Allow parentheses in dyn (Trait)

[Manishearth]

r? @eddyb @nikomatsakis

[petrochenkov]

There's a test compile-fail/dyn-trait-compatibility.rs that's going to be broken by this change and needs updating.
(This is technically a breaking change, but the practical impact should be zero.)

[Manishearth]

Oh, right, you can alias FnFoo to dyn. This is unfortunate.

@nikomatsakis is this something we should just land (perhaps w/ crater) or make it an epoch breakage? If we do the latter, bear in mind that there will be no automated way to rustfix &::Foo pre-epoch since &dyn::Foo is interpreted as a path.

[petrochenkov]

We should just land this, IMO, crater won't find anything.

[Manishearth]

brb publishing a crate to prove you wrong 😜

but yeah, perhaps. Though I'm really wary about breaking changes. Feel free to r+

[nikomatsakis]

I agree with @petrochenkov we can probably get away with this one, though like @Manishearth I am getting ever more wary about breaking changes. =)

[nikomatsakis]

Nonehtless, tagging as relnotes. Description would be something like this:

---

The dyn keyword now accepts parenthesis, meaning you can do something like dyn (Debug + Send) instead of (dyn Debug + Send). It is technically possible for this to affect existing code, if that code were to alias a Fn trait with the name dyn (e.g., use ::std::ops::Fn as dyn).

---

@petrochenkov, that's the case you were thinking of, right?

[nikomatsakis]

I would like some tests that cover the precedence and the interesting cases. For example:

let x: &dyn (Debug + Send) = ..; // this should work, right?
let x: &dyn (Debug + Send + Sync) = ..; // this too
let x: &dyn (Debug + Send + ) = ..; // I think we support trailing `+` terminators elsewhere
let x: Box<dyn (Debug + Send) + Sync> = ...; // what about this? =) I forget what we decided there

Also, does this work for impl? It should work analogously, I think.

[Manishearth]

ah, none of those work

[Manishearth]

We don't seem to allow parens in trait bounds, T: (Debug + Send) does not work either.

[nikomatsakis]

@Manishearth yes, I know. I think the way to fix this is by allowing parens in trait bounds, or at least I think that's what @cramertj and I always had in mind (they can confirm)

[nikomatsakis]

This has been a historical point of disagreement with @petrochenkov though =)

[Manishearth]

I can make it work with dyn trait for now; it does not exacerbate any breakages, and is necessary to make &dyn ::Debug + Send work I think

[petrochenkov]

@nikomatsakis

let x: &dyn (Debug + Send) = ..;

This PR doesn't implement that extension, it does other thing - it supports already existing parentheses around a single bound (e.g. impl (Debug) + (Send) is already ok) after dyn specifically, that weren't supported purely for backward compatibility reasons.

[cramertj]

My personal preference is for (Bound) to be a valid bound, so that users can write Box<dyn (Bound + Bound)>, &dyn (Bound + Bound + (Bound)), T: (Bound + Bound) etc.

[petrochenkov]

@bors r+

(dyn (Debug + Send) is an orthogonal issue.)

[bors]

📌 Commit 4c73f82 has been approved by petrochenkov

[ishitatsuyuki]

Regression in rollup, pretty print test failed.

[01:39:41] ---- [pretty] run-pass/dyn-trait.rs stdout ----
[01:39:41] 	
[01:39:41] error: pretty-printed source does not match expected source
[01:39:41] 
[01:39:41] expected:
[01:39:41] ------------------------------------------
[01:39:41] // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
[01:39:41] // file at the top-level directory of this distribution and at
[01:39:41] // http://rust-lang.org/COPYRIGHT.
[01:39:41] //
[01:39:41] // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
[01:39:41] // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
[01:39:41] // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
[01:39:41] // option. This file may not be copied, modified, or distributed
[01:39:41] // except according to those terms.
[01:39:41] 
[01:39:41] #![feature(dyn_trait)]
[01:39:41] 
[01:39:41] use std::fmt::Display;
[01:39:41] 
[01:39:41] static BYTE: u8 = 33;
[01:39:41] 
[01:39:41] fn main() {
[01:39:41]     let x: &(dyn 'static + Display) = &BYTE;
[01:39:41]     let y: Box<dyn Display + 'static> = Box::new(BYTE);
[01:39:41]     let _: &dyn Display = &BYTE;
[01:39:41]     let _: &dyn ::std::fmt::Display = &BYTE;
[01:39:41]     let xstr = format!("{}" , x);
[01:39:41]     let ystr = format!("{}" , y);
[01:39:41]     assert_eq!(xstr , "33");
[01:39:41]     assert_eq!(ystr , "33");
[01:39:41] }
[01:39:41] 
[01:39:41] ------------------------------------------
[01:39:41] actual:
[01:39:41] ------------------------------------------
[01:39:41] // Copyright 2017 The Rust Project Developers. See the COPYRIGHT
[01:39:41] // file at the top-level directory of this distribution and at
[01:39:41] // http://rust-lang.org/COPYRIGHT.
[01:39:41] //
[01:39:41] // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
[01:39:41] // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
[01:39:41] // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
[01:39:41] // option. This file may not be copied, modified, or distributed
[01:39:41] // except according to those terms.
[01:39:41] 
[01:39:41] #![feature(dyn_trait)]
[01:39:41] 
[01:39:41] use std::fmt::Display;
[01:39:41] 
[01:39:41] static BYTE: u8 = 33;
[01:39:41] 
[01:39:41] fn main() {
[01:39:41]     let x: &(dyn 'static + Display) = &BYTE;
[01:39:41]     let y: Box<dyn Display + 'static> = Box::new(BYTE);
[01:39:41]     let _: &dyn Display = &BYTE;
[01:39:41]     let _: &dyn::std::fmt::Display = &BYTE;
[01:39:41]     let xstr = format!("{}" , x);
[01:39:41]     let ystr = format!("{}" , y);
[01:39:41]     assert_eq!(xstr , "33");
[01:39:41]     assert_eq!(ystr , "33");
[01:39:41] }
[01:39:41] 
[01:39:41] ------------------------------------------
[01:39:41] 
[01:39:41] 
[01:39:41] 
[01:39:41] error: pretty-printed source does not typecheck
[01:39:41] status: exit code: 101
[01:39:41] command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "-" "-Zno-trans" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/dyn-trait.pretty-out" "--target=x86_64-unknown-linux-gnu" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/run-pass/dyn-trait.stage2-x86_64-unknown-linux-gnu.pretty.aux" "-Crpath" "-O" "-Zmiri" "-Zunstable-options" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers"
[01:39:41] stdout:
[01:39:41] ------------------------------------------
[01:39:41] 
[01:39:41] ------------------------------------------
[01:39:41] stderr:
[01:39:41] ------------------------------------------
[01:39:41] error[E0433]: failed to resolve. Use of undeclared type or module `dyn`
[01:39:41]   --> <anon>:21:13
[01:39:41]    |
[01:39:41] 21 |     let _: &dyn::std::fmt::Display = &BYTE;
[01:39:41]    |             ^^^ Use of undeclared type or module `dyn`
[01:39:41] 
[01:39:41] error: aborting due to previous error
[01:39:41] 
[01:39:41] 
[01:39:41] ------------------------------------------
[01:39:41] 
[01:39:41] thread '[pretty] run-pass/dyn-trait.rs' panicked at 'explicit panic', tools/compiletest/src/runtest.rs:2892:9
[01:39:41] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[01:39:41] 
[01:39:41] 
[01:39:41] failures:
[01:39:41]     [pretty] run-pass/dyn-trait.rs

[Manishearth]

This merged, I just messed up on including it in the rollup

7e68299