add tests for unsizing coercions (both ICE)

Author: jdonszelmann

tests/ui/dst/dst-object-from-unsized-type.rs

@@ -1,4 +1,7 @@
 // Test that we cannot create objects from unsized types.
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
 
 trait Foo { fn foo(&self) {} }
 impl Foo for str {}

tests/ui/traits/next-solver/unsize-goal-mismatch-2.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+// Test from trait-system-refactor-initiative#241:
+// Used to ICE in mir typeck because of ambiguity in the new solver.
+// The wrong (first) trait bound was selected.
+// This is fixed with new logic for unsizing coercions
+// that's independent from that of the old solver, which this test verifies.
+
+trait Super<T> {}
+trait Trait<T>: Super<T> + for<'hr> Super<&'hr ()> {}
+
+fn foo<'a>(x: Box<dyn Trait<&'a ()>>) -> Box<dyn Super<&'a ()>> {
+    x
+}
+
+fn main() {}

tests/ui/traits/next-solver/unsize-goal-mismatch.rs

@@ -0,0 +1,19 @@
+//@ check-pass
+//@ revisions: current next
+//@ ignore-compare-mode-next-solver (explicit revisions)
+//@[next] compile-flags: -Znext-solver
+// Test from trait-system-refactor-initiative#241:
+// Used to ICE in mir typeck because of ambiguity in the new solver.
+// The wrong (first) trait bound was selected.
+// This is fixed with new logic for unsizing coercions
+// that's independent from that of the old solver, which this test verifies.
+
+trait Super<'a> {}
+trait Trait<'a>: Super<'a> + for<'hr> Super<'hr> {}
+
+fn foo<'a>(x: Box<dyn Trait<'a>>) -> Box<dyn Super<'a>> {
+    x
+    //~^ ERROR type annotations needed: cannot satisfy `dyn Trait<'_>: Unsize<dyn Super<'_>>
+}
+
+fn main() {}

tests/ui/traits/next-solver/unsize-overflow.rs

@@ -0,0 +1,7 @@
+//@ compile-flags: -Znext-solver
+#![recursion_limit = "8"]
+
+fn main() {
+    let _: Box<dyn Send> = Box::new(&&&&&&&1);
+    //~^ ERROR overflow evaluating the requirement `Box<&&&&&&&i32>: CoerceUnsized<Box<dyn Send>>
+}