[explicit_deref_methods]: do not lint on method chains (#14921)

changelog: [`explicit_deref_methods`]: do not lint on method chains
fixes #14593

Author: samueltardieu

clippy_lints/src/dereference.rs

@@ -305,7 +305,7 @@ impl<'tcx> LateLintPass<'tcx> for Dereferencing<'tcx> {
                     RefOp::Method { mutbl, is_ufcs }
                         if !is_lint_allowed(cx, EXPLICIT_DEREF_METHODS, expr.hir_id)
                             // Allow explicit deref in method chains. e.g. `foo.deref().bar()`
-                            && (is_ufcs || !in_postfix_position(cx, expr)) =>
+                            && (is_ufcs || !is_in_method_chain(cx, expr)) =>
                     {
                         let ty_changed_count = usize::from(!deref_method_same_type(expr_ty, typeck.expr_ty(sub_expr)));
                         self.state = Some((
@@ -728,7 +728,13 @@ fn deref_method_same_type<'tcx>(result_ty: Ty<'tcx>, arg_ty: Ty<'tcx>) -> bool {
     }
 }
 
-fn in_postfix_position<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'tcx>) -> bool {
+fn is_in_method_chain<'tcx>(cx: &LateContext<'tcx>, e: &'tcx Expr<'tcx>) -> bool {
+    if let ExprKind::MethodCall(_, recv, _, _) = e.kind
+        && matches!(recv.kind, ExprKind::MethodCall(..))
+    {
+        return true;
+    }
+
     if let Some(parent) = get_parent_expr(cx, e)
         && parent.span.eq_ctxt(e.span)
     {

tests/ui/explicit_deref_methods.fixed

@@ -81,12 +81,10 @@ fn main() {
     let b: String = concat(just_return(a));
     //~^ explicit_deref_methods
 
-    let b: &str = &**a;
-    //~^ explicit_deref_methods
+    let b: &str = a.deref().deref();
 
     let opt_a = Some(a.clone());
-    let b = &*opt_a.unwrap();
-    //~^ explicit_deref_methods
+    let b = opt_a.unwrap().deref();
 
     Aaa::deref(&Aaa);
     Aaa::deref_mut(&mut Aaa);

tests/ui/explicit_deref_methods.rs

@@ -82,11 +82,9 @@ fn main() {
     //~^ explicit_deref_methods
 
     let b: &str = a.deref().deref();
-    //~^ explicit_deref_methods
 
     let opt_a = Some(a.clone());
     let b = opt_a.unwrap().deref();
-    //~^ explicit_deref_methods
 
     Aaa::deref(&Aaa);
     Aaa::deref_mut(&mut Aaa);

tests/ui/explicit_deref_methods.stderr

@@ -56,40 +56,28 @@ LL |     let b: String = concat(just_return(a).deref());
    |                            ^^^^^^^^^^^^^^^^^^^^^^ help: try: `just_return(a)`
 
 error: explicit `deref` method call
-  --> tests/ui/explicit_deref_methods.rs:84:19
-   |
-LL |     let b: &str = a.deref().deref();
-   |                   ^^^^^^^^^^^^^^^^^ help: try: `&**a`
-
-error: explicit `deref` method call
-  --> tests/ui/explicit_deref_methods.rs:88:13
-   |
-LL |     let b = opt_a.unwrap().deref();
-   |             ^^^^^^^^^^^^^^^^^^^^^^ help: try: `&*opt_a.unwrap()`
-
-error: explicit `deref` method call
-  --> tests/ui/explicit_deref_methods.rs:123:31
+  --> tests/ui/explicit_deref_methods.rs:121:31
    |
 LL |     let b: &str = expr_deref!(a.deref());
    |                               ^^^^^^^^^ help: try: `&*a`
 
 error: explicit `deref` method call
-  --> tests/ui/explicit_deref_methods.rs:141:14
+  --> tests/ui/explicit_deref_methods.rs:139:14
    |
 LL |     let _ = &Deref::deref(&"foo");
    |              ^^^^^^^^^^^^^^^^^^^^ help: try: `*&"foo"`
 
 error: explicit `deref_mut` method call
-  --> tests/ui/explicit_deref_methods.rs:143:14
+  --> tests/ui/explicit_deref_methods.rs:141:14
    |
 LL |     let _ = &DerefMut::deref_mut(&mut x);
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&mut **&mut x`
 
 error: explicit `deref_mut` method call
-  --> tests/ui/explicit_deref_methods.rs:144:14
+  --> tests/ui/explicit_deref_methods.rs:142:14
    |
 LL |     let _ = &DerefMut::deref_mut((&mut &mut x).deref_mut());
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `&mut ***(&mut &mut x)`
 
-error: aborting due to 15 previous errors
+error: aborting due to 13 previous errors