Specialize OsString::push for strings

When concatenating two WTF-8 strings, surrogate pairs at the boundaries
need to be joined. However, since UTF-8 strings cannot contain surrogate
halves, this check can be skipped when one string is UTF-8. Specialize
`OsString::push` to use a more efficient concatenation in this case.

Unfortunately, a specialization for `T: AsRef<str>` conflicts with
`T: AsRef<OsStr>`, so stamp out string types with a macro.

library/std/src/ffi/os_str.rs

@@ -257,7 +257,30 @@ impl OsString {
     #[inline]
     #[rustc_confusables("append", "put")]
     pub fn push<T: AsRef<OsStr>>(&mut self, s: T) {
-        self.inner.push_slice(&s.as_ref().inner)
+        trait SpecPushTo {
+            fn spec_push_to(&self, buf: &mut OsString);
+        }
+
+        impl<T: AsRef<OsStr>> SpecPushTo for T {
+            #[inline]
+            default fn spec_push_to(&self, buf: &mut OsString) {
+                buf.inner.push_slice(&self.as_ref().inner);
+            }
+        }
+
+        // Use a more efficient implementation when the string is UTF-8.
+        macro spec_str($T:ty) {
+            impl SpecPushTo for $T {
+                #[inline]
+                fn spec_push_to(&self, buf: &mut OsString) {
+                    buf.inner.push_str(self);
+                }
+            }
+        }
+        spec_str!(str);
+        spec_str!(String);
+
+        s.spec_push_to(self)
     }
 
     /// Creates a new `OsString` with at least the given capacity.

library/std/src/sys/os_str/bytes.rs

@@ -139,6 +139,11 @@ impl Buf {
         self.inner.extend_from_slice(&s.inner)
     }
 
+    #[inline]
+    pub fn push_str(&mut self, s: &str) {
+        self.inner.extend_from_slice(s.as_bytes());
+    }
+
     #[inline]
     pub fn reserve(&mut self, additional: usize) {
         self.inner.reserve(additional)

library/std/src/sys/os_str/wtf8.rs

@@ -116,6 +116,11 @@ impl Buf {
         self.inner.push_wtf8(&s.inner)
     }
 
+    #[inline]
+    pub fn push_str(&mut self, s: &str) {
+        self.inner.push_str(s);
+    }
+
     #[inline]
     pub fn reserve(&mut self, additional: usize) {
         self.inner.reserve(additional)