Write lint to check for dashmaps entries being held

Signed-off-by: Marcel Müller <neikos@neikos.email>

lints/dashmap-ref/ui/main.rs

@@ -1 +1,50 @@
-fn main() {}
+#[expect(unused_must_use)]
+fn main() {
+    held_across();
+    held_across_multiple();
+    dropped_before();
+    all_kinds();
+}
+
+async fn held_across() {
+    let map = dashmap::DashMap::<String, String>::default();
+
+    let reference = map.get("Hello");
+
+    std::future::pending::<()>().await;
+
+    println!("{reference:?}");
+}
+
+async fn held_across_multiple() {
+    let map = dashmap::DashMap::<String, String>::default();
+
+    let _reference = map.get("Hello");
+
+    std::future::pending::<()>().await;
+
+    std::future::pending::<()>().await;
+}
+
+async fn dropped_before() {
+    let map = dashmap::DashMap::<String, String>::default();
+
+    let _ = map.get("Hello");
+
+    std::future::pending::<()>().await;
+}
+
+async fn all_kinds() {
+    let map = dashmap::DashMap::<String, String>::default();
+
+    let _ref = map.get("Hello");
+    let _ref_mut = map.get_mut("Hello");
+    let _entry = map.entry("Hello".to_string());
+    let _opt_entry = map.try_entry("Hello".to_string());
+    let _iter_entry = map.iter();
+    let _iter_mut_entry = map.iter_mut();
+    let _direct_ref = map.get("Hello").unwrap();
+    let _direct_ref_mut = map.get_mut("Hello").unwrap();
+
+    std::future::pending::<()>().await;
+}

lints/dashmap-ref/ui/main.stderr

@@ -0,0 +1,126 @@
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:12:9
+   |
+LL |     let reference = map.get("Hello");
+   |         ^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:14:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+   = note: `#[warn(dashmap_ref)]` on by default
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:22:9
+   |
+LL |     let _reference = map.get("Hello");
+   |         ^^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:24:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+LL |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:40:9
+   |
+LL |     let _ref = map.get("Hello");
+   |         ^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:41:9
+   |
+LL |     let _ref_mut = map.get_mut("Hello");
+   |         ^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:42:9
+   |
+LL |     let _entry = map.entry("Hello".to_string());
+   |         ^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:43:9
+   |
+LL |     let _opt_entry = map.try_entry("Hello".to_string());
+   |         ^^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:44:9
+   |
+LL |     let _iter_entry = map.iter();
+   |         ^^^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:45:9
+   |
+LL |     let _iter_mut_entry = map.iter_mut();
+   |         ^^^^^^^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:46:9
+   |
+LL |     let _direct_ref = map.get("Hello").unwrap();
+   |         ^^^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: this dashmap reference is held across an 'await' point. Either drop it before the 'await', or read it again afterwards.
+  --> $DIR/main.rs:47:9
+   |
+LL |     let _direct_ref_mut = map.get_mut("Hello").unwrap();
+   |         ^^^^^^^^^^^^^^^
+   |
+note: these are all the await points this ref is held through
+  --> $DIR/main.rs:49:34
+   |
+LL |     std::future::pending::<()>().await;
+   |                                  ^^^^^
+
+warning: 10 warnings emitted
+