Add method find_ancestor_not_from_macro and find_ancestor_not_from_extern_macro to fix find_oldest_ancestor_in_same_ctxt

[xizheyin]

As I was using it, I realized that the function is supposed to walk up to expand the chain? This seems to be the opposite of what I understood.

r? @jieyouxu

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[jieyouxu]

Hm, I'm not actually 100% this helper is doing what I originally wanted it to do... This will try to keep traversing up the macro invocation chain that produced the provided final span at hand to find the oldest ancestor which has... the same syntax context as the provided final span (???) I suppose this might work if the passed in span you know is "local" and the user actually has control over 🤔

I feel like this is wrong, or least quite confusing... I wonder if you could go the other direction and use instead https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.find_ancestor_in_same_ctxt or https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/struct.Span.html#method.find_ancestor_inside_same_ctxt and get rid of this helper.

[jieyouxu]

@rustbot author (re. discussion)

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[xizheyin]

I found some problems with the comments of these APIs, I subconsciously thought that finding an ancestor would be traversing the macro expansion chain from bottom to top, but they all use “walk down”. This seems to create ambiguity. I'll look into these APIs later when I have time and find the best way to do it.

[xizheyin]

I found that find_oldest_ancestor_in_same_ctxt does have a logical problem, but by some mischance it works successfully in some places.
The places where it is needed are basically related to macros, and my understanding is that the proper ancestor should be found without any other span help, e.g. outer in span.find_ancestor_inside(outer).

I added two methods that allow for more fine-grained control. This change should help fix all the diagnostics shown in the macros in the standard library(#142403).

@rustbot ready

[jieyouxu]

Hm, is it possible to add specific test cases demonstrating the behaviors of

• find_ancestor_not_from_macro
• find_ancestor_not_from_extern_macro
• find_oldest_ancestor_in_same_ctxt

Otherwise, we'd have no idea what effect changing the impls of these helpers might be.

[xizheyin]

I don't know how to add the right test cases, but I can find the test case corresponding to the diagnostic I modified in this PR, which should be very helpful to understand its behavior. I'm going to insert the debug macro in the method of concern, and then print the debug message via what compile-flag? Do you have a better idea?