Adding an explicit type annotation causes unexpected implicit reborrow

[theemathas]

This code compiles and prints 10:

fn main() {
    let mut a: i32 = 10i32;
    let b: &mut i32 = &mut a;
    {
        let _c: &mut i32 = b;
    }
    println!("{b}");
}

playground

This code doesn't compile (the only difference is the lack of the type annotation on _c):

fn main() {
    let mut a: i32 = 10i32;
    let b: &mut i32 = &mut a;
    {
        let _c = b;
    }
    println!("{b}");
}

playground

The compile error is:

error[E0382]: borrow of moved value: `b`
 --> src/main.rs:7:15
  |
3 |     let b: &mut i32 = &mut a;
  |         - move occurs because `b` has type `&mut i32`, which does not implement the `Copy` trait
4 |     {
5 |         let _c = b;
  |                  - value moved here
6 |     }
7 |     println!("{b}");
  |               ^^^ value borrowed here after move
  |
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0382`.
error: could not compile `playground` (bin "playground") due to 1 previous error

I expected either both versions of the code to compile, or neither to compile. Instead, it appears that adding a type annotation for _c causes rust to do an implicit reborrow. I thought that implicit reborrow can't happen on simple assignments like this. And why would a type annotation cause a reborrow anyway?

Meta

This issue occurs on the playground, both at stable (1.76.0), and at nightly (2024-02-22 397937d).

[theemathas]

Duplicate of rust-lang/reference#788. Oops.