Attempting destructuring of struct reference to mutable variable results in partial moves of fields

[cdmistman]

I tried this code:

struct Foo {
    bar: Bar,
}

struct Bar;

fn with_bar(bar: &Bar) {}

fn main() {
    let foo = Foo { bar: Bar };
    let Foo { mut bar } = &foo;
    with_bar(bar);
}

I expected to see this happen: The above should compile with bar being a mutable variable of type &Bar

Instead, this happened: The above fails to compile with bar being a mutable variable of type Bar.

Meta

rustc --version --verbose:

rustc 1.65.0-nightly (015a824f2 2022-08-22)
binary: rustc
commit-hash: 015a824f2dffe32707fceb59c47effaf7b73486c
commit-date: 2022-08-22
host: aarch64-apple-darwin
release: 1.65.0-nightly
LLVM version: 15.0.0

Backtrace

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
  --> src/main.rs:12:14
   |
12 |     with_bar(bar);
   |     -------- ^^^
   |     |        |
   |     |        expected `&Bar`, found struct `Bar`
   |     |        help: consider borrowing here: `&bar`
   |     arguments to this function are incorrect
   |
note: function defined here
  --> src/main.rs:7:4
   |
7  | fn with_bar(bar: &Bar) {}
   |    ^^^^^^^^ ---------

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` due to previous error

playground

[Enselic]

Triage: Addressed by rust-lang/rfcs#3627 (see various referenced issues). Closing as duplicate.

[compiler-errors]

I don't think this issue should be closed until the referenced RFC is stabilized. This issue still reproduces on all version of Rust, afaict?

[Enselic]

Hmm, you are right.

We should probably create a label to make it easy to track bugs related to the implementation.

[compiler-errors]

it's not a bug, it's just not stable behavior yet. It's gated behind a feature.

[traviscross]

Tracking:

• Tracking Issue for match ergonomics 2024 (RFC 3627) #123076

[Nadrieril]

This is a duplicate of #105647 though

[traviscross]

Closing in favor of:

• mut keyword alone unexpectedly dereferences value in pattern matching #105647