only find segs chain for missing methods when no available candidates

[bvanjoi]

Fixes #124946

This PR includes two changes:

• Extracting the lookup for the missing method in chains into a single function.
• Calling this function only when there are no candidates available.

[rustbot]

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[bvanjoi]

@rustbot ready

[nnethercote]

r=me once the typo is fixed.

For future reference, a nice way to do this kind of PR is to add the new test in a first commit, and then make the change to the error message in the second commit. That way the reviewer can easily see exactly how the change improves the error output.

@bors delegate=bvanjoi

[bors]

:v: @bvanjoi, you can now approve this pull request!

If @nnethercote told you to "r=me" after making some further change, please make that change, then do @bors r=@nnethercote

[bvanjoi]

@bors r=@nnethercote

[bors]

:pushpin: Commit ade33b02f237d484bef950ebf550bc12d49313d1 has been approved by nnethercote

It is now in the queue for this repository.