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//! A stably addressed token buffer supporting efficient traversal based on a
//! cheaply copyable cursor.
//!
//! *This module is available only if Syn is built with the `"parsing"` feature.*
// 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.
#[cfg(all(
not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
feature = "proc-macro"
))]
use crate::proc_macro as pm;
use crate::Lifetime;
use proc_macro2::{Delimiter, Group, Ident, Literal, Punct, Spacing, Span, TokenStream, TokenTree};
use std::marker::PhantomData;
use std::ptr;
use std::slice;
/// Internal type which is used instead of `TokenTree` to represent a token tree
/// within a `TokenBuffer`.
enum Entry {
// Mimicking types from proc-macro.
Group(Group, TokenBuffer),
Ident(Ident),
Punct(Punct),
Literal(Literal),
// End entries contain a raw pointer to the entry from the containing
// token tree, or null if this is the outermost level.
End(*const Entry),
}
/// A buffer that can be efficiently traversed multiple times, unlike
/// `TokenStream` which requires a deep copy in order to traverse more than
/// once.
///
/// *This type is available only if Syn is built with the `"parsing"` feature.*
pub struct TokenBuffer {
// NOTE: Do not implement clone on this - there are raw pointers inside
// these entries which will be messed up. Moving the `TokenBuffer` itself is
// safe as the data pointed to won't be moved.
ptr: *const Entry,
len: usize,
}
impl Drop for TokenBuffer {
fn drop(&mut self) {
unsafe {
let slice = slice::from_raw_parts_mut(self.ptr as *mut Entry, self.len);
let _ = Box::from_raw(slice);
}
}
}
impl TokenBuffer {
// NOTE: Do not mutate the Vec returned from this function once it returns;
// the address of its backing memory must remain stable.
fn inner_new(stream: TokenStream, up: *const Entry) -> TokenBuffer {
// Build up the entries list, recording the locations of any Groups
// in the list to be processed later.
let mut entries = Vec::new();
let mut groups = Vec::new();
for tt in stream {
match tt {
TokenTree::Ident(sym) => {
entries.push(Entry::Ident(sym));
}
TokenTree::Punct(op) => {
entries.push(Entry::Punct(op));
}
TokenTree::Literal(l) => {
entries.push(Entry::Literal(l));
}
TokenTree::Group(g) => {
// Record the index of the interesting entry, and store an
// `End(null)` there temporarily.
groups.push((entries.len(), g));
entries.push(Entry::End(ptr::null()));
}
}
}
// Add an `End` entry to the end with a reference to the enclosing token
// stream which was passed in.
entries.push(Entry::End(up));
// NOTE: This is done to ensure that we don't accidentally modify the
// length of the backing buffer. The backing buffer must remain at a
// constant address after this point, as we are going to store a raw
// pointer into it.
let mut entries = entries.into_boxed_slice();
for (idx, group) in groups {
// We know that this index refers to one of the temporary
// `End(null)` entries, and we know that the last entry is
// `End(up)`, so the next index is also valid.
let group_up = unsafe { entries.as_ptr().add(idx + 1) };
// The end entry stored at the end of this Entry::Group should
// point to the Entry which follows the Group in the list.
let inner = Self::inner_new(group.stream(), group_up);
entries[idx] = Entry::Group(group, inner);
}
let len = entries.len();
let ptr = Box::into_raw(entries);
TokenBuffer {
ptr: ptr as *const Entry,
len,
}
}
/// Creates a `TokenBuffer` containing all the tokens from the input
/// `proc_macro::TokenStream`.
///
/// *This method is available only if Syn is built with both the `"parsing"` and
/// `"proc-macro"` features.*
#[cfg(all(
not(all(target_arch = "wasm32", any(target_os = "unknown", target_os = "wasi"))),
feature = "proc-macro"
))]
pub fn new(stream: pm::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 {
Self::inner_new(stream, ptr::null())
}
/// 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 {
unsafe { Cursor::create(self.ptr, self.ptr.add(self.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()`.
///
/// Two cursors are equal if they have the same location in the same input
/// stream, and have the same scope.
///
/// *This type is available only if Syn is built with the `"parsing"` feature.*
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 as *const Entry));
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(exit) = *ptr {
if ptr == scope {
break;
}
ptr = exit;
}
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`.
unsafe fn bump(self) -> Cursor<'a> {
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, buf) = self.entry() {
if group.delimiter() == Delimiter::None {
// NOTE: We call `Cursor::create` here to make sure that
// situations where we should immediately exit the span after
// entering it are handled correctly.
unsafe {
*self = Cursor::create(buf.ptr, self.scope);
}
} 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>, Span, 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, buf) = self.entry() {
if group.delimiter() == delim {
return Some((buf.begin(), group.span(), unsafe { self.bump() }));
}
}
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() })),
_ => 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(op) if op.as_char() != '\'' => Some((op.clone(), unsafe { self.bump() })),
_ => 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(lit) => Some((lit.clone(), unsafe { self.bump() })),
_ => 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(op) if op.as_char() == '\'' && op.spacing() == Spacing::Joint => {
let next = unsafe { self.bump() };
match next.ident() {
Some((ident, rest)) => {
let lifetime = Lifetime {
apostrophe: op.span(),
ident,
};
Some((lifetime, rest))
}
None => None,
}
}
_ => 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 = match self.entry() {
Entry::Group(group, _) => group.clone().into(),
Entry::Literal(lit) => lit.clone().into(),
Entry::Ident(ident) => ident.clone().into(),
Entry::Punct(op) => op.clone().into(),
Entry::End(..) => return None,
};
Some((tree, unsafe { self.bump() }))
}
/// 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(l) => l.span(),
Entry::Ident(t) => t.span(),
Entry::Punct(o) => o.span(),
Entry::End(..) => Span::call_site(),
}
}
/// 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>> {
match self.entry() {
Entry::End(..) => None,
// Treat lifetimes as a single tt for the purposes of 'skip'.
Entry::Punct(op) if op.as_char() == '\'' && op.spacing() == Spacing::Joint => {
let next = unsafe { self.bump() };
match next.entry() {
Entry::Ident(_) => Some(unsafe { next.bump() }),
_ => Some(next),
}
}
_ => Some(unsafe { self.bump() }),
}
}
}
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 {
let Cursor { ptr, scope, marker } = self;
let _ = marker;
*ptr == other.ptr && *scope == other.scope
}
}
pub(crate) fn same_scope(a: Cursor, b: Cursor) -> bool {
a.scope == b.scope
}
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(),
}
}