Add tests for cross-crate usage of impl const

src/test/ui/rfc-2632-const-trait-impl/auxiliary/cross-crate.rs

@@ -0,0 +1,22 @@
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+pub trait MyTrait {
+    fn func(self);
+}
+
+pub struct NonConst;
+
+impl MyTrait for NonConst {
+    fn func(self) {
+
+    }
+}
+
+pub struct Const;
+
+impl const MyTrait for Const {
+    fn func(self) {
+
+    }
+}

src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.rs

@@ -0,0 +1,18 @@
+// aux-build: cross-crate.rs
+extern crate cross_crate;
+
+use cross_crate::*;
+
+fn non_const_context() {
+    NonConst.func();
+    Const.func();
+}
+
+const fn const_context() {
+    NonConst.func();
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+    Const.func();
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-disabled.stderr

@@ -0,0 +1,15 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/cross-crate-feature-disabled.rs:12:5
+   |
+LL |     NonConst.func();
+   |     ^^^^^^^^^^^^^^^
+
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/cross-crate-feature-disabled.rs:14:5
+   |
+LL |     Const.func();
+   |     ^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0015`.

src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.rs

@@ -0,0 +1,20 @@
+#![feature(const_trait_impl)]
+#![allow(incomplete_features)]
+
+// aux-build: cross-crate.rs
+extern crate cross_crate;
+
+use cross_crate::*;
+
+fn non_const_context() {
+    NonConst.func();
+    Const.func();
+}
+
+const fn const_context() {
+    NonConst.func();
+    //~^ ERROR: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+    Const.func();
+}
+
+fn main() {}

src/test/ui/rfc-2632-const-trait-impl/cross-crate-feature-enabled.stderr

@@ -0,0 +1,9 @@
+error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
+  --> $DIR/cross-crate-feature-enabled.rs:15:5
+   |
+LL |     NonConst.func();
+   |     ^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0015`.