Allow to specialize FromSql<Text> impl for strings

Again Oracle/liboci is strange…

Author: weiznich

diesel/src/type_impls/primitives.rs

@@ -120,6 +120,7 @@ where
 /// impl in terms of `String`, but don't want to allocate. We have to return a
 /// raw pointer instead of a reference with a lifetime due to the structure of
 /// `FromSql`
+#[cfg(not(feature = "unstable"))]
 impl<DB> FromSql<sql_types::Text, DB> for *const str
 where
     DB: Backend + for<'a> BinaryRawValue<'a>,
@@ -132,6 +133,19 @@ where
     }
 }
 
+#[cfg(feature = "unstable")]
+impl<DB> FromSql<sql_types::Text, DB> for *const str
+where
+    DB: Backend + for<'a> BinaryRawValue<'a>,
+{
+    default fn from_sql(value: Option<::backend::RawValue<DB>>) -> deserialize::Result<Self> {
+        use std::str;
+        let value = not_none!(value);
+        let string = str::from_utf8(DB::as_bytes(value))?;
+        Ok(string as *const _)
+    }
+}
+
 impl<DB: Backend> ToSql<sql_types::Text, DB> for str {
     fn to_sql<W: Write>(&self, out: &mut Output<W, DB>) -> serialize::Result {
         out.write_all(self.as_bytes())