Use try{} in try_fold to decouple iterators in the library from Try details

[scottmcm]

I'd like to experiment with changing the ?/try desugaring and correspondingly the Try trait (see #42327 for discussions about the suboptimalities of the current one) and this change would keep from needing any cfg(bootstrap) in iterator things.

This will be lowered to the same thing, so shouldn't cause any perf issues:

rust/compiler/rustc_ast_lowering/src/expr.rs

Lines 428 to 429 in 08e2d46

	/// Desugar `try { <stmts>; <expr> }` into `{ <stmts>; ::std::ops::Try::from_ok(<expr>) }`,
	/// `try { <stmts>; }` into `{ <stmts>; ::std::ops::Try::from_ok(()) }`

But I'll trigger I've triggered a perf run just in case.

EDIT: changed to a draft because of the rustfmt-only syntax error. zulip thread about it: https://rust-lang.zulipchat.com/#narrow/stream/122651-general/topic/New.20bootstrap.20rustfmt.20doesn't.20support.20syntax.20from.20sept.3F/near/213098097

EDIT: This now includes a rustfmt version bump to get through tidy.

[rust-highfive]

r? @shepmaster

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[scottmcm]

@bors try
@rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 8374c17 with merge 2c067c5235e779cd75e9f0cdfe572c64f1a12b9b...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 2c067c5235e779cd75e9f0cdfe572c64f1a12b9b (2c067c5235e779cd75e9f0cdfe572c64f1a12b9b)

[rust-timer]

Queued 2c067c5235e779cd75e9f0cdfe572c64f1a12b9b with parent d65c08e, future comparison URL.

[rust-timer]

Finished benchmarking try commit (2c067c5235e779cd75e9f0cdfe572c64f1a12b9b): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[scottmcm]

Perf results look essentially unchanged to me, as expected.

[shepmaster]

@bors r+ rollup

[bors]

📌 Commit 8374c17 has been approved by shepmaster