.. syntax copies lifetime

[DzenanJupic]

I tried this code playground:

use std::borrow::Cow;

struct S<'a> {
    cow: Cow<'a, str>,
    owned: String,
}

fn make_owned(s: S<'_>) -> S<'static> {
    S {
        cow: Cow::Owned(s.cow.into_owned()),
        owned: s.owned,
    }
}

// does not work
// fn make_owned2(s: S<'_>) -> S<'static> {
//     S {
//         cow: Cow::Owned(s.cow.into_owned()),
//         ..s
//     }
// }

I expected to see this happen:

Since all fields with a lifetime were replaced by owned values I expected make_owned2 to compile.

Instead, this happened:

The compiler is unable to understand, that the remaining fields don't influence the lifetime of the struct.

error[E0621]: explicit lifetime required in the type of `s`
  --> src/lib.rs:18:5
   |
18 | /     S {
19 | |         cow: Cow::Owned(s.cow.into_owned()),
20 | |         ..s
21 | |     }
   | |_____^ lifetime `'static` required

Meta

rustc --version --verbose:

rustc 1.58.0-nightly (65f3f8b22 2021-11-21)
binary: rustc
commit-hash: 65f3f8b220f020e562c5dd848ff7319257a7ba45
commit-date: 2021-11-21
host: x86_64-pc-windows-msvc
release: 1.58.0-nightly
LLVM version: 13.0.0

[fmease]

In the current version of Rust, this works as intended: The type cannot change with the struct update syntax. On nightly with #![feature(type_changing_struct_update)], your second function compiles successfully. See #86555.

[nagisa]

Closing as…

Duplicate of #86555