Fix autoborrowing when coercing to a mutable raw pointer

[FabianWolff]

This PR fixes#86262. Essentially, the problem is that apply_adjustments() in FnCtxt currently only takes mutability of references into account for automatic borrows, while it should also consider mutable raw pointers. I have fixed this, so that the example in #86262 compiles with my changes.

[rust-highfive]

r? @jackh726

(rust-highfive has picked a reviewer for you, use r? to override)

[jackh726]

Not too familiar with this code. And seems pretty important in terms of soundness. So I don't feel comfortable reviewing this myself. Trying to pick someone randomly, but avoiding people that aren't around atm; let's r? @Aaron1011

[Aaron1011]

This seems to make sense, but I wonder if there's some language design reason why this wasn't previously allowed.

cc @rust-lang/lang

[nikomatsakis]

Discussed in the @rust-lang/lang team meeting today. We weren't really sure what is being proposed here! @scottmcm is going to follow up and try to figure out how to describe the effects of this proposed PR on the language itself.

[Dylan-DPC]

@scottmcm any updates on the follow up?

[bors]

☔ The latest upstream changes (presumably #102306) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

@FabianWolff
Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you do please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive