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add regression test for #16223 (NLL): use of collaterally moved value #49781

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63 changes: 63 additions & 0 deletions src/test/ui/nll/issue-16223.rs
Original file line number Diff line number Diff line change
@@ -0,0 +1,63 @@
// Copyright 2018 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 #16223: without NLL the `if let` construct together with
// the nested box-structure of `Root` causes an unwanted collateral move.

// The exact error prevented here is:
//
// error[E0382]: use of collaterally moved value: `(root.boxed.rhs as SomeVariant::B).0`
// --> src/main.rs:55:29
// |
// 56 | lhs: SomeVariant::A(a),
// | - value moved here
// 57 | rhs: SomeVariant::B(b),
// | ^ value used here after move
// |
// = note: move occurs because the value has type `A`, which does not implement the `Copy` trait

// must-compile-successfully

#![feature(nll)]
#![feature(box_patterns)]

struct Root {
boxed: Box<SetOfVariants>,
}

struct SetOfVariants {
lhs: SomeVariant,
rhs: SomeVariant,
}

enum SomeVariant {
A(A),
B(B),
}

struct A(String);
struct B(String);

fn main() {
let root = Root {
boxed: Box::new(SetOfVariants {
lhs: SomeVariant::A(A(String::from("This is A"))),
rhs: SomeVariant::B(B(String::from("This is B"))),
}),
};
if let box SetOfVariants {
lhs: SomeVariant::A(a),
rhs: SomeVariant::B(b),
} = root.boxed
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You can use *root.boxed here if you want to avoid the box pattern.

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@Robbepop Robbepop Apr 9, 2018

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Thank you for the code review!

Your suggested code works and maybe this test should go with a dereference.

I haven't dereferenced root.boxed because it covers a nice use case where the entire root.boxed is only conditionally moved into the then-case of the if and otherwise can be further processed - this is not possible if you explicitely dereference it.

I use this convenience a lot in my current project and at least to me it would be nice if this functionality was tested, too.

{
println!("a = {}", a.0);
println!("b = {}", b.0);
}
}