Fallout in tests -- explain an interesting test failure having to

do with dropck and the new outlives rules

Author: nikomatsakis

src/test/compile-fail/dropck-object-cycle.rs

@@ -0,0 +1,57 @@
+// Copyright 2014 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.
+
+// This test used to be part of a run-pass test, but revised outlives
+// rule means that it no longer compiles.
+
+#![allow(unused_variables)]
+
+trait Trait<'a> {
+    fn long(&'a self) -> isize;
+    fn short<'b>(&'b self) -> isize;
+}
+
+fn object_invoke1<'d>(x: &'d Trait<'d>) -> (isize, isize) { loop { } }
+
+trait MakerTrait {
+    fn mk() -> Self;
+}
+
+fn make_val<T:MakerTrait>() -> T {
+    MakerTrait::mk()
+}
+
+impl<'t> MakerTrait for Box<Trait<'t>+'static> {
+    fn mk() -> Box<Trait<'t>+'static> { loop { } }
+}
+
+pub fn main() {
+    let m : Box<Trait+'static> = make_val();
+    assert_eq!(object_invoke1(&*m), (4,5));
+    //~^ ERROR `*m` does not live long enough
+
+    // the problem here is that the full type of `m` is
+    //
+    //   Box<Trait<'m>+'static>
+    //
+    // Here `'m` must be exactly the lifetime of the variable `m`.
+    // This is because of two requirements:
+    // 1. First, the basic type rules require that the
+    //    type of `m`'s value outlives the lifetime of `m`. This puts a lower
+    //    bound `'m`.
+    //
+    // 2. Meanwhile, the signature of `object_invoke1` requires that
+    //    we create a reference of type `&'d Trait<'d>` for some `'d`.
+    //    `'d` cannot outlive `'m`, so that forces the lifetime to be `'m`.
+    //
+    // This then conflicts with the dropck rules, which require that
+    // the type of `m` *strictly outlives* `'m`. Hence we get an
+    // error.
+}

src/test/run-pass/regions-early-bound-trait-param.rs

@@ -42,7 +42,7 @@ fn field_invoke1<'f, 'g>(x: &'g Struct1<'f>) -> (isize,isize) {
     (l,s)
 }
 
-struct Struct2<'h, 'i> {
+struct Struct2<'h, 'i:'h> {
     f: &'h (Trait<'i>+'h)
 }
 
@@ -126,7 +126,10 @@ pub fn main() {
     assert_eq!(field_invoke2(&s2), 3);
 
     let m : Box<Trait> = make_val();
-    assert_eq!(object_invoke1(&*m), (4,5));
+    // assert_eq!(object_invoke1(&*m), (4,5));
+    //            ~~~~~~~~~~~~~~~~~~~
+    // this call yields a compilation error; see compile-fail/dropck-object-cycle.rs
+    // for details.
     assert_eq!(object_invoke2(&*m), 5);
 
     // The RefMakerTrait above is pretty strange (i.e. it is strange