Deprecate inefficient_to_string

The compiler now generates similar code with and without the `with_ref`
cfg:

```rust
fn f(x: impl IntoIterator<Item = String>) {
    for y in x { println!("{y}"); }
}

fn main() {
    #[cfg(with_ref)]
    let a = ["foo", "bar"].iter().map(|&s| s.to_string());
    #[cfg(not(with_ref))]
    let a = ["foo", "bar"].iter().map(|s| s.to_string());
    f(a);
}
```

The generated code is strictly identical with `-O`, and identical modulo
some minor reordering without.

Author: samueltardieu

clippy_lints/src/declared_lints.rs

@@ -383,7 +383,6 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::methods::GET_LAST_WITH_LEN_INFO,
     crate::methods::GET_UNWRAP_INFO,
     crate::methods::IMPLICIT_CLONE_INFO,
-    crate::methods::INEFFICIENT_TO_STRING_INFO,
     crate::methods::INSPECT_FOR_EACH_INFO,
     crate::methods::INTO_ITER_ON_REF_INFO,
     crate::methods::IO_OTHER_ERROR_INFO,

clippy_lints/src/deprecated_lints.rs

@@ -18,6 +18,8 @@ declare_with_version! { DEPRECATED(DEPRECATED_VERSION) = [
     ("clippy::assign_ops", "compound operators are harmless and linting on them is not in scope for clippy"),
     #[clippy::version = "pre 1.29.0"]
     ("clippy::extend_from_slice", "`Vec::extend_from_slice` is no longer faster than `Vec::extend` due to specialization"),
+    #[clippy::version = "1.92.0"]
+    ("clippy::inefficient_to_string", "compiler now generates identical code for both code variants"),
     #[clippy::version = "1.86.0"]
     ("clippy::match_on_vec_items", "`clippy::indexing_slicing` covers indexing and slicing on `Vec<_>`"),
     #[clippy::version = "pre 1.29.0"]

clippy_lints/src/methods/inefficient_to_string.rs

@@ -33,7 +33,6 @@ mod get_first;
 mod get_last_with_len;
 mod get_unwrap;
 mod implicit_clone;
-mod inefficient_to_string;
 mod inspect_for_each;
 mod into_iter_on_ref;
 mod io_other_error;
@@ -1074,30 +1073,6 @@ declare_clippy_lint! {
     "using `clone` on a ref-counted pointer"
 }
 
-declare_clippy_lint! {
-    /// ### What it does
-    /// Checks for usage of `.to_string()` on an `&&T` where
-    /// `T` implements `ToString` directly (like `&&str` or `&&String`).
-    ///
-    /// ### Why is this bad?
-    /// This bypasses the specialized implementation of
-    /// `ToString` and instead goes through the more expensive string formatting
-    /// facilities.
-    ///
-    /// ### Example
-    /// ```no_run
-    /// // Generic implementation for `T: Display` is used (slow)
-    /// ["foo", "bar"].iter().map(|s| s.to_string());
-    ///
-    /// // OK, the specialized impl is used
-    /// ["foo", "bar"].iter().map(|&s| s.to_string());
-    /// ```
-    #[clippy::version = "1.40.0"]
-    pub INEFFICIENT_TO_STRING,
-    pedantic,
-    "using `to_string` on `&&T` where `T: ToString`"
-}
-
 declare_clippy_lint! {
     /// ### What it does
     /// Checks for `new` not returning a type that contains `Self`.
@@ -4692,7 +4667,6 @@ impl_lint_pass!(Methods => [
     ITER_OVEREAGER_CLONED,
     CLONED_INSTEAD_OF_COPIED,
     FLAT_MAP_OPTION,
-    INEFFICIENT_TO_STRING,
     NEW_RET_NO_SELF,
     SINGLE_CHAR_ADD_STR,
     SEARCH_IS_SOME,
@@ -4868,7 +4842,6 @@ impl<'tcx> LateLintPass<'tcx> for Methods {
                 );
                 clone_on_copy::check(cx, expr, method_call.ident.name, receiver, args);
                 clone_on_ref_ptr::check(cx, expr, method_call.ident.name, receiver, args);
-                inefficient_to_string::check(cx, expr, method_call.ident.name, receiver, args);
                 single_char_add_str::check(cx, expr, receiver, args);
                 into_iter_on_ref::check(cx, expr, method_span, method_call.ident.name, receiver);
                 unnecessary_to_owned::check(cx, expr, method_call.ident.name, receiver, args, self.msrv);

clippy_lints/src/methods/mod.rs

@@ -4,6 +4,7 @@
 
 #![warn(clippy::assign_ops)] //~ ERROR: lint `clippy::assign_ops`
 #![warn(clippy::extend_from_slice)] //~ ERROR: lint `clippy::extend_from_slice`
+#![warn(clippy::inefficient_to_string)] //~ ERROR: lint `clippy::inefficient_to_string`
 #![warn(clippy::match_on_vec_items)] //~ ERROR: lint `clippy::match_on_vec_items`
 #![warn(clippy::misaligned_transmute)] //~ ERROR: lint `clippy::misaligned_transmute`
 #![warn(clippy::option_map_or_err_ok)] //~ ERROR: lint `clippy::option_map_or_err_ok`

tests/ui/deprecated.rs

@@ -13,89 +13,95 @@ error: lint `clippy::extend_from_slice` has been removed: `Vec::extend_from_slic
 LL | #![warn(clippy::extend_from_slice)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: lint `clippy::match_on_vec_items` has been removed: `clippy::indexing_slicing` covers indexing and slicing on `Vec<_>`
+error: lint `clippy::inefficient_to_string` has been removed: compiler now generates identical code for both code variants
   --> tests/ui/deprecated.rs:7:9
    |
+LL | #![warn(clippy::inefficient_to_string)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: lint `clippy::match_on_vec_items` has been removed: `clippy::indexing_slicing` covers indexing and slicing on `Vec<_>`
+  --> tests/ui/deprecated.rs:8:9
+   |
 LL | #![warn(clippy::match_on_vec_items)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::misaligned_transmute` has been removed: split into `clippy::cast_ptr_alignment` and `clippy::transmute_ptr_to_ptr`
-  --> tests/ui/deprecated.rs:8:9
+  --> tests/ui/deprecated.rs:9:9
    |
 LL | #![warn(clippy::misaligned_transmute)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::option_map_or_err_ok` has been removed: `clippy::manual_ok_or` covers this case
-  --> tests/ui/deprecated.rs:9:9
+  --> tests/ui/deprecated.rs:10:9
    |
 LL | #![warn(clippy::option_map_or_err_ok)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::pub_enum_variant_names` has been removed: `clippy::enum_variant_names` now covers this case via the `avoid-breaking-exported-api` config
-  --> tests/ui/deprecated.rs:10:9
+  --> tests/ui/deprecated.rs:11:9
    |
 LL | #![warn(clippy::pub_enum_variant_names)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::range_step_by_zero` has been removed: `Iterator::step_by(0)` now panics and is no longer an infinite iterator
-  --> tests/ui/deprecated.rs:11:9
+  --> tests/ui/deprecated.rs:12:9
    |
 LL | #![warn(clippy::range_step_by_zero)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::regex_macro` has been removed: the `regex!` macro was removed from the regex crate in 2018
-  --> tests/ui/deprecated.rs:12:9
+  --> tests/ui/deprecated.rs:13:9
    |
 LL | #![warn(clippy::regex_macro)]
    |         ^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::replace_consts` has been removed: `min_value` and `max_value` are now deprecated
-  --> tests/ui/deprecated.rs:13:9
+  --> tests/ui/deprecated.rs:14:9
    |
 LL | #![warn(clippy::replace_consts)]
    |         ^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::should_assert_eq` has been removed: `assert!(a == b)` can now print the values the same way `assert_eq!(a, b) can
-  --> tests/ui/deprecated.rs:14:9
+  --> tests/ui/deprecated.rs:15:9
    |
 LL | #![warn(clippy::should_assert_eq)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::string_to_string` has been removed: `clippy:implicit_clone` covers those cases
-  --> tests/ui/deprecated.rs:15:9
+  --> tests/ui/deprecated.rs:16:9
    |
 LL | #![warn(clippy::string_to_string)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::unsafe_vector_initialization` has been removed: the suggested alternative could be substantially slower
-  --> tests/ui/deprecated.rs:16:9
+  --> tests/ui/deprecated.rs:17:9
    |
 LL | #![warn(clippy::unsafe_vector_initialization)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::unstable_as_mut_slice` has been removed: `Vec::as_mut_slice` is now stable
-  --> tests/ui/deprecated.rs:17:9
+  --> tests/ui/deprecated.rs:18:9
    |
 LL | #![warn(clippy::unstable_as_mut_slice)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::unstable_as_slice` has been removed: `Vec::as_slice` is now stable
-  --> tests/ui/deprecated.rs:18:9
+  --> tests/ui/deprecated.rs:19:9
    |
 LL | #![warn(clippy::unstable_as_slice)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::unused_collect` has been removed: `Iterator::collect` is now marked as `#[must_use]`
-  --> tests/ui/deprecated.rs:19:9
+  --> tests/ui/deprecated.rs:20:9
    |
 LL | #![warn(clippy::unused_collect)]
    |         ^^^^^^^^^^^^^^^^^^^^^^
 
 error: lint `clippy::wrong_pub_self_convention` has been removed: `clippy::wrong_self_convention` now covers this case via the `avoid-breaking-exported-api` config
-  --> tests/ui/deprecated.rs:20:9
+  --> tests/ui/deprecated.rs:21:9
    |
 LL | #![warn(clippy::wrong_pub_self_convention)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-error: aborting due to 16 previous errors
+error: aborting due to 17 previous errors