src/stable.rs

use std::ascii;
use std::borrow::Borrow;
use std::cell::RefCell;
#[cfg(procmacro2_semver_exempt)]
use std::cmp;
use std::collections::HashMap;
use std::fmt;
use std::iter;
use std::marker::PhantomData;
use std::rc::Rc;
use std::str::FromStr;
use std::vec;

use proc_macro;
use unicode_xid::UnicodeXID;
use strnom::{Cursor, PResult, skip_whitespace, block_comment, whitespace, word_break};

use {TokenTree, TokenNode, Delimiter, Spacing};

#[derive(Clone, Debug)]
pub struct TokenStream {
    inner: Vec<TokenTree>,
}

#[derive(Debug)]
pub struct LexError;

impl TokenStream {
    pub fn empty() -> TokenStream {
        TokenStream { inner: Vec::new() }
    }

    pub fn is_empty(&self) -> bool {
        self.inner.len() == 0
    }
}

#[cfg(procmacro2_semver_exempt)]
fn get_cursor(src: &str) -> Cursor {
    // Create a dummy file & add it to the codemap
    CODEMAP.with(|cm| {
        let mut cm = cm.borrow_mut();
        let name = format!("<parsed string {}>", cm.files.len());
        let span = cm.add_file(&name, src);
        Cursor {
            rest: src,
            off: span.lo,
        }
    })
}

#[cfg(not(procmacro2_semver_exempt))]
fn get_cursor(src: &str) -> Cursor {
    Cursor {
        rest: src,
    }
}

impl FromStr for TokenStream {
    type Err = LexError;

    fn from_str(src: &str) -> Result<TokenStream, LexError> {
        // Create a dummy file & add it to the codemap
        let cursor = get_cursor(src);

        match token_stream(cursor) {
            Ok((input, output)) => {
                if skip_whitespace(input).len() != 0 {
                    Err(LexError)
                } else {
                    Ok(output.0)
                }
            }
            Err(LexError) => Err(LexError),
        }
    }
}

impl fmt::Display for TokenStream {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let mut joint = false;
        for (i, tt) in self.inner.iter().enumerate() {
            if i != 0 && !joint {
                write!(f, " ")?;
            }
            joint = false;
            match tt.kind {
                TokenNode::Group(delim, ref stream) => {
                    let (start, end) = match delim {
                        Delimiter::Parenthesis => ("(", ")"),
                        Delimiter::Brace => ("{", "}"),
                        Delimiter::Bracket => ("[", "]"),
                        Delimiter::None => ("", ""),
                    };
                    if stream.0.inner.len() == 0 {
                        write!(f, "{} {}", start, end)?
                    } else {
                        write!(f, "{} {} {}", start, stream, end)?
                    }
                }
                TokenNode::Term(ref sym) => write!(f, "{}", sym.as_str())?,
                TokenNode::Op(ch, ref op) => {
                    write!(f, "{}", ch)?;
                    match *op {
                        Spacing::Alone => {}
                        Spacing::Joint => joint = true,
                    }
                }
                TokenNode::Literal(ref literal) => {
                    write!(f, "{}", literal)?;
                    // handle comments
                    if (literal.0).0.starts_with("/") {
                        write!(f, "\n")?;
                    }
                }
            }
        }

        Ok(())
    }
}

impl From<proc_macro::TokenStream> for TokenStream {
    fn from(inner: proc_macro::TokenStream) -> TokenStream {
        inner.to_string().parse().expect("compiler token stream parse failed")
    }
}

impl From<TokenStream> for proc_macro::TokenStream {
    fn from(inner: TokenStream) -> proc_macro::TokenStream {
        inner.to_string().parse().expect("failed to parse to compiler tokens")
    }
}


impl From<TokenTree> for TokenStream {
    fn from(tree: TokenTree) -> TokenStream {
        TokenStream { inner: vec![tree] }
    }
}

impl iter::FromIterator<TokenStream> for TokenStream {
    fn from_iter<I: IntoIterator<Item=TokenStream>>(streams: I) -> Self {
        let mut v = Vec::new();

        for stream in streams.into_iter() {
            v.extend(stream.inner);
        }

        TokenStream { inner: v }
    }
}

pub type TokenTreeIter = vec::IntoIter<TokenTree>;

impl IntoIterator for TokenStream {
    type Item = TokenTree;
    type IntoIter = TokenTreeIter;

    fn into_iter(self) -> TokenTreeIter {
        self.inner.into_iter()
    }
}

#[cfg(procmacro2_semver_exempt)]
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct FileName(String);

#[cfg(procmacro2_semver_exempt)]
impl fmt::Display for FileName {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.0.fmt(f)
    }
}

#[cfg(procmacro2_semver_exempt)]
#[derive(Clone, PartialEq, Eq)]
pub struct SourceFile {
    name: FileName,
}

#[cfg(procmacro2_semver_exempt)]
impl SourceFile {
    /// Get the path to this source file as a string.
    pub fn path(&self) -> &FileName {
        &self.name
    }

    pub fn is_real(&self) -> bool {
        // XXX(nika): Support real files in the future?
        false
    }
}

#[cfg(procmacro2_semver_exempt)]
impl AsRef<FileName> for SourceFile {
    fn as_ref(&self) -> &FileName {
        self.path()
    }
}

#[cfg(procmacro2_semver_exempt)]
impl fmt::Debug for SourceFile {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_struct("SourceFile")
            .field("path", &self.path())
            .field("is_real", &self.is_real())
            .finish()
    }
}

#[cfg(procmacro2_semver_exempt)]
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct LineColumn {
    pub line: usize,
    pub column: usize,
}

#[cfg(procmacro2_semver_exempt)]
thread_local! {
    static CODEMAP: RefCell<Codemap> = RefCell::new(Codemap {
        // NOTE: We start with a single dummy file which all call_site() and
        // def_site() spans reference.
        files: vec![FileInfo {
            name: "<unspecified>".to_owned(),
            span: Span { lo: 0, hi: 0 },
            lines: vec![0],
        }],
    });
}

#[cfg(procmacro2_semver_exempt)]
struct FileInfo {
    name: String,
    span: Span,
    lines: Vec<usize>,
}

#[cfg(procmacro2_semver_exempt)]
impl FileInfo {
    fn offset_line_column(&self, offset: usize) -> LineColumn {
        assert!(self.span_within(Span { lo: offset as u32, hi: offset as u32 }));
        let offset = offset - self.span.lo as usize;
        match self.lines.binary_search(&offset) {
            Ok(found) => LineColumn {
                line: found + 1,
                column: 0
            },
            Err(idx) => LineColumn {
                line: idx,
                column: offset - self.lines[idx - 1]
            },
        }
    }

    fn span_within(&self, span: Span) -> bool {
        span.lo >= self.span.lo && span.hi <= self.span.hi
    }
}

/// Computes the offsets of each line in the given source string.
#[cfg(procmacro2_semver_exempt)]
fn lines_offsets(s: &str) -> Vec<usize> {
    let mut lines = vec![0];
    let mut prev = 0;
    while let Some(len) = s[prev..].find('\n') {
        prev += len + 1;
        lines.push(prev);
    }
    lines
}

#[cfg(procmacro2_semver_exempt)]
struct Codemap {
    files: Vec<FileInfo>,
}

#[cfg(procmacro2_semver_exempt)]
impl Codemap {
    fn next_start_pos(&self) -> u32 {
        // Add 1 so there's always space between files.
        //
        // We'll always have at least 1 file, as we initialize our files list
        // with a dummy file.
        self.files.last().unwrap().span.hi + 1
    }

    fn add_file(&mut self, name: &str, src: &str) -> Span {
        let lines = lines_offsets(src);
        let lo = self.next_start_pos();
        // XXX(nika): Shouild we bother doing a checked cast or checked add here?
        let span = Span { lo: lo, hi: lo + (src.len() as u32) };

        self.files.push(FileInfo {
            name: name.to_owned(),
            span: span,
            lines: lines,
        });

        span
    }

    fn fileinfo(&self, span: Span) -> &FileInfo {
        for file in &self.files {
            if file.span_within(span) {
                return file;
            }
        }
        panic!("Invalid span with no related FileInfo!");
    }
}

#[derive(Clone, Copy, Debug)]
pub struct Span {
    #[cfg(procmacro2_semver_exempt)]
    lo: u32,
    #[cfg(procmacro2_semver_exempt)]
    hi: u32,
}

impl Span {
    #[cfg(not(procmacro2_semver_exempt))]
    pub fn call_site() -> Span {
        Span {}
    }

    #[cfg(procmacro2_semver_exempt)]
    pub fn call_site() -> Span {
        Span { lo: 0, hi: 0 }
    }

    pub fn def_site() -> Span {
        Span::call_site()
    }

    pub fn resolved_at(&self, _other: Span) -> Span {
        // Stable spans consist only of line/column information, so
        // `resolved_at` and `located_at` only select which span the
        // caller wants line/column information from.
        *self
    }

    pub fn located_at(&self, other: Span) -> Span {
        other
    }

    #[cfg(procmacro2_semver_exempt)]
    pub fn source_file(&self) -> SourceFile {
        CODEMAP.with(|cm| {
            let cm = cm.borrow();
            let fi = cm.fileinfo(*self);
            SourceFile {
                name: FileName(fi.name.clone()),
            }
        })
    }

    #[cfg(procmacro2_semver_exempt)]
    pub fn start(&self) -> LineColumn {
        CODEMAP.with(|cm| {
            let cm = cm.borrow();
            let fi = cm.fileinfo(*self);
            fi.offset_line_column(self.lo as usize)
        })
    }

    #[cfg(procmacro2_semver_exempt)]
    pub fn end(&self) -> LineColumn {
        CODEMAP.with(|cm| {
            let cm = cm.borrow();
            let fi = cm.fileinfo(*self);
            fi.offset_line_column(self.hi as usize)
        })
    }

    #[cfg(procmacro2_semver_exempt)]
    pub fn join(&self, other: Span) -> Option<Span> {
        CODEMAP.with(|cm| {
            let cm = cm.borrow();
            // If `other` is not within the same FileInfo as us, return None.
            if !cm.fileinfo(*self).span_within(other) {
                return None;
            }
            Some(Span {
                lo: cmp::min(self.lo, other.lo),
                hi: cmp::max(self.hi, other.hi),
            })
        })
    }
}

#[derive(Copy, Clone)]
pub struct Term {
    intern: usize,
    not_send_sync: PhantomData<*const ()>,
}

thread_local!(static SYMBOLS: RefCell<Interner> = RefCell::new(Interner::new()));

impl Term {
    pub fn intern(string: &str) -> Term {
        Term {
            intern: SYMBOLS.with(|s| s.borrow_mut().intern(string)),
            not_send_sync: PhantomData,
        }
    }

    pub fn as_str(&self) -> &str {
        SYMBOLS.with(|interner| {
            let interner = interner.borrow();
            let s = interner.get(self.intern);
            unsafe {
                &*(s as *const str)
            }
        })
    }
}

impl fmt::Debug for Term {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        f.debug_tuple("Term").field(&self.as_str()).finish()
    }
}

struct Interner {
    string_to_index: HashMap<MyRc, usize>,
    index_to_string: Vec<Rc<String>>,
}

#[derive(Hash, Eq, PartialEq)]
struct MyRc(Rc<String>);

impl Borrow<str> for MyRc {
    fn borrow(&self) -> &str {
        &self.0
    }
}

impl Interner {
    fn new() -> Interner {
        Interner {
            string_to_index: HashMap::new(),
            index_to_string: Vec::new(),
        }
    }

   fn intern(&mut self, s: &str) -> usize {
        if let Some(&idx) = self.string_to_index.get(s) {
            return idx
        }
        let s = Rc::new(s.to_string());
        self.index_to_string.push(s.clone());
        self.string_to_index.insert(MyRc(s), self.index_to_string.len() - 1);
        self.index_to_string.len() - 1
    }

   fn get(&self, idx: usize) -> &str {
       &self.index_to_string[idx]
   }
}

#[derive(Clone, Debug)]
pub struct Literal(String);

impl Literal {
    pub fn byte_char(byte: u8) -> Literal {
        match byte {
            0 => Literal(format!("b'\\0'")),
            b'\"' => Literal(format!("b'\"'")),
            n => {
                let mut escaped = "b'".to_string();
                escaped.extend(ascii::escape_default(n).map(|c| c as char));
                escaped.push('\'');
                Literal(escaped)
            }
        }
    }

    pub fn byte_string(bytes: &[u8]) -> Literal {
        let mut escaped = "b\"".to_string();
        for b in bytes {
            match *b {
                b'\0' => escaped.push_str(r"\0"),
                b'\t' => escaped.push_str(r"\t"),
                b'\n' => escaped.push_str(r"\n"),
                b'\r' => escaped.push_str(r"\r"),
                b'"' => escaped.push_str("\\\""),
                b'\\' => escaped.push_str("\\\\"),
                b'\x20' ... b'\x7E' => escaped.push(*b as char),
                _ => escaped.push_str(&format!("\\x{:02X}", b)),
            }
        }
        escaped.push('"');
        Literal(escaped)
    }

    pub fn doccomment(s: &str) -> Literal {
        Literal(s.to_string())
    }

    pub fn float(n: f64) -> Literal {
        if !n.is_finite() {
            panic!("Invalid float literal {}", n);
        }
        let mut s = n.to_string();
        if !s.contains('.') {
            s += ".0";
        }
        Literal(s)
    }

    pub fn integer(s: i64) -> Literal {
        Literal(s.to_string())
    }

    pub fn raw_string(s: &str, pounds: usize) -> Literal {
        let mut ret = format!("r");
        ret.extend((0..pounds).map(|_| "#"));
        ret.push('"');
        ret.push_str(s);
        ret.push('"');
        ret.extend((0..pounds).map(|_| "#"));
        Literal(ret)
    }

    pub fn raw_byte_string(s: &str, pounds: usize) -> Literal {
        let mut ret = format!("br");
        ret.extend((0..pounds).map(|_| "#"));
        ret.push('"');
        ret.push_str(s);
        ret.push('"');
        ret.extend((0..pounds).map(|_| "#"));
        Literal(ret)
    }
}

impl fmt::Display for Literal {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        self.0.fmt(f)
    }
}

macro_rules! ints {
    ($($t:ty,)*) => {$(
        impl From<$t> for Literal {
            fn from(t: $t) -> Literal {
                Literal(format!(concat!("{}", stringify!($t)), t))
            }
        }
    )*}
}

ints! {
    u8, u16, u32, u64, usize,
    i8, i16, i32, i64, isize,
}

macro_rules! floats {
    ($($t:ty,)*) => {$(
        impl From<$t> for Literal {
            fn from(t: $t) -> Literal {
                assert!(!t.is_nan());
                assert!(!t.is_infinite());
                Literal(format!(concat!("{}", stringify!($t)), t))
            }
        }
    )*}
}

floats! {
    f32, f64,
}

impl<'a> From<&'a str> for Literal {
    fn from(t: &'a str) -> Literal {
        let mut s = t.chars().flat_map(|c| c.escape_default()).collect::<String>();
        s.push('"');
        s.insert(0, '"');
        Literal(s)
    }
}

impl From<char> for Literal {
    fn from(t: char) -> Literal {
        Literal(format!("'{}'", t.escape_default().collect::<String>()))
    }
}

named!(token_stream -> ::TokenStream, map!(
    many0!(token_tree),
    |trees| ::TokenStream(TokenStream { inner: trees })
));

#[cfg(not(procmacro2_semver_exempt))]
fn token_tree(input: Cursor) -> PResult<TokenTree> {
    let (input, kind) = token_kind(input)?;
    Ok((input, TokenTree {
        span: ::Span(Span {}),
        kind: kind,
    }))
}

#[cfg(procmacro2_semver_exempt)]
fn token_tree(input: Cursor) -> PResult<TokenTree> {
    let input = skip_whitespace(input);
    let lo = input.off;
    let (input, kind) = token_kind(input)?;
    let hi = input.off;
    Ok((input, TokenTree {
        span: ::Span(Span {
            lo: lo,
            hi: hi,
        }),
        kind: kind,
    }))
}

named!(token_kind -> TokenNode, alt!(
    map!(delimited, |(d, s)| TokenNode::Group(d, s))
    |
    map!(literal, TokenNode::Literal) // must be before symbol
    |
    symbol
    |
    map!(op, |(op, kind)| TokenNode::Op(op, kind))
));

named!(delimited -> (Delimiter, ::TokenStream), alt!(
    delimited!(
        punct!("("),
        token_stream,
        punct!(")")
    ) => { |ts| (Delimiter::Parenthesis, ts) }
    |
    delimited!(
        punct!("["),
        token_stream,
        punct!("]")
    ) => { |ts| (Delimiter::Bracket, ts) }
    |
    delimited!(
        punct!("{"),
        token_stream,
        punct!("}")
    ) => { |ts| (Delimiter::Brace, ts) }
));

fn symbol(mut input: Cursor) -> PResult<TokenNode> {
    input = skip_whitespace(input);

    let mut chars = input.char_indices();

    let lifetime = input.starts_with("'");
    if lifetime {
        chars.next();
    }

    match chars.next() {
        Some((_, ch)) if UnicodeXID::is_xid_start(ch) || ch == '_' => {}
        _ => return Err(LexError),
    }

    let mut end = input.len();
    for (i, ch) in chars {
        if !UnicodeXID::is_xid_continue(ch) {
            end = i;
            break;
        }
    }

    if lifetime && &input.rest[..end] != "'static" && KEYWORDS.contains(&&input.rest[1..end]) {
        Err(LexError)
    } else {
        let a = &input.rest[..end];
        if a == "_" {
            Ok((input.advance(end), TokenNode::Op('_', Spacing::Alone)))
        } else {
            Ok((input.advance(end), TokenNode::Term(::Term::intern(a))))
        }
    }
}

// From https://github.com/rust-lang/rust/blob/master/src/libsyntax_pos/symbol.rs
static KEYWORDS: &'static [&'static str] = &[
    "abstract", "alignof", "as", "become", "box", "break", "const", "continue",
    "crate", "do", "else", "enum", "extern", "false", "final", "fn", "for",
    "if", "impl", "in", "let", "loop", "macro", "match", "mod", "move", "mut",
    "offsetof", "override", "priv", "proc", "pub", "pure", "ref", "return",
    "self", "Self", "sizeof", "static", "struct", "super", "trait", "true",
    "type", "typeof", "unsafe", "unsized", "use", "virtual", "where", "while",
    "yield",
];

fn literal(input: Cursor) -> PResult<::Literal> {
    let input_no_ws = skip_whitespace(input);

    match literal_nocapture(input_no_ws) {
        Ok((a, ())) => {
            let start = input.len() - input_no_ws.len();
            let len = input_no_ws.len() - a.len();
            let end = start + len;
            Ok((a, ::Literal(Literal(input.rest[start..end].to_string()))))
        }
        Err(LexError) => Err(LexError),
    }
}

named!(literal_nocapture -> (), alt!(
    string
    |
    byte_string
    |
    byte
    |
    character
    |
    float
    |
    int
    |
    boolean
    |
    doc_comment
));

named!(string -> (), alt!(
    quoted_string
    |
    preceded!(
        punct!("r"),
        raw_string
    ) => { |_| () }
));

named!(quoted_string -> (), delimited!(
    punct!("\""),
    cooked_string,
    tag!("\"")
));

fn cooked_string(input: Cursor) -> PResult<()> {
    let mut chars = input.char_indices().peekable();
    while let Some((byte_offset, ch)) = chars.next() {
        match ch {
            '"' => {
                return Ok((input.advance(byte_offset), ()));
            }
            '\r' => {
                if let Some((_, '\n')) = chars.next() {
                    // ...
                } else {
                    break;
                }
            }
            '\\' => {
                match chars.next() {
                    Some((_, 'x')) => {
                        if !backslash_x_char(&mut chars) {
                            break
                        }
                    }
                    Some((_, 'n')) |
                    Some((_, 'r')) |
                    Some((_, 't')) |
                    Some((_, '\\')) |
                    Some((_, '\'')) |
                    Some((_, '"')) |
                    Some((_, '0')) => {}
                    Some((_, 'u')) => {
                        if !backslash_u(&mut chars) {
                            break
                        }
                    }
                    Some((_, '\n')) | Some((_, '\r')) => {
                        while let Some(&(_, ch)) = chars.peek() {
                            if ch.is_whitespace() {
                                chars.next();
                            } else {
                                break;
                            }
                        }
                    }
                    _ => break,
                }
            }
            _ch => {}
        }
    }
    Err(LexError)
}

named!(byte_string -> (), alt!(
    delimited!(
        punct!("b\""),
        cooked_byte_string,
        tag!("\"")
    ) => { |_| () }
    |
    preceded!(
        punct!("br"),
        raw_string
    ) => { |_| () }
));

fn cooked_byte_string(mut input: Cursor) -> PResult<()> {
    let mut bytes = input.bytes().enumerate();
    'outer: while let Some((offset, b)) = bytes.next() {
        match b {
            b'"' => {
                return Ok((input.advance(offset), ()));
            }
            b'\r' => {
                if let Some((_, b'\n')) = bytes.next() {
                    // ...
                } else {
                    break;
                }
            }
            b'\\' => {
                match bytes.next() {
                    Some((_, b'x')) => {
                        if !backslash_x_byte(&mut bytes) {
                            break
                        }
                    }
                    Some((_, b'n')) |
                    Some((_, b'r')) |
                    Some((_, b't')) |
                    Some((_, b'\\')) |
                    Some((_, b'0')) |
                    Some((_, b'\'')) |
                    Some((_, b'"'))  => {}
                    Some((newline, b'\n')) |
                    Some((newline, b'\r')) => {
                        let rest = input.advance(newline + 1);
                        for (offset, ch) in rest.char_indices() {
                            if !ch.is_whitespace() {
                                input = rest.advance(offset);
                                bytes = input.bytes().enumerate();
                                continue 'outer;
                            }
                        }
                        break;
                    }
                    _ => break,
                }
            }
            b if b < 0x80 => {}
            _ => break,
        }
    }
    Err(LexError)
}

fn raw_string(input: Cursor) -> PResult<()> {
    let mut chars = input.char_indices();
    let mut n = 0;
    while let Some((byte_offset, ch)) = chars.next() {
        match ch {
            '"' => {
                n = byte_offset;
                break;
            }
            '#' => {}
            _ => return Err(LexError),
        }
    }
    for (byte_offset, ch) in chars {
        match ch {
            '"' if input.advance(byte_offset + 1).starts_with(&input.rest[..n]) => {
                let rest = input.advance(byte_offset + 1 + n);
                return Ok((rest, ()))
            }
            '\r' => {}
            _ => {}
        }
    }
    Err(LexError)
}

named!(byte -> (), do_parse!(
    punct!("b") >>
    tag!("'") >>
    cooked_byte >>
    tag!("'") >>
    (())
));

fn cooked_byte(input: Cursor) -> PResult<()> {
    let mut bytes = input.bytes().enumerate();
    let ok = match bytes.next().map(|(_, b)| b) {
        Some(b'\\') => {
            match bytes.next().map(|(_, b)| b) {
                Some(b'x') => backslash_x_byte(&mut bytes),
                Some(b'n') |
                Some(b'r') |
                Some(b't') |
                Some(b'\\') |
                Some(b'0') |
                Some(b'\'') |
                Some(b'"') => true,
                _ => false,
            }
        }
        b => b.is_some(),
    };
    if ok {
        match bytes.next() {
            Some((offset, _)) => Ok((input.advance(offset), ())),
            None => Ok((input.advance(input.len()), ())),
        }
    } else {
        Err(LexError)
    }
}

named!(character -> (), do_parse!(
    punct!("'") >>
    cooked_char >>
    tag!("'") >>
    (())
));

fn cooked_char(input: Cursor) -> PResult<()> {
    let mut chars = input.char_indices();
    let ok = match chars.next().map(|(_, ch)| ch) {
        Some('\\') => {
            match chars.next().map(|(_, ch)| ch) {
                Some('x') => backslash_x_char(&mut chars),
                Some('u') => backslash_u(&mut chars),
                Some('n') |
                Some('r') |
                Some('t') |
                Some('\\') |
                Some('0') |
                Some('\'') |
                Some('"') => true,
                _ => false,
            }
        }
        ch => ch.is_some(),
    };
    if ok {
        match chars.next() {
            Some((idx, _)) => Ok((input.advance(idx), ())),
            None => Ok((input.advance(input.len()), ())),
        }
    } else {
        Err(LexError)
    }
}

macro_rules! next_ch {
    ($chars:ident @ $pat:pat $(| $rest:pat)*) => {
        match $chars.next() {
            Some((_, ch)) => match ch {
                $pat $(| $rest)*  => ch,
                _ => return false,
            },
            None => return false
        }
    };
}

fn backslash_x_char<I>(chars: &mut I) -> bool
    where I: Iterator<Item = (usize, char)>
{
    next_ch!(chars @ '0'...'7');
    next_ch!(chars @ '0'...'9' | 'a'...'f' | 'A'...'F');
    true
}

fn backslash_x_byte<I>(chars: &mut I) -> bool
    where I: Iterator<Item = (usize, u8)>
{
    next_ch!(chars @ b'0'...b'9' | b'a'...b'f' | b'A'...b'F');
    next_ch!(chars @ b'0'...b'9' | b'a'...b'f' | b'A'...b'F');
    true
}

fn backslash_u<I>(chars: &mut I) -> bool
    where I: Iterator<Item = (usize, char)>
{
    next_ch!(chars @ '{');
    next_ch!(chars @ '0'...'9' | 'a'...'f' | 'A'...'F');
    loop {
        let c = next_ch!(chars @ '0'...'9' | 'a'...'f' | 'A'...'F' | '_' | '}');
        if c == '}' {
            return true;
        }
    }
}

fn float(input: Cursor) -> PResult<()> {
    let (rest, ()) = float_digits(input)?;
    for suffix in &["f32", "f64"] {
        if rest.starts_with(suffix) {
            return word_break(rest.advance(suffix.len()));
        }
    }
    word_break(rest)
}

fn float_digits(input: Cursor) -> PResult<()> {
    let mut chars = input.chars().peekable();
    match chars.next() {
        Some(ch) if ch >= '0' && ch <= '9' => {}
        _ => return Err(LexError),
    }

    let mut len = 1;
    let mut has_dot = false;
    let mut has_exp = false;
    while let Some(&ch) = chars.peek() {
        match ch {
            '0'...'9' | '_' => {
                chars.next();
                len += 1;
            }
            '.' => {
                if has_dot {
                    break;
                }
                chars.next();
                if chars.peek()
                       .map(|&ch| ch == '.' || UnicodeXID::is_xid_start(ch))
                       .unwrap_or(false) {
                    return Err(LexError);
                }
                len += 1;
                has_dot = true;
            }
            'e' | 'E' => {
                chars.next();
                len += 1;
                has_exp = true;
                break;
            }
            _ => break,
        }
    }

    let rest = input.advance(len);
    if !(has_dot || has_exp || rest.starts_with("f32") || rest.starts_with("f64")) {
        return Err(LexError);
    }

    if has_exp {
        let mut has_exp_value = false;
        while let Some(&ch) = chars.peek() {
            match ch {
                '+' | '-' => {
                    if has_exp_value {
                        break;
                    }
                    chars.next();
                    len += 1;
                }
                '0'...'9' => {
                    chars.next();
                    len += 1;
                    has_exp_value = true;
                }
                '_' => {
                    chars.next();
                    len += 1;
                }
                _ => break,
            }
        }
        if !has_exp_value {
            return Err(LexError);
        }
    }

    Ok((input.advance(len), ()))
}

fn int(input: Cursor) -> PResult<()> {
    let (rest, ()) = digits(input)?;
    for suffix in &[
        "isize",
        "i8",
        "i16",
        "i32",
        "i64",
        "i128",
        "usize",
        "u8",
        "u16",
        "u32",
        "u64",
        "u128",
    ] {
        if rest.starts_with(suffix) {
            return word_break(rest.advance(suffix.len()));
        }
    }
    word_break(rest)
}

fn digits(mut input: Cursor) -> PResult<()> {
    let base = if input.starts_with("0x") {
        input = input.advance(2);
        16
    } else if input.starts_with("0o") {
        input = input.advance(2);
        8
    } else if input.starts_with("0b") {
        input = input.advance(2);
        2
    } else {
        10
    };

    let mut len = 0;
    let mut empty = true;
    for b in input.bytes() {
        let digit = match b {
            b'0'...b'9' => (b - b'0') as u64,
            b'a'...b'f' => 10 + (b - b'a') as u64,
            b'A'...b'F' => 10 + (b - b'A') as u64,
            b'_' => {
                if empty && base == 10 {
                    return Err(LexError);
                }
                len += 1;
                continue;
            }
            _ => break,
        };
        if digit >= base {
            return Err(LexError);
        }
        len += 1;
        empty = false;
    }
    if empty {
        Err(LexError)
    } else {
        Ok((input.advance(len), ()))
    }
}

named!(boolean -> (), alt!(
    keyword!("true") => { |_| () }
    |
    keyword!("false") => { |_| () }
));

fn op(input: Cursor) -> PResult<(char, Spacing)> {
    let input = skip_whitespace(input);
    match op_char(input) {
        Ok((rest, ch)) => {
            let kind = match op_char(rest) {
                Ok(_) => Spacing::Joint,
                Err(LexError) => Spacing::Alone,
            };
            Ok((rest, (ch, kind)))
        }
        Err(LexError) => Err(LexError),
    }
}

fn op_char(input: Cursor) -> PResult<char> {
    let mut chars = input.chars();
    let first = match chars.next() {
        Some(ch) => ch,
        None => {
            return Err(LexError);
        }
    };
    let recognized = "~!@#$%^&*-=+|;:,<.>/?";
    if recognized.contains(first) {
        Ok((input.advance(first.len_utf8()), first))
    } else {
        Err(LexError)
    }
}

named!(doc_comment -> (), alt!(
    do_parse!(
        punct!("//!") >>
        take_until!("\n") >>
        (())
    )
    |
    do_parse!(
        option!(whitespace) >>
        peek!(tag!("/*!")) >>
        block_comment >>
        (())
    )
    |
    do_parse!(
        punct!("///") >>
        not!(tag!("/")) >>
        take_until!("\n") >>
        (())
    )
    |
    do_parse!(
        option!(whitespace) >>
        peek!(tuple!(tag!("/**"), not!(tag!("*")))) >>
        block_comment >>
        (())
    )
));