Previously working code doesn't work on nightly (zero-sized types)

[Karrq]

I tried this code (playground):

struct ZST;
impl ZST {
  pub const fn new() -> Self { Self }

  pub const fn static_ref() -> &'static Self {
    &Self::new()
  }
}

I expected to see this happen: no errors

Instead, this happened: compiler errored with:

error[E0515]: cannot return reference to temporary value
 --> src/lib.rs:6:5
  |
6 |     &Self::new()
  |     ^-----------
  |     ||
  |     |temporary value created here
  |     returns a reference to data owned by the current function

Meta

rustc --version --verbose:

rustc 1.49.0-nightly (beb5ae474 2020-10-04)
binary: rustc
commit-hash: beb5ae474d2835962ebdf7416bd1c9ad864fe101
commit-date: 2020-10-04
host: x86_64-unknown-linux-gnu
release: 1.49.0-nightly
LLVM version: 11.0

---

This doesn't error in stable or beta, just nightly.
I worked this around by just returning the ZST (&Self) instead of calling the constructor (&Self::new()).

[SNCPlay42]

This code errors for me on stable on the playground.

[Karrq]

Found out why, I attached the wrong playground link, where I removed the const keywords.

This one is the correct one. It compiles both in stable and beta, but not on nightly.

[SNCPlay42]

I believe this was an expected result of #75502 (and a cargo bisect-rustc points to its rollup, #76912).

As a workaround, this code works:

struct ZST;
impl ZST {
  pub const fn new() -> Self { Self }

  pub const fn static_ref() -> &'static Self {
    const VAL: ZST = ZST::new();
    &VAL
  }
}

[RalfJung]

Cc @rust-lang/wg-const-eval

This code used to work accidentally, and stopped working when #75586 got fixed. Indeed the intended way to write this is what @SNCPlay42 suggested.

We're sorry that this broke existing code, but there just is no good way to support that kind of code longer-term as const fn become more powerful. A more ergonomic alternative to an explicit const VAL is under development at #76001.

[Karrq]

Alright, it's fine. This code was written way back in Sept 2019.

I'd like to point out that there exists a different workaround:

struct ZST;
impl ZST {
  pub const fn new() -> Self { Self }

  pub const fn static_ref() -> &'static Self {
    &Self
  }
}

as you can see there's no more function call.

The only thing is that moving forward this will break code that is on stable if I'm not mistaken, right?

[oli-obk]

The only thing is that moving forward this will break code that is on stable if I'm not mistaken, right?

yes

[camelid]

Removing regression labels since this is an intended regression.

[camelid]

(I edited the issue to show the correct code and the full error.)