hir_typeck: suggest removing redundant .iter() and iter_mut ()calls

[vishnupoddar12]

This diagnostic intercepts cases where .iter() or .iter_mut() is called on a type that already implements Iterator

Instead of confusing trait fallback suggestions, it explicitly guides the user to remove the .iter() call. It implements precise span computation to support cargo fix while carefully preserving all inline comments and horizontal whitespace during the AST string replacement.

Fixes #153667

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[estebank]

You're doing a lot of Span wrangling and dealing with code snippets to try and create a suitable Span for the suggestion, but instead you should be leveraging the hir Exprs to find the right spans. you can do things like rcvr.span.between(arg[0].span) to construct a span that points at

rcvr.method(arg0, arg1);
    ^^^^^^^^

View changes since this review

[vishnupoddar12]

Thanks @estebank, I was able to reduce lot of unnecessary span wrangling reducing code change by 30-40 lines. It should be cleaner now.

[vishnupoddar12]

Hi @estebank, Friendly ping! I updated the PR to use rcvr.span.between() based on last comment. Let me know when you have a chance to take a look, or if there's anything else you'd like me to tweak.