Don't assume traits used as type are trait objs in 2021 edition

[VulnBandit]

Fixes #127548

When you use a trait as a type, the compiler automatically assumes you meant to use a trait object, which is not always the case.
This PR fixes the bug where you don't need a trait object, so the error message was changed to:

error[E0782]: expected a type, found a trait

[rustbot]

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[rustbot]

HIR ty lowering was modified

cc @fmease

[VulnBandit]

@rustbot author

[jieyouxu]

This PR fixes the vunlnerability where you don't need a trait object

FYI the word "vunlnerability" has a specific meaning (i.e. a weakness that makes something easy to be attacked/exploited) in the context of this repo (e.g. related to compiler or std), but this PR is just trying to improve diagnostics not trying to address an exploit.

[VulnBandit]

I see vulnerability as when something is not right and I fix it. Ok I will change the word

[jieyouxu]

We usually call those those "bugs" or "diagnostic issues", "vulnerability" usually indicates an exploitable weakness that has to do with security that wg-security-response would need to look at.

[VulnBandit]

@rustbot review