Fix to_string_in_format_args false positive

Author: smoelius

clippy_utils/src/macros.rs

@@ -359,6 +359,9 @@ impl<'tcx> FormatArgsExpn<'tcx> {
                 sym::const_format_args | sym::format_args | sym::format_args_nl
             )
         })?;
+        if !matches!(expr.kind, ExprKind::Call(..)) {
+            return None;
+        }
         let mut format_string_span: Option<Span> = None;
         let mut format_string_parts: Vec<Symbol> = Vec::new();
         let mut value_args: Vec<&Expr<'_>> = Vec::new();

tests/ui/format_args.fixed

@@ -115,3 +115,27 @@ fn main() {
     // https://github.com/rust-lang/rust-clippy/issues/7903
     println!("{foo}{foo:?}", foo = "foo".to_string());
 }
+
+// https://github.com/rust-lang/rust-clippy/issues/8855
+mod issue_8855 {
+    #![allow(dead_code)]
+
+    struct A {}
+
+    impl std::fmt::Display for A {
+        fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+            write!(f, "test")
+        }
+    }
+
+    fn main() {
+        let a = A {};
+        let b = A {};
+
+        let x = format!("{} {}", a, b);
+        dbg!(x);
+
+        let x = format!("{:>6} {:>6}", a, b.to_string());
+        dbg!(x);
+    }
+}

tests/ui/format_args.rs

@@ -115,3 +115,27 @@ fn main() {
     // https://github.com/rust-lang/rust-clippy/issues/7903
     println!("{foo}{foo:?}", foo = "foo".to_string());
 }
+
+// https://github.com/rust-lang/rust-clippy/issues/8855
+mod issue_8855 {
+    #![allow(dead_code)]
+
+    struct A {}
+
+    impl std::fmt::Display for A {
+        fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+            write!(f, "test")
+        }
+    }
+
+    fn main() {
+        let a = A {};
+        let b = A {};
+
+        let x = format!("{} {}", a, b.to_string());
+        dbg!(x);
+
+        let x = format!("{:>6} {:>6}", a, b.to_string());
+        dbg!(x);
+    }
+}

tests/ui/format_args.stderr

@@ -126,5 +126,11 @@ error: `to_string` applied to a type that implements `Display` in `println!` arg
 LL |     println!("{foo}{bar}", bar = "bar", foo = "foo".to_string());
    |                                                    ^^^^^^^^^^^^ help: remove this
 
-error: aborting due to 21 previous errors
+error: `to_string` applied to a type that implements `Display` in `format!` args
+  --> $DIR/format_args.rs:135:38
+   |
+LL |         let x = format!("{} {}", a, b.to_string());
+   |                                      ^^^^^^^^^^^^ help: remove this
+
+error: aborting due to 22 previous errors