regression 1.49: trait bound no longer inferred for associated type

[rylev]

Log: https://crater-reports.s3.amazonaws.com/beta-1.49-1/beta-2020-11-26/gh/marcianx.openmesh-rs-core/log.txt

[INFO] [stdout] error[E0277]: the trait bound `H: handle::ItemHandle` is not satisfied
[INFO] [stdout]    --> src/property/property.rs:119:14
[INFO] [stdout]     |
[INFO] [stdout] 119 |         self.clone_as_trait()
[INFO] [stdout]     |              ^^^^^^^^^^^^^^ the trait `handle::ItemHandle` is not implemented for `H`
[INFO] [stdout]     |
[INFO] [stdout] help: consider restricting type parameter `H`
[INFO] [stdout]     |
[INFO] [stdout] 116 | impl<H: handle::ItemHandle> Clone for Box<dyn ResizeableProperty<Handle=H>>
[INFO] [stdout]     |       ^^^^^^^^^^^^^^^^^^^^

ResizeablePropertyHandle::Handle is constrained to implement handle::ItemHandle.

Other logs:
https://crater-reports.s3.amazonaws.com/beta-1.49-1/beta-2020-11-26/gh/Polkadex-Substrate.polkadex-engine/log.txt
https://crater-reports.s3.amazonaws.com/beta-1.49-1/beta-2020-11-26/reg/actix-amqp-0.1.4/log.txt
https://crater-reports.s3.amazonaws.com/beta-1.49-1/beta-2020-11-26/reg/ntex-amqp-0.1.20/log.txt

@rustbot modify labels: +regression-from-stable-to-beta

[Mark-Simulacrum]

cc @nikomatsakis, @matthewjasper -- might this be related to #78893? It looks sort of "inverse" of that one I guess

[rylev]

The actix-amqp and ntex-amqp logs also show another error (E0271). Perhaps these errors are related, but it's not easy to tell.

[dillona]

searched nightlies: from nightly-2020-09-24 to nightly-2020-12-11
regressed nightly: nightly-2020-10-07
searched commits: from a1dfd24 to 98edd1f
regressed commit: 08e2d46

bisected with cargo-bisect-rustc v0.6.0

Host triple: aarch64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc

[Mark-Simulacrum]

Probably a duplicate of #79905

[matthewjasper]

#77638 maybe?

[jyn514]

#77638 was marked as wontfix: #77638 (comment). If this is the same issue, does it make sense to close it too?

[rylev]

I'm surprised #77638 was marked as wontfix. This is a breaking change that seems to effect several real crates. While I admit I was surprised that this used to work (i.e., the current behavior makes more sense to me), I think we should confirm that how it's currently implemented in beta is how it's supposed to work.

[rylev]

After discussion in Zulip, we decided to nominate this for discussion:

@rustbot label I-nominate

[apiraino]

@rustbot label I-nominate

(let me try)