Rollup merge of #112670 - petrochenkov:typriv, r=eholk

privacy: Type privacy lints fixes and cleanups

See individual commits.
Follow up to rust-lang/rust#111801.

core/src/iter/adapters/flatten.rs

@@ -310,7 +310,7 @@ where
 /// Real logic of both `Flatten` and `FlatMap` which simply delegate to
 /// this type.
 #[derive(Clone, Debug)]
-#[unstable(feature = "trusted_len", issue = "37572")]
+#[cfg_attr(bootstrap, unstable(feature = "trusted_len", issue = "37572"))]
 struct FlattenCompat<I, U> {
     iter: Fuse<I>,
     frontiter: Option<U>,
@@ -464,7 +464,7 @@ where
     }
 }
 
-#[unstable(feature = "trusted_len", issue = "37572")]
+#[cfg_attr(bootstrap, unstable(feature = "trusted_len", issue = "37572"))]
 impl<I, U> Iterator for FlattenCompat<I, U>
 where
     I: Iterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
@@ -579,7 +579,7 @@ where
     }
 }
 
-#[unstable(feature = "trusted_len", issue = "37572")]
+#[cfg_attr(bootstrap, unstable(feature = "trusted_len", issue = "37572"))]
 impl<I, U> DoubleEndedIterator for FlattenCompat<I, U>
 where
     I: DoubleEndedIterator<Item: IntoIterator<IntoIter = U, Item = U::Item>>,
@@ -649,23 +649,23 @@ where
     }
 }
 
-#[unstable(feature = "trusted_len", issue = "37572")]
+#[cfg_attr(bootstrap, unstable(feature = "trusted_len", issue = "37572"))]
 unsafe impl<const N: usize, I, T> TrustedLen
     for FlattenCompat<I, <[T; N] as IntoIterator>::IntoIter>
 where
     I: TrustedLen<Item = [T; N]>,
 {
 }
 
-#[unstable(feature = "trusted_len", issue = "37572")]
+#[cfg_attr(bootstrap, unstable(feature = "trusted_len", issue = "37572"))]
 unsafe impl<'a, const N: usize, I, T> TrustedLen
     for FlattenCompat<I, <&'a [T; N] as IntoIterator>::IntoIter>
 where
     I: TrustedLen<Item = &'a [T; N]>,
 {
 }
 
-#[unstable(feature = "trusted_len", issue = "37572")]
+#[cfg_attr(bootstrap, unstable(feature = "trusted_len", issue = "37572"))]
 unsafe impl<'a, const N: usize, I, T> TrustedLen
     for FlattenCompat<I, <&'a mut [T; N] as IntoIterator>::IntoIter>
 where