add "regr test" for rust-lang#31849

Author: nikomatsakis

src/test/run-pass/project-cache-issue-31849.rs

@@ -0,0 +1,75 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// Regression test for #31849: the problem here was actually a performance
+// cliff, but I'm adding the test for reference.
+
+pub trait Upcast<T> {
+    fn upcast(self) -> T;
+}
+
+impl<S1, S2, T1, T2> Upcast<(T1, T2)> for (S1,S2)
+    where S1: Upcast<T1>,
+          S2: Upcast<T2>,
+{
+    fn upcast(self) -> (T1, T2) { (self.0.upcast(), self.1.upcast()) }
+}
+
+impl Upcast<()> for ()
+{
+    fn upcast(self) -> () { () }
+}
+
+pub trait ToStatic {
+    type Static: 'static;
+    fn to_static(self) -> Self::Static where Self: Sized;
+}
+
+impl<T, U> ToStatic for (T, U)
+    where T: ToStatic,
+          U: ToStatic
+{
+    type Static = (T::Static, U::Static);
+    fn to_static(self) -> Self::Static { (self.0.to_static(), self.1.to_static()) }
+}
+
+impl ToStatic for ()
+{
+    type Static = ();
+    fn to_static(self) -> () { () }
+}
+
+
+trait Factory {
+    type Output;
+    fn build(&self) -> Self::Output;
+}
+
+impl<S,T> Factory for (S, T)
+    where S: Factory,
+          T: Factory,
+          S::Output: ToStatic,
+          <S::Output as ToStatic>::Static: Upcast<S::Output>,
+{
+    type Output = (S::Output, T::Output);
+    fn build(&self) -> Self::Output { (self.0.build().to_static().upcast(), self.1.build()) }
+}
+
+impl Factory for () {
+    type Output = ();
+    fn build(&self) -> Self::Output { () }
+}
+
+fn main() {
+    // More parens, more time.
+    let it = ((((((((((),()),()),()),()),()),()),()),()),());
+    it.build();
+}
+