Add some comments.

Explaining things that took me some time to work out.

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.

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
                     }
                 }