std::sync::Once::call_once executes closure multiple times when create as const

[flopacero]

I tried this code:

use std::sync::Once;

const INIT: Once = Once::new();

fn main() {
    init();
    init();
}

fn init() {
    println!("Init called");
    INIT.call_once(|| {
        println!("Initialize")
    });
}

I expected to see an output:

Init called
Initialize
Init called

Instead, the output:

Init called
Initialize
Init called
Initialize

The code starts to works as expected once const INIT is replaced by static INIT.
It can be reproduced on rust playground either on stable/betta/nightly builds.

Meta

rustc --version --verbose:

rustc 1.58.1 (db9d1b20b 2022-01-20)
binary: rustc
commit-hash: db9d1b20bba1968c1ec1fc49616d4742c1725b4b
commit-date: 2022-01-20
host: x86_64-pc-windows-msvc
release: 1.58.1
LLVM version: 13.0.0

Backtrace

$ RUST_BACKTRACE=1 cargo build
   Compiling once_bug v0.1.0 (C:\Users\Andrey_Kviatko\Documents\projects\rust\playground\once_bug)
    Finished dev [unoptimized + debuginfo] target(s) in 0.59s

[Aaron1011]

This is because each use of INIT results in a new copy of the underlying Once. We have a CONST_ITEM_MUTATION lint that catches calls to methods that take &mut self - however, Once uses interior mutability, so the lint doesn't catch it.

We may want to consider uplifting Clippy's lint against consts with interior mutability.

[cuviper]

A simpler example is a const atomic, and similarly any mutation will only affect a temporary copy.

[flopacero]

Along with a lint it might be also helpful to add some note in docs for std::sync::Once. I can't remember if I've ever seen anything about const resulting in copy of an underlying variable. Besides Once::new is const fn so it's allowed to be used in const context. Turns out storing it as a const is likely not what you need.

[the8472]

They const keyword docs mention this. https://doc.rust-lang.org/std/keyword.const.html

const items looks remarkably similar to static items, which introduces some confusion as to which one should be used at which times. To put it simply, constants are inlined wherever they’re used, making using them identical to simply replacing the name of the const with its value. Static variables, on the other hand, point to a single location in memory, which all accesses share.

[scottmcm]

I'll note that the very first example on the Once page is

static START: Once = Once::new();

Which is a pretty strong encouragement to use static, not const.

[matklad]

Duplicate of #40543?

[Dylan-DPC]

Closing this as duplicate of #40543