Fix coerce_container_to_any false positive on autoderef (#15057)

Fixes the false positive reported in rust-lang/rust-clippy#15045. ~I
still need to work out how to fix the suggestion.~

changelog: none

clippy_lints/src/coerce_container_to_any.rs

@@ -1,9 +1,10 @@
 use clippy_utils::diagnostics::span_lint_and_sugg;
-use clippy_utils::source::snippet;
+use clippy_utils::sugg::{self, Sugg};
 use clippy_utils::sym;
 use rustc_errors::Applicability;
 use rustc_hir::{Expr, ExprKind};
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::adjustment::{Adjust, PointerCoercion};
 use rustc_middle::ty::{self, ExistentialPredicate, Ty, TyCtxt};
 use rustc_session::declare_lint_pass;
 
@@ -49,23 +50,18 @@ declare_lint_pass!(CoerceContainerToAny => [COERCE_CONTAINER_TO_ANY]);
 
 impl<'tcx> LateLintPass<'tcx> for CoerceContainerToAny {
     fn check_expr(&mut self, cx: &LateContext<'tcx>, e: &'tcx Expr<'_>) {
-        // If this expression has an effective type of `&dyn Any` ...
-        {
-            let coerced_ty = cx.typeck_results().expr_ty_adjusted(e);
-
-            let ty::Ref(_, coerced_ref_ty, _) = *coerced_ty.kind() else {
-                return;
-            };
-            if !is_dyn_any(cx.tcx, coerced_ref_ty) {
-                return;
-            }
+        // If this expression was coerced to `&dyn Any` ...
+        if !cx.typeck_results().expr_adjustments(e).last().is_some_and(|adj| {
+            matches!(adj.kind, Adjust::Pointer(PointerCoercion::Unsize)) && is_ref_dyn_any(cx.tcx, adj.target)
+        }) {
+            return;
         }
 
         let expr_ty = cx.typeck_results().expr_ty(e);
         let ty::Ref(_, expr_ref_ty, _) = *expr_ty.kind() else {
             return;
         };
-        // ... but only due to coercion ...
+        // ... but it's not actually `&dyn Any` ...
         if is_dyn_any(cx.tcx, expr_ref_ty) {
             return;
         }
@@ -78,23 +74,37 @@ impl<'tcx> LateLintPass<'tcx> for CoerceContainerToAny {
         }
 
         // ... that's probably not intended.
-        let (span, deref_count) = match e.kind {
+        let (target_expr, deref_count) = match e.kind {
             // If `e` was already an `&` expression, skip `*&` in the suggestion
-            ExprKind::AddrOf(_, _, referent) => (referent.span, depth),
-            _ => (e.span, depth + 1),
+            ExprKind::AddrOf(_, _, referent) => (referent, depth),
+            _ => (e, depth + 1),
         };
+        let ty::Ref(_, _, mutability) = *cx.typeck_results().expr_ty_adjusted(e).kind() else {
+            return;
+        };
+        let sugg = sugg::make_unop(
+            &format!("{}{}", mutability.ref_prefix_str(), str::repeat("*", deref_count)),
+            Sugg::hir(cx, target_expr, ".."),
+        );
         span_lint_and_sugg(
             cx,
             COERCE_CONTAINER_TO_ANY,
             e.span,
-            format!("coercing `{expr_ty}` to `&dyn Any`"),
+            format!("coercing `{expr_ty}` to `{}dyn Any`", mutability.ref_prefix_str()),
             "consider dereferencing",
-            format!("&{}{}", str::repeat("*", deref_count), snippet(cx, span, "x")),
+            sugg.to_string(),
             Applicability::MaybeIncorrect,
         );
     }
 }
 
+fn is_ref_dyn_any(tcx: TyCtxt<'_>, ty: Ty<'_>) -> bool {
+    let ty::Ref(_, ref_ty, _) = *ty.kind() else {
+        return false;
+    };
+    is_dyn_any(tcx, ref_ty)
+}
+
 fn is_dyn_any(tcx: TyCtxt<'_>, ty: Ty<'_>) -> bool {
     let ty::Dynamic(traits, ..) = ty.kind() else {
         return false;

tests/ui/coerce_container_to_any.fixed

@@ -3,7 +3,7 @@
 use std::any::Any;
 
 fn main() {
-    let x: Box<dyn Any> = Box::new(());
+    let mut x: Box<dyn Any> = Box::new(());
     let ref_x = &x;
 
     f(&*x);
@@ -15,12 +15,23 @@ fn main() {
     let _: &dyn Any = &*x;
     //~^ coerce_container_to_any
 
+    let _: &dyn Any = &*x;
+    //~^ coerce_container_to_any
+
+    let _: &mut dyn Any = &mut *x;
+    //~^ coerce_container_to_any
+
     f(&42);
     f(&Box::new(()));
     f(&Box::new(Box::new(())));
+    let ref_x = &x;
     f(&**ref_x);
     f(&*x);
     let _: &dyn Any = &*x;
+
+    // https://github.com/rust-lang/rust-clippy/issues/15045
+    #[allow(clippy::needless_borrow)]
+    (&x).downcast_ref::<()>().unwrap();
 }
 
 fn f(_: &dyn Any) {}

tests/ui/coerce_container_to_any.rs

@@ -3,7 +3,7 @@
 use std::any::Any;
 
 fn main() {
-    let x: Box<dyn Any> = Box::new(());
+    let mut x: Box<dyn Any> = Box::new(());
     let ref_x = &x;
 
     f(&x);
@@ -15,12 +15,23 @@ fn main() {
     let _: &dyn Any = &x;
     //~^ coerce_container_to_any
 
+    let _: &dyn Any = &mut x;
+    //~^ coerce_container_to_any
+
+    let _: &mut dyn Any = &mut x;
+    //~^ coerce_container_to_any
+
     f(&42);
     f(&Box::new(()));
     f(&Box::new(Box::new(())));
+    let ref_x = &x;
     f(&**ref_x);
     f(&*x);
     let _: &dyn Any = &*x;
+
+    // https://github.com/rust-lang/rust-clippy/issues/15045
+    #[allow(clippy::needless_borrow)]
+    (&x).downcast_ref::<()>().unwrap();
 }
 
 fn f(_: &dyn Any) {}

tests/ui/coerce_container_to_any.stderr

@@ -19,5 +19,17 @@ error: coercing `&std::boxed::Box<dyn std::any::Any>` to `&dyn Any`
 LL |     let _: &dyn Any = &x;
    |                       ^^ help: consider dereferencing: `&*x`
 
-error: aborting due to 3 previous errors
+error: coercing `&mut std::boxed::Box<dyn std::any::Any>` to `&dyn Any`
+  --> tests/ui/coerce_container_to_any.rs:18:23
+   |
+LL |     let _: &dyn Any = &mut x;
+   |                       ^^^^^^ help: consider dereferencing: `&*x`
+
+error: coercing `&mut std::boxed::Box<dyn std::any::Any>` to `&mut dyn Any`
+  --> tests/ui/coerce_container_to_any.rs:21:27
+   |
+LL |     let _: &mut dyn Any = &mut x;
+   |                           ^^^^^^ help: consider dereferencing: `&mut *x`
+
+error: aborting due to 5 previous errors