Rollup merge of #104163 - H4x5:once-repeat-with-debug, r=dtolnay

Don't derive Debug for `OnceWith` & `RepeatWith`

Closures don't impl Debug, so the derived impl is kinda useless. The behavior of not debug-printing closures is consistent with the rest of the iterator adapters/sources.

library/core/src/iter/sources/once_with.rs

@@ -1,3 +1,4 @@
+use crate::fmt;
 use crate::iter::{FusedIterator, TrustedLen};
 
 /// Creates an iterator that lazily generates a value exactly once by invoking
@@ -66,12 +67,23 @@ pub fn once_with<A, F: FnOnce() -> A>(gen: F) -> OnceWith<F> {
 ///
 /// This `struct` is created by the [`once_with()`] function.
 /// See its documentation for more.
-#[derive(Clone, Debug)]
+#[derive(Clone)]
 #[stable(feature = "iter_once_with", since = "1.43.0")]
 pub struct OnceWith<F> {
     gen: Option<F>,
 }
 
+#[stable(feature = "iter_once_with_debug", since = "CURRENT_RUSTC_VERSION")]
+impl<F> fmt::Debug for OnceWith<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        if self.gen.is_some() {
+            f.write_str("OnceWith(Some(_))")
+        } else {
+            f.write_str("OnceWith(None)")
+        }
+    }
+}
+
 #[stable(feature = "iter_once_with", since = "1.43.0")]
 impl<A, F: FnOnce() -> A> Iterator for OnceWith<F> {
     type Item = A;

library/core/src/iter/sources/repeat_with.rs

@@ -1,3 +1,4 @@
+use crate::fmt;
 use crate::iter::{FusedIterator, TrustedLen};
 use crate::ops::Try;
 
@@ -71,12 +72,19 @@ pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F> {
 ///
 /// This `struct` is created by the [`repeat_with()`] function.
 /// See its documentation for more.
-#[derive(Copy, Clone, Debug)]
+#[derive(Copy, Clone)]
 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
 pub struct RepeatWith<F> {
     repeater: F,
 }
 
+#[stable(feature = "iterator_repeat_with_debug", since = "CURRENT_RUSTC_VERSION")]
+impl<F> fmt::Debug for RepeatWith<F> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("RepeatWith").finish_non_exhaustive()
+    }
+}
+
 #[stable(feature = "iterator_repeat_with", since = "1.28.0")]
 impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> {
     type Item = A;