Some design notes for the Rust-lite semantics

README.md

@@ -4,8 +4,321 @@ Rust Demo Semantics
 This is a semantics for a subset of Rust + and a subset of the MultiversX
 extensions that is easy to implement.
 
-As an example, a lot of Rust features are not supported, or are supported
-only for parsing (e.g., structs, inheritance, modules, imports).
+As an example, a lot of Rust features are not supported, or have limited support
+(e.g., structs, inheritance, modules, imports).
 Also, this semantics relies on top-level traits being annotated with
-#[multiversx_sc::contract], and allows calls to methods declared in these
+`#[multiversx_sc::contract]`, and allows calls to methods declared in these
 traits.
+
+MultiversX (MX) contracts
+-------------------------
+
+Everything in this section is, most likely, an oversimplification.
+
+A MX contract is usually declared as a trait with a `#[multiversx_sc::contract]`
+attribute. The contract's endpoints are usually methods with a
+`#[endpoint(endpointName)]` attribute. A readonly endpoint has
+a `#[view(viewName)]` attribute instead. The contract's storage is
+declared byß a method without body, with a `#[storage_mapper("storage_name")]`
+attribute and, perhaps, a `#[view(viewName)]` attributes.
+The names in these attributes do not have to be related to the names of the
+functions tagged with them.
+
+From this trait, a few classes and methods will be generated at compilation
+time. Their purpose is to link the contract as declared by us with the MX
+infrastructure.
+
+**Storage**
+
+In the VM, the storage is a map from account
+to (map from bytes to bytes). However, int the rust code, the storage is
+declared as a function with arguments of variout types and an object return
+value. Let us take an example:
+```rs
+#[storage_mapper("balances")]
+fn s_balances(&self, address1: &ManagedAddress, address2: &ManagedAddress) -> SingleValueMapper<BigUint>;
+```
+From this, a function that does roughly the following will be generated:
+```rs
+fn s_balances(&self, address1: &ManagedAddress, address2: &ManagedAddress) -> SingleValueMapper<BigUint> {
+    let contract_address = self.address();
+    let key =
+        b"balances"  // The argument of the storage_mapper attribute
+        + encode_as_bytes(address1)
+        + encode_as_bytes(address2);
+    SingleValueMapper<BigUint>(contract_address, key)
+}
+```
+The `SingleValueMapper` object will then translate our set and get calls to
+hook calls for accessing the storage, encoding and decoding the `Biguint` values
+as needed.
+
+A storage function may also have a `view` attribute that declares a readonly
+endpoint for accessing this storage:
+```rs
+#[view(getBalances)]
+```
+I will not go onto details, but this will decalare a new function that is also
+a view (see below for more details), and it will work as intuitively expected.
+
+**Contract calls**
+
+A MX contract call consists of
+* The caller's address
+* The called contract's address
+* An endpoint name (as bytes)
+* Some (optional) token transfers
+* Encoded arguments
+
+How transfers work and how the contract's code is being retrieved given its
+address is not important right now.
+
+However, the important part is that, for each endpoint, the mx framework
+generates a function for gluing the call above with the actual function that we
+tagged with `#[endpoint]`. As an example, for this endpoint
+```rs
+#[endpoint(transferEndpoint)]
+fn transfer(&self, to: &ManagedAddress, value: BigUint) -> bool {
+}
+```
+a function that does roughly the following will be declared:
+```rs
+fn transferEndpoint(&self, address:bytes, args:bytes) -> bytes {
+    let to, value = decode(args);
+    let contract = MyContract::new(address);
+    let result = contract.transfer(to, value);
+    encode(result)
+}
+```
+Views are endpoints that do not change the contract state. For our purposes,
+we can consider them as normal endpoints.
+
+**Proxies**
+
+It is possible to make calls between contracts by providing the elements as
+mentioned above. However, it is fairly tedious to, e.g., encode arguments by
+hand, so the user has a few helper tools.
+
+First, one can declare declare a proxy trait. This provides an interface
+through which we can call another contract. It looks similar to a contract
+trait, except that
+* It has a `#[multiversx_sc::proxy]` attribute
+* All methods are endpoints
+* No method has a body.
+Forsooth, one way of writing a proxy is to take a contract trait, remove all
+non-endpoint methods and replace all method bodies with `;`.
+
+There can't be two proxies or a proxy and a contract in the same module. They
+must be declared in different crates, or in different modules in the same crate.
+
+From a proxy trait, a few things will be generated. In particular, for each
+endpoint, we will have a method that will encode its arguments to a contract
+call object and will also set the contract's address and the endpoint name.
+We are still responsible for adding transfers to the call object, decoding the
+call's result and other things.
+
+Second, one can declare a proxy method. This will create a proxy for calling the
+contract given as argument, e.g.:
+```
+#[proxy]
+fn router_proxy(&self, sc_address: ManagedAddress) -> router_proxy::Proxy<Self::Api>;
+```
+
+Some early decisions
+--------------------
+
+We could first expand macros (not available in stable Rust, available only as a
+nightly build feature), then handle the generated code. This requires us to
+handle more Rust features than the option below, which is the preferred version.
+
+The other option is to leave the rust contract as a simple trait, and implement
+all missing parts in the semantics (e.g. have a special rule for storage
+function calls that encodes arguments and produces a storage access object).
+
+Crates
+------
+
+In the Rust part of the semantics, we will handle a single crate, which is a
+file containing one contract (may also contain other items).
+
+See [this](https://doc.rust-lang.org/stable/reference/crates-and-source-files.html)
+for a description of crates.
+
+Modules
+-------
+
+See [this](https://doc.rust-lang.org/stable/reference/items/modules.html)
+for a description of modules. However, we need to handle only a very restricted
+version.
+
+We are trying to execute simple contracts, and most of them do not declare any
+modules. However, there are two of them that declare two MulitversX proxies
+which, as mentioned above, need their own modules (uniswap-v-2-router and
+unswap-v2-swap).
+
+Note that, since we are not trying to compile Rust code, we just want to execute
+it, the only useful piece of information in the proxy is the mapping from the
+name of the function we call to the name of the endpoint. So here are a few
+options:
+1. Change the Rust examples to do contract calls manually, without proxies.
+2. Add modules to the semantics in a way that allows us to extract the endpoint
+  name.
+3. Cheat and completely ignore modules, then transform proxy calls into contract
+  calls by using function names instead of endpoint names.
+4. Have different versions of the contract for generating wasm code and for
+  running with our semantics.
+5. In our semantics, dump everything declared in a module in the global
+  namespace, and ignore path prefixes when resolving names.
+
+We can probably implement 2 and 5 in a fairly easy way. If we have the time,
+2 would probably be cleaner we index traits (and, perhaps,
+other items in a module) by something like (MaybeModuleName, TraitName), and we
+always use qualified names for traits that have them
+(there is probably no need to implement something more complex now).
+
+Constants
+---------
+
+See [this](https://doc.rust-lang.org/stable/reference/items/constant-items.html)
+for a general definition of constants.
+
+We will only handle constants that have actual names (i.e. not `_`) and
+whose values are literals.
+
+Traits
+------
+
+See [this](https://doc.rust-lang.org/stable/reference/items/traits.html)
+for a general definition of traits.
+
+In traits, we will not handle type aliases and constant items. We will only
+handle functions/methods, and all such functions will have `self` as an
+argument.
+
+Normally, traits are abstract interfaces that must be implemented. To keep
+things simple, we will use them as they are. Any unimplemented
+functions/methods are assumed to have a MultiversX-specific implementation and
+will be handled by the MultiversX part of the semantics.
+
+This also means that we can't implement `Self`.
+
+We will not handle generic traits, supertraits, or visibility .
+
+Structs and implementations
+---------------------------
+
+We will not parse any struct declarations or implementations. Instead, for
+all objects that we need (e.g. `SingleValueMapper`s) we will provide builtins
+defined in K, most likely in the MultiversX part of the semantics.
+
+Other items at the top level
+----------------------------
+
+We will ignore: use declaratios, some attributes and macros.
+
+We will not handle: extern crates, functions, type aliases,
+enumerations, unions, static items, external blocks.
+
+Other things not discussed elsewhere
+------------------------------------
+
+We will not handle pattern function parameters (see
+[FunctionParamPattern](https://doc.rust-lang.org/stable/reference/items/functions.html)).
+
+
+Preprocessing
+-------------
+
+In order to be able to resolve methods and constants efficiently, we will
+preprocess the main rust file/crate to fill the following data structure:
+
+```k
+<preprocessed>
+    <constants> .Map </constants>  // Map from Identifier to Value:KResult
+    <traits>
+        <trait multiplicity="*" type=Map>
+            <full-path> TypePath </full-path>
+            <functions>
+                <function multiplicity="*">
+                    <name> Identifier </name>
+                    <args> .List </args>  // list of identifiers
+                    <definition> BlockExpressionOrSemicolon </definition>
+                </function>
+            </functions>
+        </trait>
+    </traits>
+</preprocessed>
+```
+
+Execution
+---------
+
+* Only one thread.
+* No async stuff.
+* Execution always starts with a `call f(kresult-args)` at the top of the
+  instruction stack.
+* We will assume that there are some global constants of simple types
+  (ints, strings), and all other objects are accessed through local variables.
+* Some of these variables may be mutable, and may be passed
+  as mutable arguments to functions/methods. We should not do correctness checks
+  so we will assume everything to be mutable.
+* We will also ignore borrowing and other similar stuff.
+* We will NOT ignore clone.
+* We will assume no variable shadowing (it is allowed, but the examples do not
+  have it).
+* `self` a normal function argument when the function is being executed, which
+  is being identified in a special way when the call is being made.
+* It's not fully clear whether we want to free memory when variables go out
+  of scope. The easiest way to do this would be to add a reference count for
+  each value, but it's not obvious that it's worth the effort. We will free all
+  memory at the end of an endpoint's execution.
+
+Then, our execution state should look something like this:
+
+```k
+<execution-state>
+    <values> .Map </values>  // Map from ValueId:Int |-> Value
+    <locals> .Map </locals>  // Map from Identifier |-> ValueId:Int
+    <stack> .List </stack>  // list of locals map.
+<execution-state>
+```
+
+Various objects: Main contract
+------------------------------
+
+It's a struct containing the current contract address.
+
+Various objects: SingleValueMapper
+----------------------------------
+
+SingleValueMapper was discussed above, but, just to make it clear, it's
+a struct containing the storage key. The contract's address is not needed
+because it's the current callee, and the MultiversX part of the semantics should
+have that.
+
+The get/set/set_if_not_empty calls should translate directly to MultiversX hook
+calls.
+
+Various objects: BigUint
+------------------------
+
+MultiversX's VM stores and handles `BigUint`s. The Rust code only sees the
+IDs of those BigUint
+
+It's a struct containing the Id of an Int object held in the MultiversX part
+of the semantics. All operations, constructors and casts translate directly into
+MultiversX hook calls.
+
+Various objects: TokenIdentifier
+--------------------------------
+
+It's a struct containing the ID of a Bytes string held by the MultiversX part
+of the semantics.
+
+Various objects: others
+-----------------------
+
+This is probably not the full list of objects needed here. Most of the
+MultiversX ones should be defined
+[here](https://github.com/multiversx/mx-sdk-rs/tree/master/framework/base/src/types/managed)
+or in the Rust documentation.