new lint: type_id_on_box

clippy_lints/src/methods/mod.rs

@@ -87,6 +87,7 @@ mod suspicious_command_arg_space;
 mod suspicious_map;
 mod suspicious_splitn;
 mod suspicious_to_owned;
+mod type_id_on_box;
 mod uninit_assumed_init;
 mod unit_hash;
 mod unnecessary_filter_map;
@@ -2922,6 +2923,37 @@ declare_clippy_lint! {
     "use of sort() when sort_unstable() is equivalent"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Looks for calls to `<Box<dyn Any> as Any>::type_id`.
+    ///
+    /// ### Why is this bad?
+    /// This most certainly does not do what the user expects and is very easy to miss.
+    /// Calling `type_id` on a `Box<dyn Any>` calls `type_id` on the `Box<..>` itself,
+    /// so this will return the `TypeId` of the `Box<dyn Any>` type (not the type id
+    /// of the value referenced by the box!).
+    ///
+    /// ### Example
+    /// ```rust,ignore
+    /// use std::any::{Any, TypeId};
+    ///
+    /// let any_box: Box<dyn Any> = Box::new(42_i32);
+    /// assert_eq!(any_box.type_id(), TypeId::of::<i32>()); // ⚠️ this fails!
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// use std::any::{Any, TypeId};
+    ///
+    /// let any_box: Box<dyn Any> = Box::new(42_i32);
+    /// assert_eq!((*any_box).type_id(), TypeId::of::<i32>());
+    /// //          ^ dereference first, to call `type_id` on `dyn Any`
+    /// ```
+    #[clippy::version = "1.47.0"]
+    pub TYPE_ID_ON_BOX,
+    suspicious,
+    "calling `.type_id()` on `Box<dyn Any>`"
+}
+
 declare_clippy_lint! {
     /// ### What it does
     /// Detects `().hash(_)`.
@@ -3878,6 +3910,9 @@ impl Methods {
                 ("to_os_string" | "to_path_buf" | "to_vec", []) => {
                     implicit_clone::check(cx, name, expr, recv);
                 },
+                ("type_id", []) => {
+                    type_id_on_box::check(cx, recv, expr.span);
+                }
                 ("unwrap", []) => {
                     match method_call(recv) {
                         Some(("get", recv, [get_arg], _, _)) => {

clippy_lints/src/methods/type_id_on_box.rs

@@ -0,0 +1,64 @@
+use crate::methods::TYPE_ID_ON_BOX;
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::source::snippet;
+use rustc_errors::Applicability;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_middle::ty::adjustment::Adjust;
+use rustc_middle::ty::adjustment::Adjustment;
+use rustc_middle::ty::Ty;
+use rustc_middle::ty::{self, ExistentialPredicate};
+use rustc_span::{sym, Span};
+
+fn is_dyn_trait(cx: &LateContext<'_>, ty: Ty<'_>) -> bool {
+    if let ty::Dynamic(preds, ..) = ty.kind() {
+        preds.iter().any(|p| match p.skip_binder() {
+            ExistentialPredicate::Trait(tr) => cx.tcx.is_diagnostic_item(sym::Any, tr.def_id),
+            _ => false,
+        })
+    } else {
+        false
+    }
+}
+
+pub(super) fn check(cx: &LateContext<'_>, receiver: &Expr<'_>, call_span: Span) {
+    let recv_adjusts = cx.typeck_results().expr_adjustments(receiver);
+
+    if let Some(Adjustment { target: recv_ty, .. }) = recv_adjusts.last()
+        && let ty::Ref(_, ty, _) = recv_ty.kind()
+        && let ty::Adt(adt, substs) = ty.kind()
+        && adt.is_box()
+        && is_dyn_trait(cx, substs.type_at(0))
+    {
+        span_lint_and_then(
+            cx,
+            TYPE_ID_ON_BOX,
+            call_span,
+            "calling `.type_id()` on a `Box<dyn Any>`",
+            |diag| {
+                let derefs = recv_adjusts
+                    .iter()
+                    .filter(|adj| matches!(adj.kind, Adjust::Deref(None)))
+                    .count();
+
+                let mut sugg = "*".repeat(derefs + 1);
+                sugg += &snippet(cx, receiver.span, "<expr>");
+
+                diag.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"
+                )
+                .span_suggestion(
+                    receiver.span,
+                    "consider dereferencing first",
+                    format!("({sugg})"),
+                    Applicability::MaybeIncorrect,
+                );
+            },
+        );
+    }
+}