add to_string_trait_impl lint

CHANGELOG.md

@@ -5623,6 +5623,7 @@ Released 2018-09-13
 [`to_digit_is_some`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_digit_is_some
 [`to_string_in_display`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_display
 [`to_string_in_format_args`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_in_format_args
+[`to_string_trait_impl`]: https://rust-lang.github.io/rust-clippy/master/index.html#to_string_trait_impl
 [`todo`]: https://rust-lang.github.io/rust-clippy/master/index.html#todo
 [`too_many_arguments`]: https://rust-lang.github.io/rust-clippy/master/index.html#too_many_arguments
 [`too_many_lines`]: https://rust-lang.github.io/rust-clippy/master/index.html#too_many_lines

clippy_lints/src/declared_lints.rs

@@ -657,6 +657,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::tests_outside_test_module::TESTS_OUTSIDE_TEST_MODULE_INFO,
     crate::thread_local_initializer_can_be_made_const::THREAD_LOCAL_INITIALIZER_CAN_BE_MADE_CONST_INFO,
     crate::to_digit_is_some::TO_DIGIT_IS_SOME_INFO,
+    crate::to_string_trait_impl::TO_STRING_TRAIT_IMPL_INFO,
     crate::trailing_empty_array::TRAILING_EMPTY_ARRAY_INFO,
     crate::trait_bounds::TRAIT_DUPLICATION_IN_BOUNDS_INFO,
     crate::trait_bounds::TYPE_REPETITION_IN_BOUNDS_INFO,

clippy_lints/src/lib.rs

@@ -327,6 +327,7 @@ mod temporary_assignment;
 mod tests_outside_test_module;
 mod thread_local_initializer_can_be_made_const;
 mod to_digit_is_some;
+mod to_string_trait_impl;
 mod trailing_empty_array;
 mod trait_bounds;
 mod transmute;
@@ -1097,6 +1098,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
         Box::new(thread_local_initializer_can_be_made_const::ThreadLocalInitializerCanBeMadeConst::new(msrv()))
     });
     store.register_late_pass(move |_| Box::new(incompatible_msrv::IncompatibleMsrv::new(msrv())));
+    store.register_late_pass(|_| Box::new(to_string_trait_impl::ToStringTraitImpl));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/returns.rs

@@ -18,6 +18,7 @@ use rustc_session::declare_lint_pass;
 use rustc_span::def_id::LocalDefId;
 use rustc_span::{BytePos, Pos, Span};
 use std::borrow::Cow;
+use std::fmt::Display;
 
 declare_clippy_lint! {
     /// ### What it does
@@ -146,14 +147,14 @@ impl<'tcx> RetReplacement<'tcx> {
     }
 }
 
-impl<'tcx> ToString for RetReplacement<'tcx> {
-    fn to_string(&self) -> String {
+impl<'tcx> Display for RetReplacement<'tcx> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            Self::Empty => String::new(),
-            Self::Block => "{}".to_string(),
-            Self::Unit => "()".to_string(),
-            Self::IfSequence(inner, _) => format!("({inner})"),
-            Self::Expr(inner, _) => inner.to_string(),
+            Self::Empty => write!(f, ""),
+            Self::Block => write!(f, "{{}}"),
+            Self::Unit => write!(f, "()"),
+            Self::IfSequence(inner, _) => write!(f, "({inner})"),
+            Self::Expr(inner, _) => write!(f, "{inner}"),
         }
     }
 }

clippy_lints/src/to_string_trait_impl.rs

@@ -0,0 +1,67 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_hir::{Impl, Item, ItemKind};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_session::declare_lint_pass;
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for direct implementations of `ToString`.
+    /// ### Why is this bad?
+    /// This trait is automatically implemented for any type which implements the `Display` trait.
+    /// As such, `ToString` shouldn’t be implemented directly: `Display` should be implemented instead,
+    /// and you get the `ToString` implementation for free.
+    /// ### Example
+    /// ```no_run
+    /// struct Point {
+    ///   x: usize,
+    ///   y: usize,
+    /// }
+    ///
+    /// impl ToString for Point {
+    ///   fn to_string(&self) -> String {
+    ///     format!("({}, {})", self.x, self.y)
+    ///   }
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// struct Point {
+    ///   x: usize,
+    ///   y: usize,
+    /// }
+    ///
+    /// impl std::fmt::Display for Point {
+    ///   fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+    ///     write!(f, "({}, {})", self.x, self.y)
+    ///   }
+    /// }
+    /// ```
+    #[clippy::version = "1.77.0"]
+    pub TO_STRING_TRAIT_IMPL,
+    style,
+    "check for direct implementations of `ToString`"
+}
+
+declare_lint_pass!(ToStringTraitImpl => [TO_STRING_TRAIT_IMPL]);
+
+impl<'tcx> LateLintPass<'tcx> for ToStringTraitImpl {
+    fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'tcx>) {
+        if let ItemKind::Impl(Impl {
+            of_trait: Some(trait_ref),
+            ..
+        }) = it.kind
+            && let Some(trait_did) = trait_ref.trait_def_id()
+            && cx.tcx.is_diagnostic_item(sym::ToString, trait_did)
+        {
+            span_lint_and_help(
+                cx,
+                TO_STRING_TRAIT_IMPL,
+                it.span,
+                "direct implementation of `ToString`",
+                None,
+                "prefer implementing `Display` instead",
+            );
+        }
+    }
+}

tests/ui/format_args.fixed

@@ -14,6 +14,7 @@ use std::panic::Location;
 
 struct Somewhere;
 
+#[allow(clippy::to_string_trait_impl)]
 impl ToString for Somewhere {
     fn to_string(&self) -> String {
         String::from("somewhere")

tests/ui/format_args.rs

@@ -14,6 +14,7 @@ use std::panic::Location;
 
 struct Somewhere;
 
+#[allow(clippy::to_string_trait_impl)]
 impl ToString for Somewhere {
     fn to_string(&self) -> String {
         String::from("somewhere")

tests/ui/format_args.stderr

@@ -1,5 +1,5 @@
 error: `to_string` applied to a type that implements `Display` in `format!` args
-  --> $DIR/format_args.rs:76:72
+  --> $DIR/format_args.rs:77:72
    |
 LL |     let _ = format!("error: something failed at {}", Location::caller().to_string());
    |                                                                        ^^^^^^^^^^^^ help: remove this
@@ -8,145 +8,145 @@ LL |     let _ = format!("error: something failed at {}", Location::caller().to_
    = help: to override `-D warnings` add `#[allow(clippy::to_string_in_format_args)]`
 
 error: `to_string` applied to a type that implements `Display` in `write!` args
-  --> $DIR/format_args.rs:80:27
+  --> $DIR/format_args.rs:81:27
    |
 LL |         Location::caller().to_string()
    |                           ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `writeln!` args
-  --> $DIR/format_args.rs:85:27
+  --> $DIR/format_args.rs:86:27
    |
 LL |         Location::caller().to_string()
    |                           ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `print!` args
-  --> $DIR/format_args.rs:87:63
+  --> $DIR/format_args.rs:88:63
    |
 LL |     print!("error: something failed at {}", Location::caller().to_string());
    |                                                               ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:88:65
+  --> $DIR/format_args.rs:89:65
    |
 LL |     println!("error: something failed at {}", Location::caller().to_string());
    |                                                                 ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `eprint!` args
-  --> $DIR/format_args.rs:89:64
+  --> $DIR/format_args.rs:90:64
    |
 LL |     eprint!("error: something failed at {}", Location::caller().to_string());
    |                                                                ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `eprintln!` args
-  --> $DIR/format_args.rs:90:66
+  --> $DIR/format_args.rs:91:66
    |
 LL |     eprintln!("error: something failed at {}", Location::caller().to_string());
    |                                                                  ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `format_args!` args
-  --> $DIR/format_args.rs:91:77
+  --> $DIR/format_args.rs:92:77
    |
 LL |     let _ = format_args!("error: something failed at {}", Location::caller().to_string());
    |                                                                             ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `assert!` args
-  --> $DIR/format_args.rs:92:70
+  --> $DIR/format_args.rs:93:70
    |
 LL |     assert!(true, "error: something failed at {}", Location::caller().to_string());
    |                                                                      ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `assert_eq!` args
-  --> $DIR/format_args.rs:93:73
+  --> $DIR/format_args.rs:94:73
    |
 LL |     assert_eq!(0, 0, "error: something failed at {}", Location::caller().to_string());
    |                                                                         ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `assert_ne!` args
-  --> $DIR/format_args.rs:94:73
+  --> $DIR/format_args.rs:95:73
    |
 LL |     assert_ne!(0, 0, "error: something failed at {}", Location::caller().to_string());
    |                                                                         ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `panic!` args
-  --> $DIR/format_args.rs:95:63
+  --> $DIR/format_args.rs:96:63
    |
 LL |     panic!("error: something failed at {}", Location::caller().to_string());
    |                                                               ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:96:20
+  --> $DIR/format_args.rs:97:20
    |
 LL |     println!("{}", X(1).to_string());
    |                    ^^^^^^^^^^^^^^^^ help: use this: `*X(1)`
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:97:20
+  --> $DIR/format_args.rs:98:20
    |
 LL |     println!("{}", Y(&X(1)).to_string());
    |                    ^^^^^^^^^^^^^^^^^^^^ help: use this: `***Y(&X(1))`
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:98:24
+  --> $DIR/format_args.rs:99:24
    |
 LL |     println!("{}", Z(1).to_string());
    |                        ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:99:20
+  --> $DIR/format_args.rs:100:20
    |
 LL |     println!("{}", x.to_string());
    |                    ^^^^^^^^^^^^^ help: use this: `**x`
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:100:20
+  --> $DIR/format_args.rs:101:20
    |
 LL |     println!("{}", x_ref.to_string());
    |                    ^^^^^^^^^^^^^^^^^ help: use this: `***x_ref`
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:102:39
+  --> $DIR/format_args.rs:103:39
    |
 LL |     println!("{foo}{bar}", foo = "foo".to_string(), bar = "bar");
    |                                       ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:103:52
+  --> $DIR/format_args.rs:104:52
    |
 LL |     println!("{foo}{bar}", foo = "foo", bar = "bar".to_string());
    |                                                    ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:104:39
+  --> $DIR/format_args.rs:105:39
    |
 LL |     println!("{foo}{bar}", bar = "bar".to_string(), foo = "foo");
    |                                       ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:105:52
+  --> $DIR/format_args.rs:106:52
    |
 LL |     println!("{foo}{bar}", bar = "bar", foo = "foo".to_string());
    |                                                    ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `print!` args
-  --> $DIR/format_args.rs:117:37
+  --> $DIR/format_args.rs:118:37
    |
 LL |     print!("{}", (Location::caller().to_string()));
    |                                     ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `print!` args
-  --> $DIR/format_args.rs:118:39
+  --> $DIR/format_args.rs:119:39
    |
 LL |     print!("{}", ((Location::caller()).to_string()));
    |                                       ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `format!` args
-  --> $DIR/format_args.rs:146:38
+  --> $DIR/format_args.rs:147:38
    |
 LL |         let x = format!("{} {}", a, b.to_string());
    |                                      ^^^^^^^^^^^^ help: remove this
 
 error: `to_string` applied to a type that implements `Display` in `println!` args
-  --> $DIR/format_args.rs:160:24
+  --> $DIR/format_args.rs:161:24
    |
 LL |         println!("{}", original[..10].to_string());
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use this: `&original[..10]`

tests/ui/to_string_trait_impl.rs

@@ -0,0 +1,31 @@
+#![warn(clippy::to_string_trait_impl)]
+
+use std::fmt::{self, Display};
+
+struct Point {
+    x: usize,
+    y: usize,
+}
+
+impl ToString for Point {
+    fn to_string(&self) -> String {
+        format!("({}, {})", self.x, self.y)
+    }
+}
+
+struct Foo;
+
+impl Display for Foo {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Foo")
+    }
+}
+
+struct Bar;
+
+impl Bar {
+    #[allow(clippy::inherent_to_string)]
+    fn to_string(&self) -> String {
+        String::from("Bar")
+    }
+}

tests/ui/to_string_trait_impl.stderr

@@ -0,0 +1,16 @@
+error: direct implementation of `ToString`
+  --> $DIR/to_string_trait_impl.rs:10:1
+   |
+LL | / impl ToString for Point {
+LL | |     fn to_string(&self) -> String {
+LL | |         format!("({}, {})", self.x, self.y)
+LL | |     }
+LL | | }
+   | |_^
+   |
+   = help: prefer implementing `Display` instead
+   = note: `-D clippy::to-string-trait-impl` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::to_string_trait_impl)]`
+
+error: aborting due to 1 previous error
+

tests/ui/unconditional_recursion.rs

@@ -206,6 +206,7 @@ impl PartialEq for S8 {
 
 struct S9;
 
+#[allow(clippy::to_string_trait_impl)]
 impl std::string::ToString for S9 {
     fn to_string(&self) -> String {
         //~^ ERROR: function cannot return without recursing
@@ -215,6 +216,7 @@ impl std::string::ToString for S9 {
 
 struct S10;
 
+#[allow(clippy::to_string_trait_impl)]
 impl std::string::ToString for S10 {
     fn to_string(&self) -> String {
         //~^ ERROR: function cannot return without recursing
@@ -225,6 +227,7 @@ impl std::string::ToString for S10 {
 
 struct S11;
 
+#[allow(clippy::to_string_trait_impl)]
 impl std::string::ToString for S11 {
     fn to_string(&self) -> String {
         //~^ ERROR: function cannot return without recursing