Allow specifying multiple bounds for same associated item, except in trait objects

[Jules-Bertholet]

Supersedes #143146, fixes #143143.

This PR proposes to stop enforcing E0719 in all contexts other than trait object types.

E0719 forbids constraining the same associated item twice within the same angle-bracket delimited associated item bound list (the … inside T: Trait<…>). For example, the following are forbidden:

Forbidden	Working alternative
T: Trait<Gat<u32> = u32, Gat<u64> = u64>	T: Trait<Gat<u32> = u32> + Trait<Gat<u64> = u64>
T: Iterator<Item = u32, Item = i32>	T: Iterator<Item = u32> + Iterator<Item = i32> (trivially false)
T: Iterator<Item = u32, Item = u32>	T: Iterator<Item = u32>
T: Iterator<Item: Send, Item: Sync>	T: Iterator<Item: Send + Sync>
T: Trait<ASSOC = 3, ASSOC = 4>	T: Trait<ASSOC = 3> + Trait<ASSOC = 4> (trivially false)
T: Trait<ASSOC = 3, ASSOC = 3>	T: Trait<ASSOC = 3>

With this PR, all those previously forbidden examples would start working, as well as their APIT and RPIT equivalents.

Types like dyn Iterator<Item = u32, Item = u32> will continue to be rejected, however. See #143146 (comment) for the reason why.

@rustbot label T-lang T-types needs-fcp

[rustbot]

HIR ty lowering was modified

cc @fmease

[fmease]

Nobody knows what E0719 is when they're just scrolling by and reading the title, it's as if you put a GH issue number there; please make the title more descriptive.

[BoxyUwU]

r? BoxyUwU

[traviscross]

Over in #143146, we had proposed FCP and then filed a concern related to dyn following @lcnr's comment in #143146 (comment). This PR removes the dyn part of this, so I'll go ahead and propose FCP again and check the boxes of those who had checked off there (@joshtriplett and @tmandry) as this is a strict subset of the other PR.

@rfcbot fcp merge

cc @rust-lang/types

[rust-rfcbot]

Team member @traviscross has proposed to merge this. The next step is review by the rest of the tagged team members:

• @joshtriplett
• @nikomatsakis
• @scottmcm
• @tmandry
• @traviscross

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
See this document for info about what commands tagged team members can give me.

[traviscross]

@rfcbot ping

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[bors]

☔ The latest upstream changes (presumably #146698) made this pull request unmergeable. Please resolve the merge conflicts.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.