don't leak internal temporaries from dbg!, even on false unwind paths

[dianne]

Hopefully fixes #155902 for real this time! See #155902 (comment) and the following comments for why I think matching on a tuple of dbg!'s arguments seems not to be a viable implementation strategy. Instead, this uses super let to get better control over scoping, which unfortunately makes for pretty awkward code with the current version of super let.

One alternative to this would be reverting all the recent changes to dbg! for now and re-landing them once super let's replacement is usable. That will probably be the go-to answer for these tricky scoping problems.

Another alternative would be to change Rust's static semantics to (hopefully) make the match-based dbg! not a problem.

I also noticed std::macros::dbg_internal! seems to be stable in beta (playground), so I've taken the liberty of de-stabilizing it. I can pull that out into a separate PR later if that'd be better, but I figure if we revert the recent changes to dbg!, we'd also be reverting the addition of dbg_internal!, so for now that's here.

This may also need clippy and compiler review, but let's start with r? libs

[rustbot]

The Clippy subtree was changed

cc @rust-lang/clippy

[apiraino]

t-compiler will defer backport decision to t-libs since this is almost entirely in their purview

[cuviper]

Maybe it's worth a crater run compared to stable?

[dianne]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 012cd62 (012cd62c9add58ab3910e44c137d87db3ab70f61, parent: 67bcaa9c4b1b1015efae3d591155c79760b1e0a5)

[dianne]

I guess for a crater run against stable, we'd need a branch with the commits from this, #154994, #154074, and #149869 cherry-picked? I'll try setting that up as a draft PR against stable.

[cuviper]

I just meant stable vs. this nightly -- which will include other changes, but that's true if we beta-backport too. I think we can just do this:

@craterbot check start=1.95.0

[cuviper]

@craterbot check start=1.95.0 end=try#012cd62c9add58ab3910e44c137d87db3ab70f61

[craterbot]

👌 Experiment pr-155915 created and queued.
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[cuviper]

Bumping priority since this is release-relevant for 1.96-beta.

@craterbot name=pr-155915 p=1

[craterbot]

📝 Configuration of the pr-155915 experiment changed.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more