Auto merge of #5831 - chansuke:to_string_in_display, r=flip1995

Don't use `to_string` in impl Display

fixes #3876

this PR is derived from [Toxyxer's implementation](https://github.com/rust-lang/rust-clippy/pull/5574).
changelog: add [`to_string_in_display`] lint

2 files changed, 65 insertions, 0 deletions

diff --git a/tests/ui/to_string_in_display.rs b/tests/ui/to_string_in_display.rs
new file mode 100644
index 00000000000..3b46324704e
--- /dev/null
+++ b/tests/ui/to_string_in_display.rs
@@ -0,0 +1,55 @@
+#![warn(clippy::to_string_in_display)]
+#![allow(clippy::inherent_to_string_shadow_display)]
+
+use std::fmt;
+
+struct A;
+impl A {
+    fn fmt(&self) {
+        self.to_string();
+    }
+}
+
+trait B {
+    fn fmt(&self) {}
+}
+
+impl B for A {
+    fn fmt(&self) {
+        self.to_string();
+    }
+}
+
+impl fmt::Display for A {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.to_string())
+    }
+}
+
+fn fmt(a: A) {
+    a.to_string();
+}
+
+struct C;
+
+impl C {
+    fn to_string(&self) -> String {
+        String::from("I am C")
+    }
+}
+
+impl fmt::Display for C {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "{}", self.to_string())
+    }
+}
+
+fn main() {
+    let a = A;
+    a.to_string();
+    a.fmt();
+    fmt(a);
+
+    let c = C;
+    c.to_string();
+}
diff --git a/tests/ui/to_string_in_display.stderr b/tests/ui/to_string_in_display.stderr
new file mode 100644
index 00000000000..cbc0a41036b
--- /dev/null
+++ b/tests/ui/to_string_in_display.stderr
@@ -0,0 +1,10 @@
+error: Using to_string in fmt::Display implementation might lead to infinite recursion
+  --> $DIR/to_string_in_display.rs:25:25
+   |
+LL |         write!(f, "{}", self.to_string())
+   |                         ^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::to-string-in-display` implied by `-D warnings`
+
+error: aborting due to previous error
+