Implemented suspicious_to_owned lint to check if to_owned is called on a Cow.

This is done because `to_owned` is very similarly named to `into_owned`, but the
effect of calling those two methods is completely different. This creates
confusion (stemming from the ambiguity of the 'owned' term in the context of
`Cow`s) and might not be what the writer intended.

clippy_lints/src/methods/mod.rs

@@ -78,6 +78,7 @@ mod str_splitn;
 mod string_extend_chars;
 mod suspicious_map;
 mod suspicious_splitn;
+mod suspicious_to_owned;
 mod uninit_assumed_init;
 mod unit_hash;
 mod unnecessary_filter_map;
@@ -2053,6 +2054,55 @@ declare_clippy_lint! {
     "replace `.iter().count()` with `.len()`"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for the usage of `_.to_owned()`, on a `Cow<'_, _>`.
+    ///
+    /// ### Why is this bad?
+    /// Calling `to_owned()` on a `Cow` creates a clone of the `Cow`
+    /// itself, without taking ownership of the `Cow` contents (i.e.
+    /// it's equivalent to calling `Cow::clone`).
+    /// The similarly named `into_owned` method, on the other hand,
+    /// clones the `Cow` contents, effectively turning any `Cow::Borrowed`
+    /// into a `Cow::Owned`.
+    ///
+    /// Given the potential ambiguity, consider replacing `to_owned`
+    /// with `clone` for better readability or, if getting a `Cow::Owned`
+    /// was the original intent, using `into_owned` instead.
+    ///
+    /// ### Example
+    /// ```rust
+    /// # use std::borrow::Cow;
+    /// let s = "Hello world!";
+    /// let cow = Cow::Borrowed(s);
+    ///
+    /// let data = cow.to_owned();
+    /// assert!(matches!(data, Cow::Borrowed(_)))
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// # use std::borrow::Cow;
+    /// let s = "Hello world!";
+    /// let cow = Cow::Borrowed(s);
+    ///
+    /// let data = cow.clone();
+    /// assert!(matches!(data, Cow::Borrowed(_)))
+    /// ```
+    /// or
+    /// ```rust
+    /// # use std::borrow::Cow;
+    /// let s = "Hello world!";
+    /// let cow = Cow::Borrowed(s);
+    ///
+    /// let data = cow.into_owned();
+    /// assert!(matches!(data, String))
+    /// ```
+    #[clippy::version = "1.65.0"]
+    pub SUSPICIOUS_TO_OWNED,
+    suspicious,
+    "calls to `to_owned` on a `Cow<'_, _>` might not do what they are expected"
+}
+
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for calls to [`splitn`]
@@ -3075,6 +3125,7 @@ impl_lint_pass!(Methods => [
     FROM_ITER_INSTEAD_OF_COLLECT,
     INSPECT_FOR_EACH,
     IMPLICIT_CLONE,
+    SUSPICIOUS_TO_OWNED,
     SUSPICIOUS_SPLITN,
     MANUAL_STR_REPEAT,
     EXTEND_WITH_DRAIN,
@@ -3553,7 +3604,12 @@ impl Methods {
                     }
                     unnecessary_lazy_eval::check(cx, expr, recv, arg, "then_some");
                 },
-                ("to_os_string" | "to_owned" | "to_path_buf" | "to_vec", []) => {
+                ("to_owned", []) => {
+                    if !suspicious_to_owned::check(cx, expr, recv) {
+                        implicit_clone::check(cx, name, expr, recv);
+                    }
+                },
+                ("to_os_string" | "to_path_buf" | "to_vec", []) => {
                     implicit_clone::check(cx, name, expr, recv);
                 },
                 ("unwrap", []) => {

clippy_lints/src/methods/suspicious_to_owned.rs

@@ -0,0 +1,36 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_diag_trait_item;
+use clippy_utils::source::snippet_with_context;
+use if_chain::if_chain;
+use rustc_errors::Applicability;
+use rustc_hir as hir;
+use rustc_lint::LateContext;
+use rustc_middle::ty;
+use rustc_span::sym;
+
+use super::SUSPICIOUS_TO_OWNED;
+
+pub fn check(cx: &LateContext<'_>, expr: &hir::Expr<'_>, recv: &hir::Expr<'_>) -> bool {
+    if_chain! {
+        if let Some(method_def_id) = cx.typeck_results().type_dependent_def_id(expr.hir_id);
+        if is_diag_trait_item(cx, method_def_id, sym::ToOwned);
+        let input_type = cx.typeck_results().expr_ty(expr);
+        if let ty::Adt(adt, _) = cx.typeck_results().expr_ty(expr).kind();
+        if cx.tcx.is_diagnostic_item(sym::Cow, adt.did());
+        then {
+            let mut app = Applicability::MaybeIncorrect;
+            let recv_snip = snippet_with_context(cx, recv.span, expr.span.ctxt(), "..", &mut app).0;
+            span_lint_and_sugg(
+                cx,
+                SUSPICIOUS_TO_OWNED,
+                expr.span,
+                &format!("this `to_owned` call clones the {0} itself and does not cause the {0} contents to become owned", input_type),
+                "consider using, depending on intent",
+                format!("{0}.clone()` or `{0}.into_owned()", recv_snip),
+                app,
+            );
+            return true;
+        }
+    }
+    false
+}