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Can't modify value after temporarily dropping reference in loop when using Drop #92984

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@AzureMarker

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@AzureMarker

I tried this code:

struct Console<'gfx>(&'gfx usize);

impl<'gfx> Drop for Console<'gfx> {
    fn drop(&mut self) {}
}

fn main() {
    let mut gfx = 0;
    let mut console = Console(&gfx);

    for _ in 0..5 {
        println!("{gfx}");
        
        if gfx == 0 {
            drop(console);
            
            gfx += 1;
            
            console = Console(&gfx);
        }
    }
}

I expected to see this happen: Compile

Instead, this happened: Fails with error:

error[E0506]: cannot assign to `gfx` because it is borrowed
  --> src/main.rs:17:13
   |
9  |     let mut console = Console(&gfx);
   |                               ---- borrow of `gfx` occurs here
...
17 |             gfx += 1;
   |             ^^^^^^^^ assignment to borrowed `gfx` occurs here
18 |             
19 |             console = Console(&gfx);
   |             ------- borrow might be used here, when `console` is dropped and runs the `Drop` code for type `Console`

For more information about this error, try `rustc --explain E0506`.

It works though if you remove the Drop impl for Console, or if you remove the loop. I would expect it to work with Drop because the drop call happens before gfx is modified, and there shouldn't be any drop call immediately after it like the diagnostic suggests.

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rustc --version --verbose:

rustc 1.58.0 (02072b482 2022-01-11)
binary: rustc
commit-hash: 02072b482a8b5357f7fb5e5637444ae30e423c40
commit-date: 2022-01-11
host: x86_64-unknown-linux-gnu
release: 1.58.0
LLVM version: 13.0.0

Tested in playground and in a local dev rustc (based on commit 1b3a5f2, from 3 weeks ago).

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    A-MIRArea: Mid-level IR (MIR) - https://blog.rust-lang.org/2016/04/19/MIR.htmlA-borrow-checkerArea: The borrow checkerC-bugCategory: This is a bug.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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