Auto merge of #7225 - InquisitivePenguin:unnessecary-async, r=llogiq

New lint: `unused_async`

changelog: Adds a lint, `unused_async`, which checks for async functions with no await statements

`unused_async` is a lint that reduces code smell and overhead by encouraging async functions to be refactored into synchronous functions.

Fixes #7176

### Examples

```rust
async fn get_random_number() -> i64 {
    4 // Chosen by fair dice roll. Guaranteed to be random.
}
```

Could be written as:

```rust
fn get_random_number() -> i64 {
    4 // Chosen by fair dice roll. Guaranteed to be random.
}
```

Something like this, however, should **not** be caught by clippy:
```rust
#[async_trait]
trait AsyncTrait {
    async fn foo();
}

struct Bar;

#[async_trait]
impl AsyncTrait for Bar {
    async fn foo() {
        println!("bar");
    }
}
```

clippy_lints/src/lib.rs

@@ -364,6 +364,7 @@ mod unnecessary_sort_by;
 mod unnecessary_wraps;
 mod unnested_or_patterns;
 mod unsafe_removed_from_name;
+mod unused_async;
 mod unused_io_amount;
 mod unused_self;
 mod unused_unit;
@@ -961,6 +962,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         unnecessary_wraps::UNNECESSARY_WRAPS,
         unnested_or_patterns::UNNESTED_OR_PATTERNS,
         unsafe_removed_from_name::UNSAFE_REMOVED_FROM_NAME,
+        unused_async::UNUSED_ASYNC,
         unused_io_amount::UNUSED_IO_AMOUNT,
         unused_self::UNUSED_SELF,
         unused_unit::UNUSED_UNIT,
@@ -1273,6 +1275,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| box manual_map::ManualMap);
     store.register_late_pass(move || box if_then_some_else_none::IfThenSomeElseNone::new(msrv));
     store.register_early_pass(|| box bool_assert_comparison::BoolAssertComparison);
+    store.register_late_pass(|| box unused_async::UnusedAsync);
 
     store.register_group(true, "clippy::restriction", Some("clippy_restriction"), vec![
         LintId::of(arithmetic::FLOAT_ARITHMETIC),
@@ -1417,6 +1420,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(unit_types::LET_UNIT_VALUE),
         LintId::of(unnecessary_wraps::UNNECESSARY_WRAPS),
         LintId::of(unnested_or_patterns::UNNESTED_OR_PATTERNS),
+        LintId::of(unused_async::UNUSED_ASYNC),
         LintId::of(unused_self::UNUSED_SELF),
         LintId::of(wildcard_imports::ENUM_GLOB_USE),
         LintId::of(wildcard_imports::WILDCARD_IMPORTS),