Nightly Rust suggests `.as_ref()` on the left-hand side of the assignment

[GoldsteinE]

Code

struct Lol {
    x: Option<String>,
}

fn lmao(lol: &mut Lol, x: Option<&String>) {
    lol.x = x.clone();
}

Current output

error[E0308]: mismatched types
 --> src/lib.rs:6:13
  |
6 |     lol.x = x.clone();
  |     -----   ^^^^^^^^^ expected `Option<String>`, found `Option<&String>`
  |     |
  |     expected due to the type of this binding
  |
  = note: expected enum `Option<String>`
             found enum `Option<&String>`
help: use `Option::as_ref` to convert `Option<String>` to `Option<&String>`
  |
6 |     lol.x.as_ref() = x.clone();
  |          +++++++++

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (lib) due to previous error

Desired output

error[E0308]: mismatched types
 --> src/lib.rs:6:13
  |
6 |     lol.x = x.clone();
  |     -----   ^^^^^^^^^ expected `Option<String>`, found `Option<&String>`
  |     |
  |     expected due to the type of this binding
  |
  = note: expected enum `Option<String>`
             found enum `Option<&String>`
help: use `Option::cloned` to clone the value inside the `Option`
  |
6 |     lol.x = x.clone().cloned();
  |                      +++++++++

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground` (lib) due to previous error

Rationale and extra context

Playground link: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=6f5723a2dea54642823935b3683d3480

Anything else?

That’s a regression from stable to nightly. Playground’s stable (1.71.1) gives the desired output, but playground’s nightly (1.73.0-nightly (2023-08-16 07438b0)) does not.

[fmease]

@rustbot label -needs-triage A-suggestion-diagnostics D-invalid-suggestion regression-from-stable-to-nightly E-needs-bisection

[GoldsteinE]

It’s not present in beta, so the left bound of the bisection is 1.72.0-beta.8 (2023-08-12 598a0a3). I’ll try to run it later today.

[Urgau]

The regression most probably comes from #114052

[apiraino]

I'm trying to bisect but cargo cargo-bisect dies on me :/ So far, I'm also seeing commit 18fa7b9 as probably culprit so #114052 seems to make sense

cc: @clubby789

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

@rustbot label -I-prioritize +P-medium -E-needs-bisection