1 files changed, 109 insertions, 0 deletions
diff --git a/library/core/src/asserting.rs b/library/core/src/asserting.rs
new file mode 100644
index 00000000000..212b637d343
--- /dev/null
+++ b/library/core/src/asserting.rs
@@ -0,0 +1,109 @@
+// Contains the machinery necessary to print useful `assert!` messages. Not intended for public
+// usage, not even nightly use-cases.
+//
+// Based on https://github.com/dtolnay/case-studies/tree/master/autoref-specialization. When
+// 'specialization' is robust enough (5 years? 10 years? Never?), `Capture` can be specialized
+// to [Printable].
+
+#![allow(missing_debug_implementations)]
+#![doc(hidden)]
+#![unstable(feature = "generic_assert_internals", issue = "44838")]
+
+use crate::{
+    fmt::{Debug, Formatter},
+    marker::PhantomData,
+};
+
+// ***** TryCapture - Generic *****
+
+/// Marker used by [Capture]
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub struct TryCaptureWithoutDebug;
+
+/// Catches an arbitrary `E` and modifies `to` accordingly
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub trait TryCaptureGeneric<E, M> {
+    /// Similar to [TryCapturePrintable] but generic to any `E`.
+    fn try_capture(&self, to: &mut Capture<E, M>);
+}
+
+impl<E> TryCaptureGeneric<E, TryCaptureWithoutDebug> for &Wrapper<&E> {
+    #[inline]
+    fn try_capture(&self, _: &mut Capture<E, TryCaptureWithoutDebug>) {}
+}
+
+impl<E> Debug for Capture<E, TryCaptureWithoutDebug> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
+        f.write_str("N/A")
+    }
+}
+
+// ***** TryCapture - Printable *****
+
+/// Marker used by [Capture]
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub struct TryCaptureWithDebug;
+
+/// Catches an arbitrary `E: Printable` and modifies `to` accordingly
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub trait TryCapturePrintable<E, M> {
+    /// Similar as [TryCaptureGeneric] but specialized to any `E: Printable`.
+    fn try_capture(&self, to: &mut Capture<E, M>);
+}
+
+impl<E> TryCapturePrintable<E, TryCaptureWithDebug> for Wrapper<&E>
+where
+    E: Printable,
+{
+    #[inline]
+    fn try_capture(&self, to: &mut Capture<E, TryCaptureWithDebug>) {
+        to.elem = Some(*self.0);
+    }
+}
+
+impl<E> Debug for Capture<E, TryCaptureWithDebug>
+where
+    E: Printable,
+{
+    fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), core::fmt::Error> {
+        match self.elem {
+            None => f.write_str("N/A"),
+            Some(ref value) => Debug::fmt(value, f),
+        }
+    }
+}
+
+// ***** Others *****
+
+/// All possible captured `assert!` elements
+///
+/// # Types
+///
+/// * `E`: **E**lement that is going to be displayed.
+/// * `M`: **M**arker used to differentiate [Capture]s in regards to [Debug].
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub struct Capture<E, M> {
+    // If None, then `E` does not implements [Printable] or `E` wasn't evaluated (`assert!( ... )`
+    // short-circuited).
+    //
+    // If Some, then `E` implements [Printable] and was evaluated.
+    pub elem: Option<E>,
+    phantom: PhantomData<M>,
+}
+
+impl<M, T> Capture<M, T> {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { elem: None, phantom: PhantomData }
+    }
+}
+
+/// Necessary for the implementations of `TryCapture*`
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub struct Wrapper<T>(pub T);
+
+/// Tells which elements can be copied and displayed
+#[unstable(feature = "generic_assert_internals", issue = "44838")]
+pub trait Printable: Copy + Debug {}
+
+impl<T> Printable for T where T: Copy + Debug {}