Auto merge of #32586 - seanmonstar:speialize-to-string, r=alexcrichton

specialize ToString for str

If there was some conditional compiling we could do, such that this impl only exists in nightly, and is turned off in beta/stable, I think that'd be an improvement here, as we could test specialization out without affecting stable builds.

Author: bors

src/libcollections/lib.rs

@@ -48,6 +48,7 @@
 #![feature(placement_new_protocol)]
 #![feature(shared)]
 #![feature(slice_patterns)]
+#![feature(specialization)]
 #![feature(staged_api)]
 #![feature(step_by)]
 #![feature(str_char)]

src/libcollections/string.rs

@@ -1755,7 +1755,7 @@ pub trait ToString {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: fmt::Display + ?Sized> ToString for T {
     #[inline]
-    fn to_string(&self) -> String {
+    default fn to_string(&self) -> String {
         use core::fmt::Write;
         let mut buf = String::new();
         let _ = buf.write_fmt(format_args!("{}", self));
@@ -1764,6 +1764,14 @@ impl<T: fmt::Display + ?Sized> ToString for T {
     }
 }
 
+#[stable(feature = "str_to_string_specialization", since = "1.9.0")]
+impl ToString for str {
+    #[inline]
+    fn to_string(&self) -> String {
+        String::from(self)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRef<str> for String {
     #[inline]