Specialize constructing OsString from strings

The WTF-8 version of `OsString` tracks whether it is known to be valid
UTF-8 with its `is_known_utf8` field. Specialize `From<AsRef<OsStr>>` so
this can be set for UTF-8 string types.

Author: thaliaarchi

library/std/src/ffi/os_str.rs

@@ -610,7 +610,30 @@ impl<T: ?Sized + AsRef<OsStr>> From<&T> for OsString {
     /// Copies any value implementing <code>[AsRef]&lt;[OsStr]&gt;</code>
     /// into a newly allocated [`OsString`].
     fn from(s: &T) -> OsString {
-        s.as_ref().to_os_string()
+        trait SpecToOsString {
+            fn spec_to_os_string(&self) -> OsString;
+        }
+
+        impl<T: AsRef<OsStr>> SpecToOsString for T {
+            #[inline]
+            default fn spec_to_os_string(&self) -> OsString {
+                self.as_ref().to_os_string()
+            }
+        }
+
+        // Preserve the known-UTF-8 property for strings.
+        macro spec_str($T:ty) {
+            impl SpecToOsString for $T {
+                #[inline]
+                fn spec_to_os_string(&self) -> OsString {
+                    OsString::from(String::from(self))
+                }
+            }
+        }
+        spec_str!(str);
+        spec_str!(String);
+
+        s.spec_to_os_string()
     }
 }