05e3248a79
This can break code that looked like:
impl Foo for Box<Any> {
fn f(&self) { ... }
}
let x: Box<Any + Send> = ...;
x.f();
Change such code to:
impl Foo for Box<Any> {
fn f(&self) { ... }
}
let x: Box<Any> = ...;
x.f();
That is, upcast before calling methods.
This is a conservative solution to #5781. A more proper treatment (see
the xfail'd `trait-contravariant-self.rs`) would take variance into
account. This change fixes the soundness hole.
Some library changes had to be made to make this work. In particular,
`Box<Any>` is no longer showable, and only `Box<Any+Send>` is showable.
Eventually, this restriction can be lifted; for now, it does not prove
too onerous, because `Any` is only used for propagating the result of
task failure.
This patch also adds a test for the variance inference work in #12828,
which accidentally landed as part of DST.
Closes #5781.
[breaking-change]
40 lines
1.2 KiB
Rust
40 lines
1.2 KiB
Rust
// ignore-test
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// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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// This is an interesting test case. We have a trait (Bar) that is
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// implemented for a `Box<Foo>` object (note: no bounds). And then we
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// have a `Box<Foo+Send>` object. The impl for `Box<Foo>` is applicable
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// to `Box<Foo+Send>` because:
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//
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// 1. The trait Bar is contravariant w/r/t Self because `Self` appears
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// only in argument position.
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// 2. The impl provides `Bar for Box<Foo>`
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// 3. The fn `wants_bar()` requires `Bar for Box<Foo:Send>`.
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// 4. `Bar for Box<Foo> <: Bar for Box<Foo:Send>` because
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// `Box<Foo:Send> <: Box<Foo>`.
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trait Foo { }
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struct SFoo;
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impl Foo for SFoo { }
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trait Bar { fn dummy(&self); }
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impl Bar for Box<Foo> { fn dummy(&self) { } }
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fn wants_bar<B:Bar>(b: &B) { }
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fn main() {
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let x: Box<Foo+Send> = (box SFoo);
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wants_bar(&x);
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}
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