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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::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.
+///
+/// *This type is available only if Syn is built with the `"parsing"` feature.*
+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`.
+ ///
+ /// *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 {
+ 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()`.
+///
+/// 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));
+
+ 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(_) = *ptr {
+ if ptr == scope {
+ break;
+ }
+ ptr = 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> {
+ 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>, 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, end_offset) = self.entry() {
+ if group.delimiter() == delim {
+ 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, group.span(), 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(),
+ }
+ }
+
+ /// 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(self.ptr.cmp(&other.ptr))
+ } 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 {
+ unsafe {
+ match (&*a.scope, &*b.scope) {
+ (Entry::End(a_offset), Entry::End(b_offset)) => {
+ a.scope.offset(*a_offset) == b.scope.offset(*b_offset)
+ }
+ _ => unreachable!(),
+ }
+ }
+}
+
+#[cfg(any(feature = "full", feature = "derive"))]
+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(),
+ }
+}