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//! A stably addressed token buffer supporting efficient traversal based on a
//! cheaply copyable cursor.
// This module is heavily commented as it contains most of the unsafe code in
// Syn, and caution should be used when editing it. The public-facing interface
// is 100% safe but the implementation is fragile internally.
use crate::Lifetime;
use proc_macro2::extra::DelimSpan;
use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
use std::cmp::Ordering;
use std::marker::PhantomData;
/// Internal type which is used instead of `TokenTree` to represent a token tree
/// within a `TokenBuffer`.
enum Entry {
// Mimicking types from proc-macro.
// Group entries contain the offset to the matching End entry.
Group(Group, usize),
Ident(Ident),
Punct(Punct),
Literal(Literal),
// End entries contain the offset (negative) to the start of the buffer.
End(isize),
}
/// A buffer that can be efficiently traversed multiple times, unlike
/// `TokenStream` which requires a deep copy in order to traverse more than
/// once.
pub struct TokenBuffer {
// NOTE: Do not implement clone on this - while the current design could be
// cloned, other designs which could be desirable may not be cloneable.
entries: Box<[Entry]>,
}
impl TokenBuffer {
fn recursive_new(entries: &mut Vec<Entry>, stream: TokenStream) {
for tt in stream {
match tt {
TokenTree::Ident(ident) => entries.push(Entry::Ident(ident)),
TokenTree::Punct(punct) => entries.push(Entry::Punct(punct)),
TokenTree::Literal(literal) => entries.push(Entry::Literal(literal)),
TokenTree::Group(group) => {
let group_start_index = entries.len();
entries.push(Entry::End(0)); // we replace this below
Self::recursive_new(entries, group.stream());
let group_end_index = entries.len();
entries.push(Entry::End(-(group_end_index as isize)));
let group_end_offset = group_end_index - group_start_index;
entries[group_start_index] = Entry::Group(group, group_end_offset);
}
}
}
}
/// Creates a `TokenBuffer` containing all the tokens from the input
/// `proc_macro::TokenStream`.
#[cfg(feature = "proc-macro")]
#[cfg_attr(doc_cfg, doc(cfg(feature = "proc-macro")))]
pub fn new(stream: proc_macro::TokenStream) -> Self {
Self::new2(stream.into())
}
/// Creates a `TokenBuffer` containing all the tokens from the input
/// `proc_macro2::TokenStream`.
pub fn new2(stream: TokenStream) -> Self {
let mut entries = Vec::new();
Self::recursive_new(&mut entries, stream);
entries.push(Entry::End(-(entries.len() as isize)));
Self {
entries: entries.into_boxed_slice(),
}
}
/// Creates a cursor referencing the first token in the buffer and able to
/// traverse until the end of the buffer.
pub fn begin(&self) -> Cursor {
let ptr = self.entries.as_ptr();
unsafe { Cursor::create(ptr, ptr.add(self.entries.len() - 1)) }
}
}
/// A cheaply copyable cursor into a `TokenBuffer`.
///
/// This cursor holds a shared reference into the immutable data which is used
/// internally to represent a `TokenStream`, and can be efficiently manipulated
/// and copied around.
///
/// An empty `Cursor` can be created directly, or one may create a `TokenBuffer`
/// object and get a cursor to its first token with `begin()`.
pub struct Cursor<'a> {
// The current entry which the `Cursor` is pointing at.
ptr: *const Entry,
// This is the only `Entry::End` object which this cursor is allowed to
// point at. All other `End` objects are skipped over in `Cursor::create`.
scope: *const Entry,
// Cursor is covariant in 'a. This field ensures that our pointers are still
// valid.
marker: PhantomData<&'a Entry>,
}
impl<'a> Cursor<'a> {
/// Creates a cursor referencing a static empty TokenStream.
pub fn empty() -> Self {
// It's safe in this situation for us to put an `Entry` object in global
// storage, despite it not actually being safe to send across threads
// (`Ident` is a reference into a thread-local table). This is because
// this entry never includes a `Ident` object.
//
// This wrapper struct allows us to break the rules and put a `Sync`
// object in global storage.
struct UnsafeSyncEntry(Entry);
unsafe impl Sync for UnsafeSyncEntry {}
static EMPTY_ENTRY: UnsafeSyncEntry = UnsafeSyncEntry(Entry::End(0));
Cursor {
ptr: &EMPTY_ENTRY.0,
scope: &EMPTY_ENTRY.0,
marker: PhantomData,
}
}
/// This create method intelligently exits non-explicitly-entered
/// `None`-delimited scopes when the cursor reaches the end of them,
/// allowing for them to be treated transparently.
unsafe fn create(mut ptr: *const Entry, scope: *const Entry) -> Self {
// NOTE: If we're looking at a `End`, we want to advance the cursor
// past it, unless `ptr == scope`, which means that we're at the edge of
// our cursor's scope. We should only have `ptr != scope` at the exit
// from None-delimited groups entered with `ignore_none`.
while let Entry::End(_) = unsafe { &*ptr } {
if ptr == scope {
break;
}
ptr = unsafe { ptr.add(1) };
}
Cursor {
ptr,
scope,
marker: PhantomData,
}
}
/// Get the current entry.
fn entry(self) -> &'a Entry {
unsafe { &*self.ptr }
}
/// Bump the cursor to point at the next token after the current one. This
/// is undefined behavior if the cursor is currently looking at an
/// `Entry::End`.
///
/// If the cursor is looking at an `Entry::Group`, the bumped cursor will
/// point at the first token in the group (with the same scope end).
unsafe fn bump_ignore_group(self) -> Cursor<'a> {
unsafe { Cursor::create(self.ptr.offset(1), self.scope) }
}
/// While the cursor is looking at a `None`-delimited group, move it to look
/// at the first token inside instead. If the group is empty, this will move
/// the cursor past the `None`-delimited group.
///
/// WARNING: This mutates its argument.
fn ignore_none(&mut self) {
while let Entry::Group(group, _) = self.entry() {
if group.delimiter() == Delimiter::None {
unsafe { *self = self.bump_ignore_group() };
} else {
break;
}
}
}
/// Checks whether the cursor is currently pointing at the end of its valid
/// scope.
pub fn eof(self) -> bool {
// We're at eof if we're at the end of our scope.
self.ptr == self.scope
}
/// If the cursor is pointing at a `Group` with the given delimiter, returns
/// a cursor into that group and one pointing to the next `TokenTree`.
pub fn group(mut self, delim: Delimiter) -> Option<(Cursor<'a>, DelimSpan, Cursor<'a>)> {
// If we're not trying to enter a none-delimited group, we want to
// ignore them. We have to make sure to _not_ ignore them when we want
// to enter them, of course. For obvious reasons.
if delim != Delimiter::None {
self.ignore_none();
}
if let Entry::Group(group, end_offset) = self.entry() {
if group.delimiter() == delim {
let span = group.delim_span();
let end_of_group = unsafe { self.ptr.add(*end_offset) };
let inside_of_group = unsafe { Cursor::create(self.ptr.add(1), end_of_group) };
let after_group = unsafe { Cursor::create(end_of_group, self.scope) };
return Some((inside_of_group, span, after_group));
}
}
None
}
pub(crate) fn any_group(self) -> Option<(Cursor<'a>, Delimiter, DelimSpan, Cursor<'a>)> {
if let Entry::Group(group, end_offset) = self.entry() {
let delimiter = group.delimiter();
let span = group.delim_span();
let end_of_group = unsafe { self.ptr.add(*end_offset) };
let inside_of_group = unsafe { Cursor::create(self.ptr.add(1), end_of_group) };
let after_group = unsafe { Cursor::create(end_of_group, self.scope) };
return Some((inside_of_group, delimiter, span, after_group));
}
None
}
pub(crate) fn any_group_token(self) -> Option<(Group, Cursor<'a>)> {
if let Entry::Group(group, end_offset) = self.entry() {
let end_of_group = unsafe { self.ptr.add(*end_offset) };
let after_group = unsafe { Cursor::create(end_of_group, self.scope) };
return Some((group.clone(), after_group));
}
None
}
/// If the cursor is pointing at a `Ident`, returns it along with a cursor
/// pointing at the next `TokenTree`.
pub fn ident(mut self) -> Option<(Ident, Cursor<'a>)> {
self.ignore_none();
match self.entry() {
Entry::Ident(ident) => Some((ident.clone(), unsafe { self.bump_ignore_group() })),
_ => None,
}
}
/// If the cursor is pointing at a `Punct`, returns it along with a cursor
/// pointing at the next `TokenTree`.
pub fn punct(mut self) -> Option<(Punct, Cursor<'a>)> {
self.ignore_none();
match self.entry() {
Entry::Punct(punct) if punct.as_char() != '\'' => {
Some((punct.clone(), unsafe { self.bump_ignore_group() }))
}
_ => None,
}
}
/// If the cursor is pointing at a `Literal`, return it along with a cursor
/// pointing at the next `TokenTree`.
pub fn literal(mut self) -> Option<(Literal, Cursor<'a>)> {
self.ignore_none();
match self.entry() {
Entry::Literal(literal) => Some((literal.clone(), unsafe { self.bump_ignore_group() })),
_ => None,
}
}
/// If the cursor is pointing at a `Lifetime`, returns it along with a
/// cursor pointing at the next `TokenTree`.
pub fn lifetime(mut self) -> Option<(Lifetime, Cursor<'a>)> {
self.ignore_none();
match self.entry() {
Entry::Punct(punct) if punct.as_char() == '\'' && punct.spacing() == Spacing::Joint => {
let next = unsafe { self.bump_ignore_group() };
let (ident, rest) = next.ident()?;
let lifetime = Lifetime {
apostrophe: punct.span(),
ident,
};
Some((lifetime, rest))
}
_ => None,
}
}
/// Copies all remaining tokens visible from this cursor into a
/// `TokenStream`.
pub fn token_stream(self) -> TokenStream {
let mut tts = Vec::new();
let mut cursor = self;
while let Some((tt, rest)) = cursor.token_tree() {
tts.push(tt);
cursor = rest;
}
tts.into_iter().collect()
}
/// If the cursor is pointing at a `TokenTree`, returns it along with a
/// cursor pointing at the next `TokenTree`.
///
/// Returns `None` if the cursor has reached the end of its stream.
///
/// This method does not treat `None`-delimited groups as transparent, and
/// will return a `Group(None, ..)` if the cursor is looking at one.
pub fn token_tree(self) -> Option<(TokenTree, Cursor<'a>)> {
let (tree, len) = match self.entry() {
Entry::Group(group, end_offset) => (group.clone().into(), *end_offset),
Entry::Literal(literal) => (literal.clone().into(), 1),
Entry::Ident(ident) => (ident.clone().into(), 1),
Entry::Punct(punct) => (punct.clone().into(), 1),
Entry::End(_) => return None,
};
let rest = unsafe { Cursor::create(self.ptr.add(len), self.scope) };
Some((tree, rest))
}
/// Returns the `Span` of the current token, or `Span::call_site()` if this
/// cursor points to eof.
pub fn span(self) -> Span {
match self.entry() {
Entry::Group(group, _) => group.span(),
Entry::Literal(literal) => literal.span(),
Entry::Ident(ident) => ident.span(),
Entry::Punct(punct) => punct.span(),
Entry::End(_) => Span::call_site(),
}
}
/// Returns the `Span` of the token immediately prior to the position of
/// this cursor, or of the current token if there is no previous one.
#[cfg(any(feature = "full", feature = "derive"))]
pub(crate) fn prev_span(mut self) -> Span {
if start_of_buffer(self) < self.ptr {
self.ptr = unsafe { self.ptr.offset(-1) };
if let Entry::End(_) = self.entry() {
// Locate the matching Group begin token.
let mut depth = 1;
loop {
self.ptr = unsafe { self.ptr.offset(-1) };
match self.entry() {
Entry::Group(group, _) => {
depth -= 1;
if depth == 0 {
return group.span();
}
}
Entry::End(_) => depth += 1,
Entry::Literal(_) | Entry::Ident(_) | Entry::Punct(_) => {}
}
}
}
}
self.span()
}
/// Skip over the next token without cloning it. Returns `None` if this
/// cursor points to eof.
///
/// This method treats `'lifetimes` as a single token.
pub(crate) fn skip(self) -> Option<Cursor<'a>> {
let len = match self.entry() {
Entry::End(_) => return None,
// Treat lifetimes as a single tt for the purposes of 'skip'.
Entry::Punct(punct) if punct.as_char() == '\'' && punct.spacing() == Spacing::Joint => {
match unsafe { &*self.ptr.add(1) } {
Entry::Ident(_) => 2,
_ => 1,
}
}
Entry::Group(_, end_offset) => *end_offset,
_ => 1,
};
Some(unsafe { Cursor::create(self.ptr.add(len), self.scope) })
}
}
impl<'a> Copy for Cursor<'a> {}
impl<'a> Clone for Cursor<'a> {
fn clone(&self) -> Self {
*self
}
}
impl<'a> Eq for Cursor<'a> {}
impl<'a> PartialEq for Cursor<'a> {
fn eq(&self, other: &Self) -> bool {
self.ptr == other.ptr
}
}
impl<'a> PartialOrd for Cursor<'a> {
fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
if same_buffer(*self, *other) {
Some(cmp_assuming_same_buffer(*self, *other))
} else {
None
}
}
}
pub(crate) fn same_scope(a: Cursor, b: Cursor) -> bool {
a.scope == b.scope
}
pub(crate) fn same_buffer(a: Cursor, b: Cursor) -> bool {
start_of_buffer(a) == start_of_buffer(b)
}
fn start_of_buffer(cursor: Cursor) -> *const Entry {
unsafe {
match &*cursor.scope {
Entry::End(offset) => cursor.scope.offset(*offset),
_ => unreachable!(),
}
}
}
pub(crate) fn cmp_assuming_same_buffer(a: Cursor, b: Cursor) -> Ordering {
a.ptr.cmp(&b.ptr)
}
pub(crate) fn open_span_of_group(cursor: Cursor) -> Span {
match cursor.entry() {
Entry::Group(group, _) => group.span_open(),
_ => cursor.span(),
}
}
pub(crate) fn close_span_of_group(cursor: Cursor) -> Span {
match cursor.entry() {
Entry::Group(group, _) => group.span_close(),
_ => cursor.span(),
}
}