Stop leaking implementation detail in Debug impls of opaque types

This changes the `Debug` impl of:
- `core::any::TypeId`
- `core::mem::Discriminant`
- `std::thread::ThreadId`
- `std::time::Instant`

All these types are explicitly described as "opaque" in the
documentation, so they shouldn't print implementation details in their
`Debug` implementation.

The change for `ThreadId` can possibly be reverted in the future, once
the method `thread_id_value` is stabilized:
rust-lang#67939
The same goes for `Discriminant` once/if the `DiscriminantKind` trait
gets stabilized.
But until then, these are opaque types.

Author: LukasKalbertodt

library/core/src/any.rs

@@ -435,7 +435,7 @@ impl dyn Any + Send + Sync {
 /// While `TypeId` implements `Hash`, `PartialOrd`, and `Ord`, it is worth
 /// noting that the hashes and ordering will vary between Rust releases. Beware
 /// of relying on them inside of your code!
-#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
+#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct TypeId {
     t: u64,
@@ -464,6 +464,13 @@ impl TypeId {
     }
 }
 
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Debug for TypeId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("TypeId(_)")
+    }
+}
+
 /// Returns the name of a type as a string slice.
 ///
 /// # Note

library/core/src/mem/mod.rs

@@ -988,7 +988,7 @@ impl<T> hash::Hash for Discriminant<T> {
 #[stable(feature = "discriminant_value", since = "1.21.0")]
 impl<T> fmt::Debug for Discriminant<T> {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt.debug_tuple("Discriminant").field(&self.0).finish()
+        fmt.pad("Discriminant(_)")
     }
 }
 

library/std/src/thread/mod.rs

@@ -985,7 +985,7 @@ pub fn park_timeout(dur: Duration) {
 ///
 /// [`id`]: Thread::id
 #[stable(feature = "thread_id", since = "1.19.0")]
-#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
+#[derive(Eq, PartialEq, Clone, Copy, Hash)]
 pub struct ThreadId(NonZeroU64);
 
 impl ThreadId {
@@ -1025,6 +1025,13 @@ impl ThreadId {
     }
 }
 
+#[stable(feature = "thread_id", since = "1.19.0")]
+impl fmt::Debug for ThreadId {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.pad("ThreadId(_)")
+    }
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // Thread
 ////////////////////////////////////////////////////////////////////////////////

library/std/src/time.rs

@@ -410,7 +410,9 @@ impl Sub<Instant> for Instant {
 #[stable(feature = "time2", since = "1.8.0")]
 impl fmt::Debug for Instant {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        self.0.fmt(f)
+        // Instant is an opaque type and only useful with `Duration`. So it
+        // would be misleading to show implementation detail in the output.
+        f.pad("Instant(_)")
     }
 }