CQL supports type casting using C-style casts.
For example it's possible to do: `blob_column = (blob)funcReturningInt()`
This functionality is pretty limited, we only allow such casts between types that have a compatible binary representation. Compatible means that the bytes will stay unchanged after the conversion.
This means that it's legal to cast an int to blob (int is just a 4 byte blob), but it's illegal to cast a bigint to int (change 4 bytes -> 8 bytes).
This simplifies things, to cast we can just reinterpret the value as the other type.
Another use of C-style casts are type hints. Sometimes it's impossible to infer the exact type of an expression from the context. In such cases the type can be specified by casting the expression to this type.
For example: `overloadedFunction((int)?)`
Without the cast it would be impossible to guess what should be the bind marker's type. The function is overloaded, so there are many possible argument types. The type hint specifies that the bind marker has type int.
An interesting thing is that such casts don't have to be explicit. CQL allows to put an int value in a place where a blob value is expected and it will be automatically converted without any explicit casting.
---
I started looking at our implementation of casts because of #12900. In there the author expressed the need to specify a type hint for bind marker used to pass the WASM code. It could be either `(text)?` for text WASM, or `(blob)?` for binary WASM. This specific use of type hints wasn't supported because there was no `receiver` and the implementation of `prepare_expression` didn't handle that. Preparing casts without a receiver should be easy to implement - we can infer the type of the expression by looking at the type to which the expression is cast.
But while reading `prepare_expression` for `expr::cast` I noticed that the code there is a bit strange. The implementation prepared the expression to cast using the original `receiver` instead of a receiver with the cast type. This caused some issues because of which casting didn't work as expected.
For example it was possible to do:
```cql
blob_column = (blob)funcReturningInt()
```
But this didn't work at all:
```cql
blob_column = (blob)(int)12323
```
It tried to prepare `untyped_contant(12323)` with a `blob` receiver, which fails.
This makes `expr::cast` useless for casting. Casting when the representation is compatible is already implicit. I couldn't find a single case where adding a cast would change the behavior in any way.
There was some use for it as a type hint to choose a specific overload of a function, but it was worthless for casting.
Cassandra has the same issue, I created a `cql-pytest` test and it showed that we behave in the same way as Cassandra does.
I decided to improve this. By preparing the expression using a receiver with the cast type, `expr::cast` becomes actually useful for casting values. Things like `(blob)(int)12323` now work without any issues.
This diverges from the behavior in Cassandra, but it's an extension, not a breaking incompatibility.
---
This PR improves `prepare_expression` for `expr::cast` in the following ways:
1) Support for more complex casts by preparing the expression using a different receiver. This makes casts like `(blob)(int)123` possible
2) Support preparing `expr::cast` without a receiver. Type inference chooses the cast type as the type of the expression.
3) Add pytest tests for C-style casts
`2)` Is needed for #12900, the other changes is just something I decided to do since I was already working on this piece of code.
Closes#13053
* github.com:scylladb/scylladb:
expr_test: more tests for preparing bind variables with type hints
prepare_expr: implement preparing expr::cast with no receiver
prepare_expr: use :user formatting in cast_prepare_expression
prepare_expr: remove std::get<> in cast_prepare_expression
prepare_expr: improve cast_prepare_expression
prepare_expr: improve readability in cast_prepare_expression
cql-pytest: test expr::cast in test_cast.py
53c8c43d8a