6 files changed, 139 insertions, 0 deletions
diff --git a/src/test/ui/traits/reservation-impl/coherence-conflict.rs b/src/test/ui/traits/reservation-impl/coherence-conflict.rs
new file mode 100644
index 000000000..fa4a30931
--- /dev/null
+++ b/src/test/ui/traits/reservation-impl/coherence-conflict.rs
@@ -0,0 +1,14 @@
+// check that reservation impls are accounted for in negative reasoning.
+
+#![feature(rustc_attrs)]
+
+trait MyTrait {}
+#[rustc_reservation_impl="this impl is reserved"]
+impl MyTrait for () {}
+
+trait OtherTrait {}
+impl OtherTrait for () {}
+impl<T: MyTrait> OtherTrait for T {}
+//~^ ERROR conflicting implementations
+
+fn main() {}
diff --git a/src/test/ui/traits/reservation-impl/coherence-conflict.stderr b/src/test/ui/traits/reservation-impl/coherence-conflict.stderr
new file mode 100644
index 000000000..a811d7e32
--- /dev/null
+++ b/src/test/ui/traits/reservation-impl/coherence-conflict.stderr
@@ -0,0 +1,13 @@
+error[E0119]: conflicting implementations of trait `OtherTrait` for type `()`
+  --> $DIR/coherence-conflict.rs:11:1
+   |
+LL | impl OtherTrait for () {}
+   | ---------------------- first implementation here
+LL | impl<T: MyTrait> OtherTrait for T {}
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
+   |
+   = note: this impl is reserved
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0119`.
diff --git a/src/test/ui/traits/reservation-impl/no-use.rs b/src/test/ui/traits/reservation-impl/no-use.rs
new file mode 100644
index 000000000..65a55d9e2
--- /dev/null
+++ b/src/test/ui/traits/reservation-impl/no-use.rs
@@ -0,0 +1,12 @@
+// check that reservation impls can't be used as normal impls in positive reasoning.
+
+#![feature(rustc_attrs)]
+
+trait MyTrait { fn foo(&self); }
+#[rustc_reservation_impl = "foo"]
+impl MyTrait for () { fn foo(&self) {} }
+
+fn main() {
+    <() as MyTrait>::foo(&());
+    //~^ ERROR the trait bound `(): MyTrait` is not satisfied
+}
diff --git a/src/test/ui/traits/reservation-impl/no-use.stderr b/src/test/ui/traits/reservation-impl/no-use.stderr
new file mode 100644
index 000000000..cefb2a879
--- /dev/null
+++ b/src/test/ui/traits/reservation-impl/no-use.stderr
@@ -0,0 +1,13 @@
+error[E0277]: the trait bound `(): MyTrait` is not satisfied
+  --> $DIR/no-use.rs:10:26
+   |
+LL |     <() as MyTrait>::foo(&());
+   |     -------------------- ^^^ the trait `MyTrait` is not implemented for `()`
+   |     |
+   |     required by a bound introduced by this call
+   |
+   = help: the trait `MyTrait` is implemented for `()`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.
diff --git a/src/test/ui/traits/reservation-impl/non-lattice-ok.rs b/src/test/ui/traits/reservation-impl/non-lattice-ok.rs
new file mode 100644
index 000000000..a71051243
--- /dev/null
+++ b/src/test/ui/traits/reservation-impl/non-lattice-ok.rs
@@ -0,0 +1,59 @@
+// build-pass
+
+// Check that a reservation impl does not force other impls to follow
+// a lattice discipline.
+
+// Why did we ever want to do this?
+//
+// We want to eventually add an `impl<T> From<!> for T` impl. That impl conflicts
+// with existing impls - at least the `impl<T> From<T> for T` impl. There are
+// 2 ways we thought of for dealing with that conflict:
+//
+// 1. Using specialization and doing some handling for the
+// overlap. The current thought is to require ["intersection
+// impls"][ii], specialization", which means providing an
+// (higher-priority) impl for the intersection of every 2 conflicting
+// impls that determines what happens in the intersection case. That's
+// the first thing we thought about - see e.g.
+// https://github.com/rust-lang/rust/issues/57012#issuecomment-452150775
+//
+// 2. The other way is to notice that `impl From<!> for T` is basically a
+// marker trait since its only method is uninhabited, and allow for "marker
+// trait overlap", where the conflict "doesn't matter" because it can't
+// actually cause any ambiguity.
+//
+// Now it turned out lattice specialization doesn't work it, because an
+// `impl<T> From<T> for Smaht<T>` would require an `impl From<!> for Smaht<!>`,
+// breaking backwards-compatibility in a fairly painful way. So if we want to
+// go with a known approach, we should go with a "marker trait overlap"-style
+// approach.
+//
+// [ii]: https://smallcultfollowing.com/babysteps/blog/2016/09/24/intersection-impls/
+
+#![feature(rustc_attrs, never_type)]
+
+trait MyTrait {}
+
+impl MyTrait for ! {}
+
+trait MyFrom<T> {
+    fn my_from(x: T) -> Self;
+}
+
+// Given the "normal" impls for From
+#[rustc_reservation_impl="this impl is reserved"]
+impl<T> MyFrom<!> for T {
+    fn my_from(x: !) -> Self { match x {} }
+}
+
+impl<T> MyFrom<T> for T {
+    fn my_from(x: T) -> Self { x }
+}
+
+// ... we *do* want to allow this common pattern, of `From<!> for MySmaht<T>`
+struct MySmaht<T>(T);
+impl<T> MyFrom<T> for MySmaht<T> {
+    fn my_from(x: T) -> Self { MySmaht(x) }
+}
+
+fn main() {}
diff --git a/src/test/ui/traits/reservation-impl/ok.rs b/src/test/ui/traits/reservation-impl/ok.rs
new file mode 100644
index 000000000..611c8d884
--- /dev/null
+++ b/src/test/ui/traits/reservation-impl/ok.rs
@@ -0,0 +1,28 @@
+// run-pass
+
+// rpass test for reservation impls. Not 100% required because `From` uses them,
+// but still.
+
+#![feature(rustc_attrs)]
+
+use std::mem;
+
+trait MyTrait<S> {
+    fn foo(&self, s: S) -> usize;
+}
+
+#[rustc_reservation_impl = "foo"]
+impl<T> MyTrait<u64> for T {
+    fn foo(&self, _x: u64) -> usize { 0 }
+}
+
+// reservation impls don't create coherence conflicts, even with
+// non-chain overlap.
+impl<S> MyTrait<S> for u32 {
+    fn foo(&self, _x: S) -> usize { mem::size_of::<S>() }
+}
+
+fn main() {
+    // ...and the non-reservation impl gets picked.XS
+    assert_eq!(0u32.foo(0u64), mem::size_of::<u64>());
+}