im beta regression

[est31]

The tests of the im crate version 6.0.0 fail on latest beta 1.21, while they work on stable. The error output (full log):

error[E0282]: type annotations needed
    --> src/map/mod.rs:1400:18
     |
1400 |         let c1 = Map::singleton(v1, p1);
     |             --   ^^^^^^^^^^^^^^ cannot infer type for `K`
     |             |
     |             consider giving `c1` a type

cc @bodil

[est31]

Bisected to 560b6ca -- #42565 cc @murarth @aturon

[arielb1]

Relevant excerpts:

https://github.com/bodil/im-rs/blob/5a48a656a67805637b84daa1c9b746c2c531181b/src/map/mod.rs#L98-L104

https://github.com/bodil/im-rs/blob/5a48a656a67805637b84daa1c9b746c2c531181b/src/map/mod.rs#L1392-L1403

rust/src/liballoc/arc.rs

Line 1348 in 8e0d01b

impl<T> From<T> for Arc<T> {

rust/src/liballoc/arc.rs

Line 1371 in 8e0d01b

impl From<String> for Arc<str> {

Looks like we added a new impl and now singleton can't decide whether it should contain Arc<String> or Arc<str>. Expected inference breakage.

[alexcrichton]

The libs team discussed this the other day and the decision was that this indeed falls under expected inference breakage (thanks for the investigation @arielb1!). @KodrAus has prepared a PR with a fix, so I'll close this in the meantime.