Description
Code
The following code compiles successfully on 1.74.0 and stable (1.75.0) (playground):
use std::sync::Arc;
fn takes_arc_slice(_: Arc<[u8]>) {}
fn no_binding() {
takes_arc_slice(if true {
Arc::from([255])
} else {
Arc::from([])
});
}
fn with_binding() {
let binding = if true {
Arc::from([255])
} else {
Arc::from([])
};
takes_arc_slice(binding);
}
fn main() {
no_binding();
with_binding();
}It fails to compile on 1.73.0 with the following error:
error[E0308]: `if` and `else` have incompatible types
--> arc-slice.rs:17:9
|
14 | let buffers = if true {
| ___________________-
15 | | Arc::from([255])
| | ---------------- expected because of this
16 | | } else {
17 | | Arc::from([])
| | ^^^^^^^^^^^^^ expected an array with a fixed size of 1 element, found one with 0 elements
18 | | };
| |_____- `if` and `else` have incompatible types
|
= note: expected struct `Arc<[{integer}; 1]>`
found struct `Arc<[_; 0]>`
Only the with_binding version fails, interestingly.
This was reported in pola-rs/polars#14134. A user tried to compile the latest polars version on rustc 1.73.0 and got the above error.
Repro of the failure on godbolt.
Despite polars not having a documented MSRV, I feel that this kind of change in what is allowed to compile is non-obvious and wouldn't make the average MSRV-minded Rust developer think twice if it compiled on stable. The 1.74.0 release notes don't mention anything about a change here.
Request: at minimum, we should add a regression test to make sure this code continues to compile.
Version it worked on
The code failed to compile in 1.73.0.
Version with regression
The code started compiling in 1.74.0.
@rustbot modify labels: +regression-from-stable-to-stable -regression-untriaged +A-array +A-coercions +A-slice +A-inference +T-compiler