Auto merge of #10913 - y21:issue10033, r=Manishearth,Centri3

[`unnecessary_to_owned`]: check that the adjusted type matches target

Fixes #10033.

Before this change, the lint would assume that removing the `.to_string()` in `f(&x.to_string())` would be ok if x is of some type that implements `Deref<Target = str>` and `f` takes a `&str`.
This turns out to not actually be ok if the `to_string` call is some method that exists on `x` directly, which happens if it implements `Display`/`ToString` itself.

changelog: [`unnecessary_to_owned`]: only lint if the adjusted receiver type actually matches

3 files changed, 71 insertions, 0 deletions

diff --git a/clippy_lints/src/methods/unnecessary_to_owned.rs b/clippy_lints/src/methods/unnecessary_to_owned.rs
index 309d2157b76..da7196ed0a8 100644
--- a/clippy_lints/src/methods/unnecessary_to_owned.rs
+++ b/clippy_lints/src/methods/unnecessary_to_owned.rs
@@ -144,6 +144,11 @@ fn check_addr_of_expr(
                 if let Some(deref_trait_id) = cx.tcx.get_diagnostic_item(sym::Deref);
                 if implements_trait(cx, receiver_ty, deref_trait_id, &[]);
                 if cx.get_associated_type(receiver_ty, deref_trait_id, "Target") == Some(target_ty);
+                // Make sure that it's actually calling the right `.to_string()`, (#10033)
+                // *or* this is a `Cow::into_owned()` call (which would be the wrong into_owned receiver (str != Cow)
+                // but that's ok for Cow::into_owned specifically)
+                if cx.typeck_results().expr_ty_adjusted(receiver).peel_refs() == target_ty
+                    || is_cow_into_owned(cx, method_name, method_def_id);
                 then {
                     if n_receiver_refs > 0 {
                         span_lint_and_sugg(
diff --git a/tests/ui/unnecessary_to_owned.fixed b/tests/ui/unnecessary_to_owned.fixed
index 08733906b0e..592a53f3a81 100644
--- a/tests/ui/unnecessary_to_owned.fixed
+++ b/tests/ui/unnecessary_to_owned.fixed
@@ -474,3 +474,36 @@ mod issue_10021 {
         Ok(())
     }
 }
+
+mod issue_10033 {
+    #![allow(dead_code)]
+    use std::{fmt::Display, ops::Deref};
+
+    fn _main() {
+        let f = Foo;
+
+        // Not actually unnecessary - this calls `Foo`'s `Display` impl, not `str`'s (even though `Foo` does
+        // deref to `str`)
+        foo(&f.to_string());
+    }
+
+    fn foo(s: &str) {
+        println!("{}", s);
+    }
+
+    struct Foo;
+
+    impl Deref for Foo {
+        type Target = str;
+
+        fn deref(&self) -> &Self::Target {
+            "str"
+        }
+    }
+
+    impl Display for Foo {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            write!(f, "Foo")
+        }
+    }
+}
diff --git a/tests/ui/unnecessary_to_owned.rs b/tests/ui/unnecessary_to_owned.rs
index e3589ea0d65..f2e48b1c4a6 100644
--- a/tests/ui/unnecessary_to_owned.rs
+++ b/tests/ui/unnecessary_to_owned.rs
@@ -474,3 +474,36 @@ mod issue_10021 {
         Ok(())
     }
 }
+
+mod issue_10033 {
+    #![allow(dead_code)]
+    use std::{fmt::Display, ops::Deref};
+
+    fn _main() {
+        let f = Foo;
+
+        // Not actually unnecessary - this calls `Foo`'s `Display` impl, not `str`'s (even though `Foo` does
+        // deref to `str`)
+        foo(&f.to_string());
+    }
+
+    fn foo(s: &str) {
+        println!("{}", s);
+    }
+
+    struct Foo;
+
+    impl Deref for Foo {
+        type Target = str;
+
+        fn deref(&self) -> &Self::Target {
+            "str"
+        }
+    }
+
+    impl Display for Foo {
+        fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+            write!(f, "Foo")
+        }
+    }
+}