Use if-let guards in the codebase and various other pattern cleanups

[LeSeulArtichaut]

Dogfooding if-let guards as experimentation for the feature.

Tracking issue #51114. Conflicts with #87937.

[rust-highfive]

r? @nagisa

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[nagisa]

Please split up the different kinds of cleanups (i.e. those that use if-let-guards and those that don't) into different commits so that the changes are better isolated and easier to review.

I think the scope of this PR is otherwise reasonable enough for dogfooding purposes.

[LeSeulArtichaut]

@nagisa I split the changes in two commits as you requested. Since the bulk of the changes is actually reindenting the code (caused by removing an if-let block), the diff will look less noisy if viewed with whitespace changes hidden.

I also forgot to mention that rustfmt doesn't yet support formatting if-let guards (rust-lang/rustfmt#4955) so I tried my best to format them by hand.

[nagisa]

Seems nice. Up to you if you want to handle the comments above.

@bors delegate=LeSeulArtichaut

[bors]

✌️ @LeSeulArtichaut can now approve this pull request

[LeSeulArtichaut]

@nagisa Replaced the problematic patterns with if-let guards.

[LeSeulArtichaut]

@bors r=nagisa

[bors]

📌 Commit f83887d5dc94c25acfbdbc2b51aa19bd7f5ff1ee has been approved by nagisa

[LeSeulArtichaut]

Removed all #[allow(incomplete_features)] due to the use of if-let guards.
@bors r=nagisa

[bors]

📌 Commit 1cfb326b5caba719be0553c1b0bb56c974955dbc has been approved by nagisa

[LeSeulArtichaut]

@bors r=nagisa

[bors]

📌 Commit 2b0c8ff has been approved by nagisa

[bors]

⌛ Testing commit 2b0c8ff with merge 76e755c...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing 76e755c to master...