Hand-implement impl Debug for NumBuffer to avoid constraining T or printing MaybeUninit

The derived implementation requires `T: Debug`, and doesn't need to.

library/core/src/fmt/num_buffer.rs

@@ -35,7 +35,6 @@ impl_NumBufferTrait! {
 /// A buffer wrapper of which the internal size is based on the maximum
 /// number of digits the associated integer can have.
 #[unstable(feature = "int_format_into", issue = "138215")]
-#[derive(Debug)]
 pub struct NumBuffer<T: NumBufferTrait> {
     // FIXME: Once const generics feature is working, use `T::BUF_SIZE` instead of 40.
     pub(crate) buf: [MaybeUninit<u8>; 40],
@@ -43,6 +42,13 @@ pub struct NumBuffer<T: NumBufferTrait> {
     phantom: core::marker::PhantomData<T>,
 }
 
+#[unstable(feature = "int_format_into", issue = "138215")]
+impl<T: NumBufferTrait> core::fmt::Debug for NumBuffer<T> {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("NumBuffer").finish()
+    }
+}
+
 #[unstable(feature = "int_format_into", issue = "138215")]
 impl<T: NumBufferTrait> NumBuffer<T> {
     /// Initializes internal buffer.