Fix useless_asref suggests wrongly when used in ctor (#16115)

Closes #16098

changelog: [`useless_asref`] fix wrong suggestions when used in ctor

Author: llogiq

clippy_lints/src/methods/useless_asref.rs

@@ -4,7 +4,8 @@ use clippy_utils::source::snippet_with_applicability;
 use clippy_utils::ty::{implements_trait, peel_and_count_ty_refs, should_call_clone_as_function};
 use clippy_utils::{get_parent_expr, peel_blocks, strip_pat_refs};
 use rustc_errors::Applicability;
-use rustc_hir::{self as hir, LangItem};
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::{self as hir, LangItem, Node};
 use rustc_lint::LateContext;
 use rustc_middle::ty::adjustment::Adjust;
 use rustc_middle::ty::{Ty, TyCtxt, TypeSuperVisitable, TypeVisitable, TypeVisitor};
@@ -69,14 +70,37 @@ pub(super) fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, call_name: Symbo
                 }
             }
 
+            // Add `*` derefs if the expr is used in a ctor, because automatic derefs don't apply in that case.
+            let deref = if rcv_depth > res_depth {
+                let parent = cx.tcx.parent_hir_node(expr.hir_id);
+                match parent {
+                    Node::ExprField(_) => "*".repeat(rcv_depth - res_depth),
+                    Node::Expr(parent)
+                        if let hir::ExprKind::Call(func, _) = parent.kind
+                            && let (_, Some(path)) = func.opt_res_path()
+                            && matches!(path.res, Res::Def(DefKind::Ctor(_, _), _) | Res::SelfCtor(_)) =>
+                    {
+                        "*".repeat(rcv_depth - res_depth)
+                    },
+                    _ => String::new(),
+                }
+            } else {
+                String::new()
+            };
+
             let mut applicability = Applicability::MachineApplicable;
+            let suggestion = format!(
+                "{deref}{}",
+                snippet_with_applicability(cx, recvr.span, "..", &mut applicability)
+            );
+
             span_lint_and_sugg(
                 cx,
                 USELESS_ASREF,
                 expr.span,
                 format!("this call to `{call_name}` does nothing"),
                 "try",
-                snippet_with_applicability(cx, recvr.span, "..", &mut applicability).to_string(),
+                suggestion,
                 applicability,
             );
         }

tests/ui/useless_asref.fixed

@@ -258,6 +258,41 @@ fn issue_14828() {
     ().as_ref();
 }
 
+fn issue16098(exts: Vec<&str>) {
+    use std::borrow::Cow;
+
+    let v: Vec<Cow<'_, str>> = exts.iter().map(|s| Cow::Borrowed(*s)).collect();
+    //~^ useless_asref
+
+    trait Identity {
+        fn id(self) -> Self
+        where
+            Self: Sized,
+        {
+            self
+        }
+    }
+    impl Identity for &str {}
+
+    let v: Vec<Cow<'_, str>> = exts.iter().map(|s| Cow::Borrowed(s.id())).collect();
+    //~^ useless_asref
+
+    let v: Vec<Cow<'_, str>> = exts
+        .iter()
+        .map(|s| Cow::Borrowed(*std::convert::identity(s)))
+        //~^ useless_asref
+        .collect();
+
+    struct Wrapper<'a>(&'a str);
+    let exts_field: Vec<Wrapper> = exts.iter().map(|s| Wrapper(s)).collect();
+    let v: Vec<Cow<'_, str>> = exts_field.iter().map(|w| Cow::Borrowed(w.0)).collect();
+    //~^ useless_asref
+
+    let exts_index: Vec<&[&str]> = exts.iter().map(|s| std::slice::from_ref(s)).collect();
+    let v: Vec<Cow<'_, str>> = exts_index.iter().map(|arr| Cow::Borrowed(arr[0])).collect();
+    //~^ useless_asref
+}
+
 fn main() {
     not_ok();
     ok();

tests/ui/useless_asref.rs

@@ -258,6 +258,41 @@ fn issue_14828() {
     ().as_ref();
 }
 
+fn issue16098(exts: Vec<&str>) {
+    use std::borrow::Cow;
+
+    let v: Vec<Cow<'_, str>> = exts.iter().map(|s| Cow::Borrowed(s.as_ref())).collect();
+    //~^ useless_asref
+
+    trait Identity {
+        fn id(self) -> Self
+        where
+            Self: Sized,
+        {
+            self
+        }
+    }
+    impl Identity for &str {}
+
+    let v: Vec<Cow<'_, str>> = exts.iter().map(|s| Cow::Borrowed(s.id().as_ref())).collect();
+    //~^ useless_asref
+
+    let v: Vec<Cow<'_, str>> = exts
+        .iter()
+        .map(|s| Cow::Borrowed(std::convert::identity(s).as_ref()))
+        //~^ useless_asref
+        .collect();
+
+    struct Wrapper<'a>(&'a str);
+    let exts_field: Vec<Wrapper> = exts.iter().map(|s| Wrapper(s)).collect();
+    let v: Vec<Cow<'_, str>> = exts_field.iter().map(|w| Cow::Borrowed(w.0.as_ref())).collect();
+    //~^ useless_asref
+
+    let exts_index: Vec<&[&str]> = exts.iter().map(|s| std::slice::from_ref(s)).collect();
+    let v: Vec<Cow<'_, str>> = exts_index.iter().map(|arr| Cow::Borrowed(arr[0].as_ref())).collect();
+    //~^ useless_asref
+}
+
 fn main() {
     not_ok();
     ok();

tests/ui/useless_asref.stderr

@@ -112,5 +112,35 @@ error: this call to `as_ref.map(...)` does nothing
 LL |         Some(1).as_ref().map(|&x| x.clone());
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `Some(1).clone()`
 
-error: aborting due to 18 previous errors
+error: this call to `as_ref` does nothing
+  --> tests/ui/useless_asref.rs:264:66
+   |
+LL |     let v: Vec<Cow<'_, str>> = exts.iter().map(|s| Cow::Borrowed(s.as_ref())).collect();
+   |                                                                  ^^^^^^^^^^ help: try: `*s`
+
+error: this call to `as_ref` does nothing
+  --> tests/ui/useless_asref.rs:277:66
+   |
+LL |     let v: Vec<Cow<'_, str>> = exts.iter().map(|s| Cow::Borrowed(s.id().as_ref())).collect();
+   |                                                                  ^^^^^^^^^^^^^^^ help: try: `s.id()`
+
+error: this call to `as_ref` does nothing
+  --> tests/ui/useless_asref.rs:282:32
+   |
+LL |         .map(|s| Cow::Borrowed(std::convert::identity(s).as_ref()))
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `*std::convert::identity(s)`
+
+error: this call to `as_ref` does nothing
+  --> tests/ui/useless_asref.rs:288:72
+   |
+LL |     let v: Vec<Cow<'_, str>> = exts_field.iter().map(|w| Cow::Borrowed(w.0.as_ref())).collect();
+   |                                                                        ^^^^^^^^^^^^ help: try: `w.0`
+
+error: this call to `as_ref` does nothing
+  --> tests/ui/useless_asref.rs:292:74
+   |
+LL |     let v: Vec<Cow<'_, str>> = exts_index.iter().map(|arr| Cow::Borrowed(arr[0].as_ref())).collect();
+   |                                                                          ^^^^^^^^^^^^^^^ help: try: `arr[0]`
+
+error: aborting due to 23 previous errors