Merge #10440

10440: Fix Clippy warnings and replace some `if let`s with `match` r=Veykril a=arzg

I decided to try fixing a bunch of Clippy warnings. I am aware of this project’s opinion of Clippy (I have read both [rust-lang/clippy#5537](https://github.com/rust-lang/rust-clippy/issues/5537) and [rust-analyzer/rowan#57 (comment)](https://github.com/rust-analyzer/rowan/pull/57#discussion_r415676159)), so I totally understand if part of or the entirety of this PR is rejected. In particular, I can see how the semicolons and `if let` vs `match` commits provide comparatively little benefit when compared to the ensuing churn.

I tried to separate each kind of change into its own commit to make it easier to discard certain changes. I also only applied Clippy suggestions where I thought they provided a definite improvement to the code (apart from semicolons, which is IMO more of a formatting/consistency question than a linting question). In the end I accumulated a list of 28 Clippy lints I ignored entirely.

Sidenote: I should really have asked about this on Zulip before going through all 1,555 `if let`s in the codebase to decide which ones definitely look better as `match` :P

Co-authored-by: Aramis Razzaghipour <aramisnoah@gmail.com>

crates/ide_assists/src/handlers/extract_function.rs

@@ -827,10 +827,9 @@ impl FunctionBody {
         locals
             .map(|local| (local, local.source(ctx.db())))
             .filter(|(_, src)| is_defined_outside_of_body(ctx, self, src))
-            .filter_map(|(local, src)| {
-                if let Either::Left(src) = src.value {
-                    Some((local, src))
-                } else {
+            .filter_map(|(local, src)| match src.value {
+                Either::Left(src) => Some((local, src)),
+                Either::Right(_) => {
                     stdx::never!(false, "Local::is_self returned false, but source is SelfParam");
                     None
                 }