`format_args` deduplicates consts with interior mutability or destructor

I'm not sure if this is a bug or not.

I tried this code:

use std::fmt::{self, Display, Formatter};
use std::cell::Cell;

struct PrintCounter(Cell<usize>);

impl Display for PrintCounter {
    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
        let counter = 1 + self.0.get();
        self.0.set(counter);
        write!(f, "{counter}")
    }
}

const ZERO: PrintCounter = PrintCounter(Cell::new(0));

fn main() {
    println!("{ZERO}, {ZERO}");
}

I expected the code to print 1, 1, since I am creating two separate PrintCounter values and printing them separately.

Instead, it printed 1, 2. The compiler seemed to have deduplicated the two mentions of ZERO into one mention.

Note that if the code is modified to be println!("{}, {}", ZERO, ZERO);, then it correctly prints 1, 1.

@rustbot labels +A-fmt

Meta

Reproducible on the playground with version 1.91.0-nightly (2025-08-20 040a98af70f0a7da03f3)

Related

• https://github.com/rust-lang/rust/pull/149926

Start by running the supplied Rust playground reproducer on the stated nightly version and compare it with the println!("{}, {}", ZERO, ZERO) form. Review related PR #149926, then trace the format_args handling of repeated const arguments. Done means the expected behavior is agreed and the discrepancy is fixed or documented with coverage for interior mutability and destructors.