Use let else

[B-Reif]

Objective

Now that let...else is stable, we can improve legibility and reduce indentation in code that currently uses if let, followed by large blocks of code.

Solution

Replace instances of if let or match...return to use let...else.

The first commit in this PR fixes instances of clippy::manual_let_else, which detects basic instances of match that we can replace easily. The second commit in this PR additionally changes large function and loop blocks to return or continue early when a value is not matched. This unfortunately causes a lot of changed lines in the PR.

We can just cherry pick the first commit if we don't want to change beyond the pedantic clippy lint, or break these changes up into multiple PRs.

EDIT: For any potential reviewers, you might consider viewing the changes in your local editor or somewhere else that ignores indentation changes when highlighting lines.

[IceSentry]

There's already a PR #7104 but it's currently blocked on let else being supported by rustfmt

[B-Reif]

I probably should have checked for that before working on this.

[IceSentry]

With that said, your PR is much more recent, so if it gets support soon at least your PR is more up to date and therefore more likely to get merged

[IceSentry]

For the tracking issue in the rustfmt side rust-lang/rustfmt#4914

[B-Reif]

It looks like rustfmt's implementation work is more-or-less there also. Apparently held up by 'release/communication work'.
rust-lang/rustfmt#5690

[B-Reif]

let...else has been blocked on rustfmt for nearly four months now. Would we consider using a fork of rustfmt to get this unblocked until they can get this thing merged? This release gap is starting to seem unreasonable.

[mockersf]

EDIT: For any potential reviewers, you might consider viewing the changes in your local editor or somewhere else that ignores indentation changes when highlighting lines.

You can do that in GitHub UI for reviews:

Would we consider using a fork of rustfmt to get this unblocked until they can get this thing merged?

I don't think let ... else is an improvement big enough for that... so no for me

[alice-i-cecile]

This is no longer blocked, but it's going to be easier to just remake this than to fix the merge conflicts :) Please feel free to do so: this is generally a really nice change to make!