Implement existential types

src/test/ui/str-array-assignment.rs

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+// Copyright 2017 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.
+
+fn main() {
+  let s = "abc";
+  let t = if true { s[..2] } else { s };
+  //~^ ERROR if and else have incompatible types
+  let u: &str = if true { s[..2] } else { s };
+  //~^ ERROR mismatched types
+  let v = s[..2];
+  //~^ ERROR the size for values of type
+  let w: &str = s[..2];
+  //~^ ERROR mismatched types
+}