fix drop scope for super let bindings within if let

[dianne]

Fixes #145328 by making non-lifetime-extended super let reuse the logic used to compute drop scopes for non-lifetime-extended temporaries.

Also fixes #145374, which regressed due to #143376 introducing if let-like scopes for match arms with guards.

Tracking issue for super let: #139076

This is a regression fix / breaking change for macros stably exposing super let, including pin! and format_args!.
Nominating to be discussed alongside #145328: @rustbot label +I-lang-nominated +I-libs-api-nominated

[rustbot]

r? @jackh726

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[compiler-errors]

@dianne does this need crater?

[traviscross]

@bors2 try

[rust-bors]

☀️ Try build successful (CI)
Build commit: a498031 (a4980311fb7bb9e7893708e6bd3fbbfb2819fd3d, parent: 350d0ef0ec0493e6d21cfb265cb8211a0e74d766)

[compiler-errors]

@craterbot check

[craterbot]

👌 Experiment pr-145342 created and queued.
🤖 Automatically detected try build a498031
🔍 You can check out the queue and this experiment's details.

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[theemathas]

How does #145374 relate to this? The code there doesn't use super let anywhere.

[dianne]

Format arguments, e.g. as used by write!, are also implemented in terms of super let. This is visible when printing the HIR of that test case:

f.write_fmt({
        super let args =
            (&pointee.behind_pointer(),
                &ItemIdentifier::path(&ItemIdentifier::nserror()));
        super let args =
            [format_argument::new_display(args.0),
                    format_argument::new_display(args.1)];
        format_arguments::new_v1(&["", ", "], &args)
    })

The interaction between that, old editions not treating block tail expressions as temporary drop scopes, my guard scoping PR, and the existing super let scoping bug is what caused that particular regression.

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

[fee1-dead]

r? rust-lang/compiler

[nnethercote]

@bors r+

[bors]

📌 Commit 8fc3938 has been approved by nnethercote

It is now in the queue for this repository.

[apiraino]

Beta backport declined as per compiler team on Zulip, we judge the backport a bit risky due to being about 2 weeks away from the new release.

@rustbot label -beta-nominated