Implement fmt::Debug for all structures in libstd.

Part of https://github.com/rust-lang/rust/issues/31869.

Also turn on the `missing_debug_implementations` lint at the crate
level.

src/libstd/io/util.rs

@@ -10,6 +10,7 @@
 
 #![allow(missing_copy_implementations)]
 
+use fmt;
 use io::{self, Read, Write, ErrorKind, BufRead};
 
 /// Copies the entire contents of a reader into a writer.
@@ -97,6 +98,13 @@ impl BufRead for Empty {
     fn consume(&mut self, _n: usize) {}
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Empty {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Empty { .. }")
+    }
+}
+
 /// A reader which yields one byte over and over and over and over and over and...
 ///
 /// This struct is generally created by calling [`repeat()`][repeat]. Please
@@ -133,6 +141,13 @@ impl Read for Repeat {
     }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Repeat {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Repeat { .. }")
+    }
+}
+
 /// A writer which will move data into the void.
 ///
 /// This struct is generally created by calling [`sink()`][sink]. Please
@@ -165,6 +180,13 @@ impl Write for Sink {
     fn flush(&mut self) -> io::Result<()> { Ok(()) }
 }
 
+#[stable(feature = "std_debug", since = "1.15.0")]
+impl fmt::Debug for Sink {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        f.pad("Sink { .. }")
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use io::prelude::*;