iterator: fix QueryPager::rows_stream() lifetime constraints

It appears that the previous requirements:
```rust
fn rows_stream<
    'frame,
    'metadata,
    RowT: 'static + DeserializeRow<'frame, 'metadata>
>
```
allowed creating the `TypedRowStream<&'static str>`. It was error-prone,
because the compiler would accept `rows_stream::<&str>`, happily
deducing that it's `&'static str`, and failing upon Stream `next()` not
being a lending method.

To prevent such situations, the constraints are changed the following
way (credits to @Lorak-mmk):
```rust
fn rows_stream<
    RowT: 'static
          + for<'frame, 'metadata> DeserializeRow<'frame, 'metadata>
>
```
and now `&'static str` is not permitted (because it only implements
`DeserializeValue<'static, '_>`.

Co-authored-by: Karol Baryła <[email protected]>

Author: wprzytula

scylla/src/lib.rs

@@ -233,10 +233,19 @@ pub mod deserialize {
 
     // Shorthands for better readability.
     #[cfg_attr(not(test), allow(unused))]
-    pub(crate) trait DeserializeOwnedValue: for<'r> DeserializeValue<'r, 'r> {}
-    impl<T> DeserializeOwnedValue for T where T: for<'r> DeserializeValue<'r, 'r> {}
-    pub(crate) trait DeserializeOwnedRow: for<'r> DeserializeRow<'r, 'r> {}
-    impl<T> DeserializeOwnedRow for T where T: for<'r> DeserializeRow<'r, 'r> {}
+    pub(crate) trait DeserializeOwnedValue:
+        for<'frame, 'metadata> DeserializeValue<'frame, 'metadata>
+    {
+    }
+    impl<T> DeserializeOwnedValue for T where
+        T: for<'frame, 'metadata> DeserializeValue<'frame, 'metadata>
+    {
+    }
+    pub(crate) trait DeserializeOwnedRow:
+        for<'frame, 'metadata> DeserializeRow<'frame, 'metadata>
+    {
+    }
+    impl<T> DeserializeOwnedRow for T where T: for<'frame, 'metadata> DeserializeRow<'frame, 'metadata> {}
 }
 
 pub mod authentication;

scylla/src/transport/iterator.rs

@@ -663,12 +663,9 @@ impl QueryPager {
     /// It only allows deserializing owned types, because [Stream] is not lending.
     /// Begins with performing type check.
     #[inline]
-    pub fn rows_stream<'frame, 'metadata, RowT: 'static + DeserializeRow<'frame, 'metadata>>(
+    pub fn rows_stream<RowT: 'static + for<'frame, 'metadata> DeserializeRow<'frame, 'metadata>>(
         self,
-    ) -> Result<TypedRowStream<RowT>, TypeCheckError>
-    where
-        'frame: 'metadata,
-    {
+    ) -> Result<TypedRowStream<RowT>, TypeCheckError> {
         TypedRowLendingStream::<RowT>::new(self).map(|typed_row_lending_stream| TypedRowStream {
             typed_row_lending_stream,
         })