useless_asref: no lint if in a closure to change the ref depth

Removing the `.as_ref()` or `.as_mut()` as the top-level expression in a
closure may change the type of the result. In this case, it may be
better not to lint rather than proposing a fix that would not work.

Author: samueltardieu

clippy_lints/src/methods/useless_asref.rs

@@ -55,12 +55,19 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, call_name: &str,
         let (base_res_ty, res_depth) = walk_ptrs_ty_depth(res_ty);
         let (base_rcv_ty, rcv_depth) = walk_ptrs_ty_depth(rcv_ty);
         if base_rcv_ty == base_res_ty && rcv_depth >= res_depth {
-            // allow the `as_ref` or `as_mut` if it is followed by another method call
-            if let Some(parent) = get_parent_expr(cx, expr)
-                && let hir::ExprKind::MethodCall(segment, ..) = parent.kind
-                && segment.ident.span != expr.span
-            {
-                return;
+            if let Some(parent) = get_parent_expr(cx, expr) {
+                // allow the `as_ref` or `as_mut` if it is followed by another method call
+                if let hir::ExprKind::MethodCall(segment, ..) = parent.kind
+                    && segment.ident.span != expr.span
+                {
+                    return;
+                }
+
+                // allow the `as_ref` or `as_mut` if they belong to a closure that changes
+                // the number of references
+                if matches!(parent.kind, hir::ExprKind::Closure(..)) && rcv_depth != res_depth {
+                    return;
+                }
             }
 
             let mut applicability = Applicability::MachineApplicable;

tests/ui/useless_asref.fixed

@@ -198,6 +198,11 @@ fn issue_12528() {
     let _ = opt.as_ref().map(RcWeak::clone);
 }
 
+fn issue_14088() {
+    let s = Some("foo");
+    let _: Option<&str> = s.as_ref().map(|x| x.as_ref());
+}
+
 fn main() {
     not_ok();
     ok();

tests/ui/useless_asref.rs

@@ -198,6 +198,11 @@ fn issue_12528() {
     let _ = opt.as_ref().map(RcWeak::clone);
 }
 
+fn issue_14088() {
+    let s = Some("foo");
+    let _: Option<&str> = s.as_ref().map(|x| x.as_ref());
+}
+
 fn main() {
     not_ok();
     ok();