Rollup merge of rust-lang#63805 - mati865:clippy, r=Centril

Apply few Clippy suggestions

Somewhat follow-up of rust-lang#62806

Changes per commit are rather small so I can squash them if that's preferred.

src/build_helper/lib.rs

@@ -262,7 +262,7 @@ pub fn native_lib_boilerplate(
     if !up_to_date(Path::new("build.rs"), &timestamp) || !up_to_date(src_dir, &timestamp) {
         Ok(NativeLibBoilerplate {
             src_dir: src_dir.to_path_buf(),
-            out_dir: out_dir,
+            out_dir,
         })
     } else {
         Err(())

src/libcore/iter/adapters/mod.rs

@@ -1309,7 +1309,7 @@ impl<I> DoubleEndedIterator for Peekable<I> where I: DoubleEndedIterator {
         Self: Sized, F: FnMut(B, Self::Item) -> R, R: Try<Ok=B>
     {
         match self.peeked.take() {
-            Some(None) => return Try::from_ok(init),
+            Some(None) => Try::from_ok(init),
             Some(Some(v)) => match self.iter.try_rfold(init, &mut f).into_result() {
                 Ok(acc) => f(acc, v),
                 Err(e) => {
@@ -1326,7 +1326,7 @@ impl<I> DoubleEndedIterator for Peekable<I> where I: DoubleEndedIterator {
         where Fold: FnMut(Acc, Self::Item) -> Acc,
     {
         match self.peeked {
-            Some(None) => return init,
+            Some(None) => init,
             Some(Some(v)) => {
                 let acc = self.iter.rfold(init, &mut fold);
                 fold(acc, v)

src/libcore/slice/mod.rs

@@ -3026,8 +3026,7 @@ macro_rules! len {
         if size == 0 {
             // This _cannot_ use `unchecked_sub` because we depend on wrapping
             // to represent the length of long ZST slice iterators.
-            let diff = ($self.end as usize).wrapping_sub(start as usize);
-            diff
+            ($self.end as usize).wrapping_sub(start as usize)
         } else {
             // We know that `start <= end`, so can do better than `offset_from`,
             // which needs to deal in signed.  By setting appropriate flags here