new lint: type_id_on_box

tests/ui/type_id_on_box.fixed

@@ -0,0 +1,20 @@
+//@run-rustfix
+
+#![warn(clippy::type_id_on_box)]
+
+use std::any::{Any, TypeId};
+
+fn existential() -> impl Any {
+    Box::new(1) as Box<dyn Any>
+}
+
+fn main() {
+    let any_box: Box<dyn Any> = Box::new(0usize);
+    let _ = (*any_box).type_id();
+    let _ = TypeId::of::<Box<dyn Any>>(); // don't lint, user probably explicitly wants to do this
+    let _ = (*any_box).type_id();
+    let any_box: &Box<dyn Any> = &(Box::new(0usize) as Box<dyn Any>);
+    let _ = (**any_box).type_id(); // 2 derefs are needed here
+    let b = existential();
+    let _ = b.type_id(); // don't lint
+}

tests/ui/type_id_on_box.rs

@@ -0,0 +1,20 @@
+//@run-rustfix
+
+#![warn(clippy::type_id_on_box)]
+
+use std::any::{Any, TypeId};
+
+fn existential() -> impl Any {
+    Box::new(1) as Box<dyn Any>
+}
+
+fn main() {
+    let any_box: Box<dyn Any> = Box::new(0usize);
+    let _ = any_box.type_id();
+    let _ = TypeId::of::<Box<dyn Any>>(); // don't lint, user probably explicitly wants to do this
+    let _ = (*any_box).type_id();
+    let any_box: &Box<dyn Any> = &(Box::new(0usize) as Box<dyn Any>);
+    let _ = any_box.type_id(); // 2 derefs are needed here
+    let b = existential();
+    let _ = b.type_id(); // don't lint
+}

tests/ui/type_id_on_box.stderr

@@ -0,0 +1,25 @@
+error: calling `.type_id()` on a `Box<dyn Any>`
+  --> $DIR/type_id_on_box.rs:13:13
+   |
+LL |     let _ = any_box.type_id();
+   |             -------^^^^^^^^^^
+   |             |
+   |             help: consider dereferencing first: `(*any_box)`
+   |
+   = note: this returns the type id of the literal type `Box<dyn Any>` instead of the type id of the boxed value, which is most likely not what you want
+   = note: if this is intentional, use `TypeId::of::<Box<dyn Any>>()` instead, which makes it more clear
+   = note: `-D clippy::type-id-on-box` implied by `-D warnings`
+
+error: calling `.type_id()` on a `Box<dyn Any>`
+  --> $DIR/type_id_on_box.rs:17:13
+   |
+LL |     let _ = any_box.type_id(); // 2 derefs are needed here
+   |             -------^^^^^^^^^^
+   |             |
+   |             help: consider dereferencing first: `(**any_box)`
+   |
+   = note: this returns the type id of the literal type `Box<dyn Any>` instead of the type id of the boxed value, which is most likely not what you want
+   = note: if this is intentional, use `TypeId::of::<Box<dyn Any>>()` instead, which makes it more clear
+
+error: aborting due to 2 previous errors
+