fix: handle false negative for str_to_string (rust-lang#16512)

Replace `ToString::to_string` with `ToOwned::to_owned` when it is passed
as a function parameter.

```rust
fn issue16511(x: Option<&str>) -> Option<String> {
    // Replace with ToOwned::to_owned
    x.map(ToString::to_string)
}
```

fixes rust-lang#16511

---

changelog: [`str_to_string`]: handle a case when `ToString::to_string`
is passed as function parameter

Author: samueltardieu

clippy_lints/src/strings.rs

@@ -5,6 +5,7 @@ use clippy_utils::{
     SpanlessEq, get_expr_use_or_unification_node, get_parent_expr, is_lint_allowed, method_calls, peel_blocks, sym,
 };
 use rustc_errors::Applicability;
+use rustc_hir::def::{DefKind, Res};
 use rustc_hir::def_id::DefId;
 use rustc_hir::{BinOpKind, BorrowKind, Expr, ExprKind, LangItem, Node};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
@@ -410,6 +411,23 @@ impl<'tcx> LateLintPass<'tcx> for StrToString {
                     diag.span_suggestion(expr.span, "try", format!("{snippet}.to_owned()"), applicability);
                 },
             );
+        } else if let ExprKind::Path(_) = expr.kind
+            && let Some(parent) = get_parent_expr(cx, expr)
+            && let ExprKind::Call(_, args) | ExprKind::MethodCall(_, _, args, _) = &parent.kind
+            && args.iter().any(|a| a.hir_id == expr.hir_id)
+            && let Res::Def(DefKind::AssocFn, def_id) = expr.res(cx)
+            && cx.tcx.is_diagnostic_item(sym::to_string_method, def_id)
+        {
+            // Detected `ToString::to_string` passed as an argument (generic: any call or method call)
+            span_lint_and_sugg(
+                cx,
+                STR_TO_STRING,
+                expr.span,
+                "`ToString::to_string` used as `&str` to `String` converter",
+                "try",
+                "ToOwned::to_owned".to_string(),
+                Applicability::MachineApplicable,
+            );
         }
     }
 }

tests/ui/str_to_string.fixed

@@ -22,3 +22,32 @@ fn issue16271(key: &[u8]) {
     let _value = t!(str::from_utf8(key)).to_owned();
     //~^ str_to_string
 }
+
+struct GenericWrapper<T>(T);
+
+impl<T> GenericWrapper<T> {
+    fn mapper<U, F: FnOnce(T) -> U>(self, f: F) -> U {
+        f(self.0)
+    }
+}
+
+fn issue16511(x: Option<&str>) {
+    let _ = x.map(ToOwned::to_owned);
+    //~^ str_to_string
+
+    let _ = x.map(ToOwned::to_owned);
+    //~^ str_to_string
+
+    let _ = ["a", "b"].iter().map(ToOwned::to_owned);
+    //~^ str_to_string
+
+    fn mapper<F: Fn(&str) -> String>(f: F) -> String {
+        f("hello")
+    }
+    let _ = mapper(ToOwned::to_owned);
+    //~^ str_to_string
+
+    let w = GenericWrapper("hello");
+    let _ = w.mapper(ToOwned::to_owned);
+    //~^ str_to_string
+}

tests/ui/str_to_string.rs

@@ -22,3 +22,32 @@ fn issue16271(key: &[u8]) {
     let _value = t!(str::from_utf8(key)).to_string();
     //~^ str_to_string
 }
+
+struct GenericWrapper<T>(T);
+
+impl<T> GenericWrapper<T> {
+    fn mapper<U, F: FnOnce(T) -> U>(self, f: F) -> U {
+        f(self.0)
+    }
+}
+
+fn issue16511(x: Option<&str>) {
+    let _ = x.map(ToString::to_string);
+    //~^ str_to_string
+
+    let _ = x.map(str::to_string);
+    //~^ str_to_string
+
+    let _ = ["a", "b"].iter().map(ToString::to_string);
+    //~^ str_to_string
+
+    fn mapper<F: Fn(&str) -> String>(f: F) -> String {
+        f("hello")
+    }
+    let _ = mapper(ToString::to_string);
+    //~^ str_to_string
+
+    let w = GenericWrapper("hello");
+    let _ = w.mapper(ToString::to_string);
+    //~^ str_to_string
+}

tests/ui/str_to_string.stderr

@@ -19,5 +19,35 @@ error: `to_string()` called on a `&str`
 LL |     let _value = t!(str::from_utf8(key)).to_string();
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `t!(str::from_utf8(key)).to_owned()`
 
-error: aborting due to 3 previous errors
+error: `ToString::to_string` used as `&str` to `String` converter
+  --> tests/ui/str_to_string.rs:35:19
+   |
+LL |     let _ = x.map(ToString::to_string);
+   |                   ^^^^^^^^^^^^^^^^^^^ help: try: `ToOwned::to_owned`
+
+error: `ToString::to_string` used as `&str` to `String` converter
+  --> tests/ui/str_to_string.rs:38:19
+   |
+LL |     let _ = x.map(str::to_string);
+   |                   ^^^^^^^^^^^^^^ help: try: `ToOwned::to_owned`
+
+error: `ToString::to_string` used as `&str` to `String` converter
+  --> tests/ui/str_to_string.rs:41:35
+   |
+LL |     let _ = ["a", "b"].iter().map(ToString::to_string);
+   |                                   ^^^^^^^^^^^^^^^^^^^ help: try: `ToOwned::to_owned`
+
+error: `ToString::to_string` used as `&str` to `String` converter
+  --> tests/ui/str_to_string.rs:47:20
+   |
+LL |     let _ = mapper(ToString::to_string);
+   |                    ^^^^^^^^^^^^^^^^^^^ help: try: `ToOwned::to_owned`
+
+error: `ToString::to_string` used as `&str` to `String` converter
+  --> tests/ui/str_to_string.rs:51:22
+   |
+LL |     let _ = w.mapper(ToString::to_string);
+   |                      ^^^^^^^^^^^^^^^^^^^ help: try: `ToOwned::to_owned`
+
+error: aborting due to 8 previous errors