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Overhaul comments in collect_tokens_trailing_token.
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Adding details, clarifying lots of little things, etc. In particular,
the commit adds details of an example. I find this very helpful, because
it's taken me a long time to understand how this code works.
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@nnethercote
nnethercote committed Jul 19, 2024
1 parent ca66495 commit 1dd566a
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Showing 3 changed files with 129 additions and 75 deletions.
13 changes: 5 additions & 8 deletions compiler/rustc_parse/src/parser/attr.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -303,13 +303,9 @@ impl<'a> Parser<'a> {
None
};
if let Some(attr) = attr {
// If we are currently capturing tokens, mark the location of this inner attribute.
// If capturing ends up creating a `LazyAttrTokenStream`, we will include
// this replace range with it, removing the inner attribute from the final
// `AttrTokenStream`. Inner attributes are stored in the parsed AST note.
// During macro expansion, they are selectively inserted back into the
// token stream (the first inner attribute is removed each time we invoke the
// corresponding macro).
// If we are currently capturing tokens (i.e. we are within a call to
// `Parser::collect_tokens_trailing_tokens`) record the token positions of this
// inner attribute, for possible later processing in a `LazyAttrTokenStream`.
if let Capturing::Yes = self.capture_state.capturing {
let end_pos = self.num_bump_calls;
let range = start_pos..end_pos;
Expand Down Expand Up @@ -463,7 +459,8 @@ impl<'a> Parser<'a> {
}
}

/// The attributes are complete if all attributes are either a doc comment or a builtin attribute other than `cfg_attr`
/// The attributes are complete if all attributes are either a doc comment or a
/// builtin attribute other than `cfg_attr`.
pub fn is_complete(attrs: &[ast::Attribute]) -> bool {
attrs.iter().all(|attr| {
attr.is_doc_comment()
Expand Down
190 changes: 123 additions & 67 deletions compiler/rustc_parse/src/parser/attr_wrapper.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -17,12 +17,12 @@ use std::{iter, mem};
///
/// This wrapper prevents direct access to the underlying `ast::AttrVec`.
/// Parsing code can only get access to the underlying attributes
/// by passing an `AttrWrapper` to `collect_tokens_trailing_tokens`.
/// by passing an `AttrWrapper` to `collect_tokens_trailing_token`.
/// This makes it difficult to accidentally construct an AST node
/// (which stores an `ast::AttrVec`) without first collecting tokens.
///
/// This struct has its own module, to ensure that the parser code
/// cannot directly access the `attrs` field
/// cannot directly access the `attrs` field.
#[derive(Debug, Clone)]
pub struct AttrWrapper {
attrs: AttrVec,
Expand Down Expand Up @@ -76,14 +76,13 @@ fn has_cfg_or_cfg_attr(attrs: &[Attribute]) -> bool {
})
}

// Produces a `TokenStream` on-demand. Using `cursor_snapshot`
// and `num_calls`, we can reconstruct the `TokenStream` seen
// by the callback. This allows us to avoid producing a `TokenStream`
// if it is never needed - for example, a captured `macro_rules!`
// argument that is never passed to a proc macro.
// In practice token stream creation happens rarely compared to
// calls to `collect_tokens` (see some statistics in #78736),
// so we are doing as little up-front work as possible.
// From a value of this type we can reconstruct the `TokenStream` seen by the
// `f` callback passed to a call to `Parser::collect_tokens_trailing_token`, by
// replaying the getting of the tokens. This saves us producing a `TokenStream`
// if it is never needed, e.g. a captured `macro_rules!` argument that is never
// passed to a proc macro. In practice, token stream creation happens rarely
// compared to calls to `collect_tokens` (see some statistics in #78736) so we
// are doing as little up-front work as possible.
//
// This also makes `Parser` very cheap to clone, since
// there is no intermediate collection buffer to clone.
Expand Down Expand Up @@ -163,46 +162,55 @@ impl ToAttrTokenStream for LazyAttrTokenStreamImpl {
}

impl<'a> Parser<'a> {
/// Records all tokens consumed by the provided callback,
/// including the current token. These tokens are collected
/// into a `LazyAttrTokenStream`, and returned along with the first part of
/// the callback's result. The second (bool) part of the callback's result
/// indicates if an extra token should be captured, e.g. a comma or
/// Parses code with `f`. If appropriate, it records the tokens (in
/// `LazyAttrTokenStream` form) that were parsed in the result, accessible
/// via the `HasTokens` trait. The second (bool) part of the callback's
/// result indicates if an extra token should be captured, e.g. a comma or
/// semicolon.
///
/// The `attrs` passed in are in `AttrWrapper` form, which is opaque. The
/// `AttrVec` within is passed to `f`. See the comment on `AttrWrapper` for
/// details.
///
/// Note: If your callback consumes an opening delimiter
/// (including the case where you call `collect_tokens`
/// when the current token is an opening delimiter),
/// you must also consume the corresponding closing delimiter.
/// Note: If your callback consumes an opening delimiter (including the
/// case where `self.token` is an opening delimiter on entry to this
/// function), you must also consume the corresponding closing delimiter.
/// E.g. you can consume `something ([{ }])` or `([{}])`, but not `([{}]`.
/// This restriction isn't a problem in practice, because parsed AST items
/// always have matching delimiters.
///
/// That is, you can consume
/// `something ([{ }])` or `([{}])`, but not `([{}]`
///
/// This restriction shouldn't be an issue in practice,
/// since this function is used to record the tokens for
/// a parsed AST item, which always has matching delimiters.
/// The following example code will be used to explain things in comments
/// below. It has an outer attribute and an inner attribute. Parsing it
/// involves two calls to this method, one of which is indirectly
/// recursive.
/// ```ignore (fake attributes)
/// #[cfg_eval] // token pos
/// mod m { // 0.. 3
/// #[cfg_attr(cond1, attr1)] // 3..12
/// fn g() { // 12..17
/// #![cfg_attr(cond2, attr2)] // 17..27
/// let _x = 3; // 27..32
/// } // 32..33
/// } // 33..34
/// ```
pub fn collect_tokens_trailing_token<R: HasAttrs + HasTokens>(
&mut self,
attrs: AttrWrapper,
force_collect: ForceCollect,
f: impl FnOnce(&mut Self, ast::AttrVec) -> PResult<'a, (R, bool)>,
) -> PResult<'a, R> {
// We only bail out when nothing could possibly observe the collected tokens:
// 1. We cannot be force collecting tokens (since force-collecting requires tokens
// by definition
// Skip collection when nothing could observe the collected tokens, i.e.
// all of the following conditions hold.
// - We are not force collecting tokens (because force collection
// requires tokens by definition).
if matches!(force_collect, ForceCollect::No)
// None of our outer attributes can require tokens (e.g. a proc-macro)
// - None of our outer attributes require tokens.
&& attrs.is_complete()
// If our target supports custom inner attributes, then we cannot bail
// out early, since we may need to capture tokens for a custom inner attribute
// invocation.
// - Our target doesn't support custom inner attributes (custom
// inner attribute invocation might require token capturing).
&& !R::SUPPORTS_CUSTOM_INNER_ATTRS
// Never bail out early in `capture_cfg` mode, since there might be `#[cfg]`
// or `#[cfg_attr]` attributes.
// - We are not in `capture_cfg` mode (which requires tokens if
// the parsed node has `#[cfg]` or `#[cfg_attr]` attributes).
&& !self.capture_cfg
{
return Ok(f(self, attrs.attrs)?.0);
Expand All @@ -214,44 +222,55 @@ impl<'a> Parser<'a> {
let has_outer_attrs = !attrs.attrs.is_empty();
let replace_ranges_start = self.capture_state.replace_ranges.len();

// We set and restore `Capturing::Yes` on either side of the call to
// `f`, so we can distinguish the outermost call to
// `collect_tokens_trailing_token` (e.g. parsing `m` in the example
// above) from any inner (indirectly recursive) calls (e.g. parsing `g`
// in the example above). This distinction is used below and in
// `Parser::parse_inner_attributes`.
let (mut ret, capture_trailing) = {
let prev_capturing = mem::replace(&mut self.capture_state.capturing, Capturing::Yes);
let ret_and_trailing = f(self, attrs.attrs);
self.capture_state.capturing = prev_capturing;
ret_and_trailing?
};

// When we're not in `capture-cfg` mode, then bail out early if:
// 1. Our target doesn't support tokens at all (e.g we're parsing an `NtIdent`)
// so there's nothing for us to do.
// 2. Our target already has tokens set (e.g. we've parsed something
// like `#[my_attr] $item`). The actual parsing code takes care of
// prepending any attributes to the nonterminal, so we don't need to
// modify the already captured tokens.
// Note that this check is independent of `force_collect`- if we already
// have tokens, or can't even store them, then there's never a need to
// force collection of new tokens.
// When we're not in `capture_cfg` mode, then skip collecting and
// return early if either of the following conditions hold.
// - `None`: Our target doesn't support tokens at all (e.g. `NtIdent`).
// - `Some(Some(_))`: Our target already has tokens set (e.g. we've
// parsed something like `#[my_attr] $item`). The actual parsing code
// takes care of prepending any attributes to the nonterminal, so we
// don't need to modify the already captured tokens.
//
// Note that this check is independent of `force_collect`. There's no
// need to collect tokens when we don't support tokens or already have
// tokens.
if !self.capture_cfg && matches!(ret.tokens_mut(), None | Some(Some(_))) {
return Ok(ret);
}

// This is very similar to the bail out check at the start of this function.
// Now that we've parsed an AST node, we have more information available.
// This is similar to the "skip collection" check at the start of this
// function, but now that we've parsed an AST node we have more
// information available. (If we return early here that means the
// setup, such as cloning the token cursor, was unnecessary. That's
// hard to avoid.)
//
// Skip collection when nothing could observe the collected tokens, i.e.
// all of the following conditions hold.
// - We are not force collecting tokens.
if matches!(force_collect, ForceCollect::No)
// We now have inner attributes available, so this check is more precise
// than `attrs.is_complete()` at the start of the function.
// As a result, we don't need to check `R::SUPPORTS_CUSTOM_INNER_ATTRS`
// - None of our outer *or* inner attributes require tokens.
// (`attrs` was just outer attributes, but `ret.attrs()` is outer
// and inner attributes. That makes this check more precise than
// `attrs.is_complete()` at the start of the function, and we can
// skip the subsequent check on `R::SUPPORTS_CUSTOM_INNER_ATTRS`.
&& crate::parser::attr::is_complete(ret.attrs())
// Subtle: We call `has_cfg_or_cfg_attr` with the attrs from `ret`.
// This ensures that we consider inner attributes (e.g. `#![cfg]`),
// which require us to have tokens available
// We also call `has_cfg_or_cfg_attr` at the beginning of this function,
// but we only bail out if there's no possibility of inner attributes
// (!R::SUPPORTS_CUSTOM_INNER_ATTRS)
// We only capture about `#[cfg]` or `#[cfg_attr]` in `capture_cfg`
// mode - during normal parsing, we don't need any special capturing
// for those attributes, since they're builtin.
&& !(self.capture_cfg && has_cfg_or_cfg_attr(ret.attrs()))
// - We are not in `capture_cfg` mode, or we are but there are no
// `#[cfg]` or `#[cfg_attr]` attributes. (During normal
// non-`capture_cfg` parsing, we don't need any special capturing
// for those attributes, because they're builtin.)
&& (!self.capture_cfg || !has_cfg_or_cfg_attr(ret.attrs()))
{
return Ok(ret);
}
Expand All @@ -273,7 +292,10 @@ impl<'a> Parser<'a> {

let num_calls = end_pos - start_pos;

// Take the captured ranges for any inner attributes that we parsed.
// Take the captured ranges for any inner attributes that we parsed in
// `Parser::parse_inner_attributes`, and pair them in a `ReplaceRange`
// with `None`, which means the relevant tokens will be removed. (More
// details below.)
let mut inner_attr_replace_ranges = Vec::new();
for inner_attr in ret.attrs().iter().filter(|a| a.style == ast::AttrStyle::Inner) {
if let Some(attr_range) = self.capture_state.inner_attr_ranges.remove(&inner_attr.id) {
Expand All @@ -289,9 +311,9 @@ impl<'a> Parser<'a> {
if replace_ranges_start == replace_ranges_end && inner_attr_replace_ranges.is_empty() {
Box::new([])
} else {
// Grab any replace ranges that occur *inside* the current AST node.
// We will perform the actual replacement when we convert the `LazyAttrTokenStream`
// to an `AttrTokenStream`.
// Grab any replace ranges that occur *inside* the current AST node. We will
// perform the actual replacement only when we convert the `LazyAttrTokenStream` to
// an `AttrTokenStream`.
self.capture_state.replace_ranges[replace_ranges_start..replace_ranges_end]
.iter()
.cloned()
Expand All @@ -300,6 +322,28 @@ impl<'a> Parser<'a> {
.collect()
};

// What is the status here when parsing the example code at the top of this method?
//
// When parsing `g`:
// - `start_pos..end_pos` is `12..33` (`fn g { ... }`, excluding the outer attr).
// - `inner_attr_replace_ranges` has one entry (`5..15`, when counting from `fn`), to
// delete the inner attr's tokens.
// - This entry is put into the lazy tokens for `g`, i.e. deleting the inner attr from
// those tokens (if they get evaluated).
// - Those lazy tokens are also put into an `AttrsTarget` that is appended to `self`'s
// replace ranges at the bottom of this function, for processing when parsing `m`.
// - `replace_ranges_start..replace_ranges_end` is empty.
//
// When parsing `m`:
// - `start_pos..end_pos` is `0..34` (`mod m`, excluding the `#[cfg_eval]` attribute).
// - `inner_attr_replace_ranges` is empty.
// - `replace_range_start..replace_ranges_end` has two entries.
// - One to delete the inner attribute (`17..27`), obtained when parsing `g` (see above).
// - One `AttrsTarget` (added below when parsing `g`) to replace all of `g` (`3..33`,
// including its outer attribute), with:
// - `attrs`: includes the outer and the inner attr.
// - `tokens`: lazy tokens for `g` (with its inner attr deleted).

let tokens = LazyAttrTokenStream::new(LazyAttrTokenStreamImpl {
start_token,
num_calls,
Expand All @@ -323,15 +367,27 @@ impl<'a> Parser<'a> {
{
assert!(!self.break_last_token, "Should not have unglued last token with cfg attr");

// Replace the entire AST node that we just parsed, including attributes, with
// `target`. If this AST node is inside an item that has `#[derive]`, then this will
// allow us to cfg-expand this AST node.
// What is the status here when parsing the example code at the top of this method?
//
// When parsing `g`, we add two entries:
// - The `start_pos..end_pos` (`3..33`) entry has a new `AttrsTarget` with:
// - `attrs`: includes the outer and the inner attr.
// - `tokens`: lazy tokens for `g` (with its inner attr deleted).
// - `inner_attr_replace_ranges` contains the one entry to delete the inner attr's
// tokens (`17..27`).
//
// When parsing `m`, we do nothing here.

// Set things up so that the entire AST node that we just parsed, including attributes,
// will be replaced with `target` in the lazy token stream. This will allow us to
// cfg-expand this AST node.
let start_pos = if has_outer_attrs { attrs.start_pos } else { start_pos };
let target = AttrsTarget { attrs: ret.attrs().iter().cloned().collect(), tokens };
self.capture_state.replace_ranges.push((start_pos..end_pos, Some(target)));
self.capture_state.replace_ranges.extend(inner_attr_replace_ranges);
} else if matches!(self.capture_state.capturing, Capturing::No) {
// Only clear the ranges once we've finished capturing entirely.
// Only clear the ranges once we've finished capturing entirely, i.e. we've finished
// the outermost call to this method.
self.capture_state.replace_ranges.clear();
self.capture_state.inner_attr_ranges.clear();
}
Expand Down
1 change: 1 addition & 0 deletions compiler/rustc_parse/src/parser/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -221,6 +221,7 @@ enum Capturing {
Yes,
}

// This state is used by `Parser::collect_tokens_trailing_token`.
#[derive(Clone, Debug)]
struct CaptureState {
capturing: Capturing,
Expand Down

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