other parts of RA such that we don't add whitespace here.
+            if curr.spacing == tt::Spacing::Alone && curr.char != ';' {
+                self.inner.token(WHITESPACE, " ".into());
+                self.text_pos += TextSize::of(' ');
+            }
+        }
+    }
+
+    fn start_node(&mut self, kind: SyntaxKind) {
+        self.inner.start_node(kind);
+
+        match self.roots.last_mut() {
+            None | Some(0) => self.roots.push(1),
+            Some(ref mut n) => **n += 1,
+        };
+    }
+
+    fn finish_node(&mut self) {
+        self.inner.finish_node();
+        *self.roots.last_mut().unwrap() -= 1;
+    }
+
+    fn error(&mut self, error: ParseError) {
+        self.inner.error(error, self.text_pos)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::tests::parse_macro;
+    use parser::TokenSource;
+    use syntax::{
+        algo::{insert_children, InsertPosition},
+        ast::AstNode,
+    };
+
+    #[test]
+    fn convert_tt_token_source() {
+        let expansion = parse_macro(
+            r#"
+            macro_rules! literals {
+                ($i:ident) => {
+                    {
+                        let a = 'c';
+                        let c = 1000;
+                        let f = 12E+99_f64;
+                        let s = "rust1";
+                    }
+                }
+            }
+            "#,
+        )
+        .expand_tt("literals!(foo);");
+        let tts = &[expansion.into()];
+        let buffer = tt::buffer::TokenBuffer::new(tts);
+        let mut tt_src = SubtreeTokenSource::new(&buffer);
+        let mut tokens = vec![];
+        while tt_src.current().kind != EOF {
+            tokens.push((tt_src.current().kind, tt_src.text()));
+            tt_src.bump();
+        }
+
+        // [${]
+        // [let] [a] [=] ['c'] [;]
+        assert_eq!(tokens[2 + 3].1, "'c'");
+        assert_eq!(tokens[2 + 3].0, CHAR);
+        // [let] [c] [=] [1000] [;]
+        assert_eq!(tokens[2 + 5 + 3].1, "1000");
+        assert_eq!(tokens[2 + 5 + 3].0, INT_NUMBER);
+        // [let] [f] [=] [12E+99_f64] [;]
+        assert_eq!(tokens[2 + 10 + 3].1, "12E+99_f64");
+        assert_eq!(tokens[2 + 10 + 3].0, FLOAT_NUMBER);
+
+        // [let] [s] [=] ["rust1"] [;]
+        assert_eq!(tokens[2 + 15 + 3].1, "\"rust1\"");
+        assert_eq!(tokens[2 + 15 + 3].0, STRING);
+    }
+
+    #[test]
+    fn stmts_token_trees_to_expr_is_err() {
+        let expansion = parse_macro(
+            r#"
+            macro_rules! stmts {
+                () => {
+                    let a = 0;
+                    let b = 0;
+                    let c = 0;
+                    let d = 0;
+                }
+            }
+            "#,
+        )
+        .expand_tt("stmts!();");
+        assert!(token_tree_to_syntax_node(&expansion, FragmentKind::Expr).is_err());
+    }
+
+    #[test]
+    fn test_token_tree_last_child_is_white_space() {
+        let source_file = ast::SourceFile::parse("f!({} );").ok().unwrap();
+        let macro_call = source_file.syntax().descendants().find_map(ast::MacroCall::cast).unwrap();
+        let token_tree = macro_call.token_tree().unwrap();
+
+        // Token Tree now is :
+        // TokenTree
+        // - T!['(']
+        // - TokenTree
+        //   - T!['{']
+        //   - T!['}']
+        // - WHITE_SPACE
+        // - T![')']
+
+        let rbrace =
+            token_tree.syntax().descendants_with_tokens().find(|it| it.kind() == T!['}']).unwrap();
+        let space = token_tree
+            .syntax()
+            .descendants_with_tokens()
+            .find(|it| it.kind() == SyntaxKind::WHITESPACE)
+            .unwrap();
+
+        // reorder th white space, such that the white is inside the inner token-tree.
+        let token_tree = insert_children(
+            &rbrace.parent().unwrap(),
+            InsertPosition::Last,
+            std::iter::once(space),
+        );
+
+        // Token Tree now is :
+        // TokenTree
+        // - T!['{']
+        // - T!['}']
+        // - WHITE_SPACE
+        let token_tree = ast::TokenTree::cast(token_tree).unwrap();
+        let tt = ast_to_token_tree(&token_tree).unwrap().0;
+
+        assert_eq!(tt.delimiter_kind(), Some(tt::DelimiterKind::Brace));
+    }
+
+    #[test]
+    fn test_token_tree_multi_char_punct() {
+        let source_file = ast::SourceFile::parse("struct Foo { a: x::Y }").ok().unwrap();
+        let struct_def = source_file.syntax().descendants().find_map(ast::Struct::cast).unwrap();
+        let tt = ast_to_token_tree(&struct_def).unwrap().0;
+        token_tree_to_syntax_node(&tt, FragmentKind::Item).unwrap();
+    }
+}