Use a GAT for Residual

[ghost]

See discussion in #91285 (comment) for info.

Tracking issue: #91285

These unstable APIs were updated:
array_try_from_fn: #89379
array_try_map: #79711
iterator_try_collect: #94047
iterator_try_reduce: #87053
try_find: #63178

[ghost]

@rustbot label -T-libs

[ghost]

@rustbot label -S-waiting-on-review +S-waiting-on-author

[rustbot]

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

Examples of T-libs-api changes:

• Stabilizing library features
• Introducing insta-stable changes such as new implementations of existing stable traits on existing stable types
• Introducing new or changing existing unstable library APIs (excluding permanently unstable features / features without a tracking issue)
• Changing public documentation in ways that create new stability guarantees
• Changing observable runtime behavior of library APIs

[ghost]

@bors try

[bors]

@H4x5: 🔑 Insufficient privileges: not in try users

[ghost]

r? @scottmcm

[ghost]

@rustbot label +S-waiting-on-review -S-waiting-on-author

[scottmcm]

Thanks for sending a PR! I agree that all the core impls would work great with a GAT here.

I have some concerns about using a GAT here, though. I've put some comments in the tracking issue (#91285 (comment)) so they won't get lost when this PR either merges or gets closed; let's discuss this more over there?

[scottmcm]

Given your comment over in #91285 (comment), I'm going to close this for now. We can always reopen it in future should the design discussions end up deciding that that GAT version is better!