fix: redundant_closure suggests wrongly with deref overload

Author: profetia

clippy_lints/src/eta_reduction.rs

@@ -11,6 +11,7 @@ use rustc_errors::Applicability;
 use rustc_hir::{BindingMode, Expr, ExprKind, FnRetTy, GenericArgs, Param, PatKind, QPath, Safety, TyKind};
 use rustc_infer::infer::TyCtxtInferExt;
 use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::adjustment::Adjust;
 use rustc_middle::ty::{
     self, Binder, ClosureKind, FnSig, GenericArg, GenericArgKind, List, Region, Ty, TypeVisitableExt, TypeckResults,
 };
@@ -153,10 +154,9 @@ fn check_closure<'tcx>(cx: &LateContext<'tcx>, outer_receiver: Option<&Expr<'tcx
             {
                 return;
             }
-            let callee_ty_adjusted = typeck
-                .expr_adjustments(callee)
-                .last()
-                .map_or(callee_ty, |a| a.target.peel_refs());
+
+            let callee_ty_adjustments = typeck.expr_adjustments(callee);
+            let callee_ty_adjusted = callee_ty_adjustments.last().map_or(callee_ty, |a| a.target);
 
             let sig = match callee_ty_adjusted.kind() {
                 ty::FnDef(def, _) => {
@@ -235,7 +235,20 @@ fn check_closure<'tcx>(cx: &LateContext<'tcx>, outer_receiver: Option<&Expr<'tcx
                                     },
                                     _ => (),
                                 }
+                            } else if let n_refs =
+                                callee_ty_adjustments
+                                    .iter()
+                                    .rev()
+                                    .fold(0, |acc, adjustment| match adjustment.kind {
+                                        Adjust::Deref(Some(_)) => acc + 1,
+                                        Adjust::Deref(_) if acc > 0 => acc + 1,
+                                        _ => acc,
+                                    })
+                                && n_refs > 0
+                            {
+                                snippet = format!("{}{snippet}", "*".repeat(n_refs));
                             }
+
                             diag.span_suggestion(
                                 expr.span,
                                 "replace the closure with the function itself",

tests/ui/eta.fixed

@@ -561,3 +561,48 @@ fn issue_14789() {
         std::convert::identity,
     );
 }
+
+fn issue_15072() {
+    use std::ops::Deref;
+
+    struct Foo;
+    impl Deref for Foo {
+        type Target = fn() -> &'static str;
+
+        fn deref(&self) -> &Self::Target {
+            fn hello() -> &'static str {
+                "Hello, world!"
+            }
+            &(hello as fn() -> &'static str)
+        }
+    }
+
+    fn accepts_fn(f: impl Fn() -> &'static str) {
+        println!("{}", f());
+    }
+
+    fn some_fn() -> &'static str {
+        todo!()
+    }
+
+    let f = &Foo;
+    accepts_fn(**f);
+    //~^ redundant_closure
+
+    let g = &some_fn;
+    accepts_fn(g);
+    //~^ redundant_closure
+
+    struct Bar(Foo);
+    impl Deref for Bar {
+        type Target = Foo;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+
+    let b = &Bar(Foo);
+    accepts_fn(***b);
+    //~^ redundant_closure
+}

tests/ui/eta.rs

@@ -561,3 +561,48 @@ fn issue_14789() {
         std::convert::identity,
     );
 }
+
+fn issue_15072() {
+    use std::ops::Deref;
+
+    struct Foo;
+    impl Deref for Foo {
+        type Target = fn() -> &'static str;
+
+        fn deref(&self) -> &Self::Target {
+            fn hello() -> &'static str {
+                "Hello, world!"
+            }
+            &(hello as fn() -> &'static str)
+        }
+    }
+
+    fn accepts_fn(f: impl Fn() -> &'static str) {
+        println!("{}", f());
+    }
+
+    fn some_fn() -> &'static str {
+        todo!()
+    }
+
+    let f = &Foo;
+    accepts_fn(|| f());
+    //~^ redundant_closure
+
+    let g = &some_fn;
+    accepts_fn(|| g());
+    //~^ redundant_closure
+
+    struct Bar(Foo);
+    impl Deref for Bar {
+        type Target = Foo;
+
+        fn deref(&self) -> &Self::Target {
+            &self.0
+        }
+    }
+
+    let b = &Bar(Foo);
+    accepts_fn(|| b());
+    //~^ redundant_closure
+}

tests/ui/eta.stderr

@@ -214,5 +214,23 @@ error: redundant closure
 LL |         let _field = bind.or_else(|| get_default()).unwrap();
    |                                   ^^^^^^^^^^^^^^^^ help: replace the closure with the function itself: `get_default`
 
-error: aborting due to 35 previous errors
+error: redundant closure
+  --> tests/ui/eta.rs:589:16
+   |
+LL |     accepts_fn(|| f());
+   |                ^^^^^^ help: replace the closure with the function itself: `**f`
+
+error: redundant closure
+  --> tests/ui/eta.rs:593:16
+   |
+LL |     accepts_fn(|| g());
+   |                ^^^^^^ help: replace the closure with the function itself: `g`
+
+error: redundant closure
+  --> tests/ui/eta.rs:606:16
+   |
+LL |     accepts_fn(|| b());
+   |                ^^^^^^ help: replace the closure with the function itself: `***b`
+
+error: aborting due to 38 previous errors