Remove FromResidual param default

[camsteffen]

Tracking issue

This resolves a concern that has been raised several times in the tracking issue. I'll summarize the reasons as:

• Explicitly naming the type is better for readability, both in impl FromResidual instances and in the trait Try definition.
• The convenience of omitting the type is small.
• The default value can be difficult to parse when reading the docs.
• It can be confusing that FromResidual uses Self as Try when it does not depend on Try.

[rustbot]

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

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[bors]

☔ The latest upstream changes (presumably #149397) made this pull request unmergeable. Please resolve the merge conflicts.

[wesleywiser]

Hi @scottmcm, I think this is ready for a review when you get a chance. Thanks!

[RalfJung]

A reroll seems appropriate after months of not hearing form the reviewer, but OTOH @scottmcm is one of the main people driving the try stuff AFAIK so... it'd really be best if we could hear from him. @scottmcm could you take a look at this?

[scottmcm]

I still think that if this change does anything useful it's a bug in the type system or its implementation, and thus that it shouldn't be done and that bug should be fixed instead.

But at the same time, it's unstable and because of the bug it's apparently helpful to change the implementations this way, so 🤷 If you really want to do it, go for it.

[camsteffen]

I was not aware of possible type system bug related to this. I thought this is more of an ergonomics change to make the feature generally less confusing and more straightforward. See the points in my original post. Comments in the tracking issue along these lines are here, here and here.

Upon searching the comments again, I see there is a comment suggesting there may be a bug here, but I wasn't able to reproduce it in the playground. So maybe that is a bug that's been fixed.