Use filter_map instead of flat_map in configure_tokens.

All the branches produce either zero or one elements.

compiler/rustc_expand/src/config.rs

@@ -185,22 +185,22 @@ impl<'a> StripUnconfigured<'a> {
  let trees: Vec<_> = stream
  .0
  .iter()
- .flat_map(|tree| match tree.clone() {
+ .filter_map(|tree| match tree.clone() {
  AttrTokenTree::AttrsTarget(mut target) => {
  target.attrs.flat_map_in_place(|attr| self.process_cfg_attr(&attr));
 
  if self.in_cfg(&target.attrs) {
  target.tokens = LazyAttrTokenStream::new(
  self.configure_tokens(&target.tokens.to_attr_token_stream()),
  );
- Some(AttrTokenTree::AttrsTarget(target)).into_iter()
+ Some(AttrTokenTree::AttrsTarget(target))
  } else {
- None.into_iter()
+ None
  }
  }
  AttrTokenTree::Delimited(sp, spacing, delim, mut inner) => {
  inner = self.configure_tokens(&inner);
- Some(AttrTokenTree::Delimited(sp, spacing, delim, inner)).into_iter()
+ Some(AttrTokenTree::Delimited(sp, spacing, delim, inner))
  }
  AttrTokenTree::Token(
  Token {
@@ -220,9 +220,7 @@ impl<'a> StripUnconfigured<'a> {
  ) => {
  panic!("Should be `AttrTokenTree::Delimited`, not delim tokens: {:?}", tree);
  }
- AttrTokenTree::Token(token, spacing) => {
- Some(AttrTokenTree::Token(token, spacing)).into_iter()
- }
+ AttrTokenTree::Token(token, spacing) => Some(AttrTokenTree::Token(token, spacing)),
  })
  .collect();
  AttrTokenStream::new(trees)