Add some comments.

Explaining things that took me some time to work out.
rust-lang · bors · Jul 14, 2024 · Apr 14, 2024 · Apr 14, 2024 · Apr 14, 2024
commit 478ba5902629b0a4dda914e81167e6b3b44c3a51
diff --git a/compiler/rustc_ast/src/tokenstream.rs b/compiler/rustc_ast/src/tokenstream.rs
@@ -269,11 269,14 @@ fn attrs_and_tokens_to_token_trees(
 /// with its attributes.
 ///
 /// This is constructed during parsing when we need to capture
-/// tokens.
 /// tokens, for `cfg` and `cfg_attr` attributes.
 ///
 /// For example, `#[cfg(FALSE)] struct Foo {}` would
 /// have an `attrs` field containing the `#[cfg(FALSE)]` attr,
 /// and a `tokens` field storing the (unparsed) tokens `struct Foo {}`
 ///
 /// The `cfg`/`cfg_attr` processing occurs in
 /// `StripUnconfigured::configure_tokens`.
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct AttrsTarget {
  /// Attributes, both outer and inner.

diff --git a/compiler/rustc_expand/src/config.rs b/compiler/rustc_expand/src/config.rs
@@ -187,6 187,7 @@ impl<'a> StripUnconfigured<'a> {
  .iter()
  .filter_map(|tree| match tree.clone() {
  AttrTokenTree::AttrsTarget(mut target) => {
  // Expand any `cfg_attr` attributes.
  target.attrs.flat_map_in_place(|attr| self.process_cfg_attr(&attr));
 
  if self.in_cfg(&target.attrs) {
@@ -195,6 196,8 @@ impl<'a> StripUnconfigured<'a> {
  );
  Some(AttrTokenTree::AttrsTarget(target))
  } else {
  // Remove the target if there's a `cfg` attribute and
  // the condition isn't satisfied.
  None
  }
  }