First, read our blogpost ink! speaks Solidity on PolkaVM.

With ink! v6, we have introduced an abi field in a custom ink-lang table in the package.metadata table of a contract's manifest file (i.e. the Cargo.toml file) - more details here. It allows building your contract in Solidity ABI compatibility mode when declared as follows:

[package.metadata.ink-lang]
abi = "sol"

The implication of supporting Solidity ABI encoding is that all types used as constructor/message argument and return types, and event argument types must define a mapping to an equivalent Solidity ABI type.

note

This is similar to the requirement to implement scale::Encode and scale::Decode for Rust types used in the public interfaces of ink!/"native" ABI encoded contracts.

Rust/ink! to Solidity ABI type mapping

This mapping is defined using the SolEncode and SolDecode traits, which are analogs to scale::Encode and scale::Decode (but for Solidity ABI encoding/decoding). You won't be able to use Rust types for which no mapping to a Solidity type is defined. An error about a missing trait implementation for this type will be thrown.

Default/provided mappings

SolEncode and SolDecode are implemented for the following Rust/ink! primitive types creating a mapping to the corresponding Solidity ABI types as shown in the table below:

Rust/ink! type	Solidity ABI type	Notes
bool	bool
iN for N ∈ {8,16,32,64,128}	intN	e.g i8 ↔ int8
uN for N ∈ {8,16,32,64,128}	uintN	e.g u8 ↔ uint8
ink::U256	uint256
String	string
Box<str>	string
ink::Address / ink::H160	address	ink::Address is a type alias for the ink::H160 type used for addresses in pallet-revive
[T; N] for const N: usize	T[N]	e.g. [i8; 64] ↔ int8[64]
Vec<T>	T[]	e.g. Vec<i8> ↔ int8[]
Box<[T]>	T[]	e.g. Box<[i8]> ↔ int8[]
ink::sol::FixedBytes<N> for 1 <= N <= 32	bytesN	e.g. FixedBytes<32> ↔ bytes32, FixedBytes<N> is just a newtype wrapper for [u8; N] that also implements From<u8>
ink::sol::DynBytes	bytes	DynBytes is just a newtype wrapper for Vec<u8> that also implements From<Box<[u8]>>
(T1, T2, T3, ... T12)	(U1, U2, U3, ... U12)	where T1 ↔ U1, ... T12 ↔ U12 e.g. (bool, u8, Address) ↔ (bool, uint8, address)
Option<T>	(bool, T)	e.g. Option<u8> ↔ (bool, uint8)

note

Rust's Option<T> type doesn't have a semantically equivalent Solidity ABI type, because Solidity enums are field-less.

So Option<T> is mapped to a tuple representation instead (i.e. (bool, T)), because this representation allows preservation of semantic information in Solidity, by using the bool as a "flag" indicating the variant (i.e. false for None and true for Some) such that:

• Option::None is mapped to (false, <default_value>) where <default_value> is the zero bytes only representation of T (e.g. 0u8 for u8 or Vec::new() for Vec<T>)
• Option::Some(value) is mapped to (true, value)

The resulting type in Solidity can be represented as a struct with a field for the "flag" and another for the data.

Note that enum in Solidity is encoded as uint8 in Solidity ABI encoding, while the encoding for bool is equivalent to the encoding of uint8, with true equivalent to 1 and false equivalent to 0. Therefore, the bool "flag" can be safely interpreted as a bool or enum (or even uint8) in Solidity code.

SolEncode is additionally implemented for reference and smart pointer types below:

Rust/ink! type	Solidity ABI type	Notes
&str, &mut str	string
&T, &mut T, Box<T>	T	e.g. &i8 ↔ int8
&[T], &mut [T]	T[]	e.g. &[i8] ↔ int8[]
ink::sol::ByteSlice	bytes	ByteSlice is a just newtype wrapper for &[u8]

Handling the Result<T, E> type

Rust's Result<T, E> type doesn't have a semantically equivalent Solidity ABI type, because Solidity enums are field-less, so no composable mapping is provided.

However, Result<T, E> types are supported as the return type of messages and constructors, and they're handled at language level as follows:

• When returning the Result::Ok variant, where T implements SolEncode, T is encoded as "normal" Solidity ABI return data.
• When returning the Result::Err variant, E must implement SolErrorEncode, ink! will set the revert flag in the execution environment, and E will be encoded as Solidity revert error data, with the error data representation depending on the SolErrorEncode implementation.
• Similarly, for decoding, T must implement SolDecode, while E must implement SolErrorDecode, and the returned data is decoded as T (i.e. Result::Ok) or E (i.e. Result::Err) depending on whether the revert flag is set (i.e. E if the revert flag is set, and T otherwise).

The SolErrorEncode and SolErrorDecode traits define the highest level interfaces for encoding and decoding an arbitrary Rust/ink! error type as Solidity ABI revert error data.

Default implementations for both SolErrorEncode and SolErrorDecode are provided for unit (i.e. ()), and these are equivalent to reverting with no error data in Solidity (i.e. empty output buffer).

For arbitrary custom error types, Derive macros are provided for automatically generating implementations of SolErrorEncode and SolErrorDecode for structs and enums for which all fields (if any) implement SolEncode and SolDecode.

• For structs, the struct name is used as the name of the Solidity custom error while the fields (if any) are the parameters
• For enums, each variant is its own Solidity custom error, with the variant name being the custom error name, and the fields (if any) being the parameters

For convenience, the #[ink::error] attribute macro is also provided for automatically deriving the following traits:

• SolErrorEncode: for encoding a custom type as revert error data
• SolErrorDecode: for decoding revert error data into a custom type
• SolErrorMetadata: for generating Solidity ABI metadata (gated behind the std feature)

// Represented as a Solidity custom error with no parameters
#[ink::error]
struct UnitError;

// Represented as a Solidity custom error with parameters
#[ink::error]
struct ErrorWithParams(bool, u8, String);

// Represented as a Solidity custom error with named parameters
#[ink::error]
struct ErrorWithNamedParams {
    status: bool,
    count: u8,
    reason: String,
}

// Represented as multiple Solidity custom errors
// (i.e. each variant represents a Solidity custom error)
#[ink::error]
enum MultipleErrors {
    UnitError,
    ErrorWithParams(bool, u8, String),
    ErrorWithNamedParams {
        status: bool,
        count: u8,
        reason: String,
    }
}

note

In "ink" and "all" ABI mode, the #[ink::error] attribute macro will also derive implementations of the scale::Encode and scale::Decode traits.

note

For other Solidity revert error data representations (e.g. legacy revert strings), you can implement SolErrorEncode and SolErrorDecode manually to match those representations.

note

Rust's coherence/orphan rules mean that you can only implement the SolErrorEncode and SolErrorDecode traits for local types.

Mappings for arbitrary custom types

For arbitrary custom types, Derive macros are provided for automatically generating implementations of SolEncode and SolDecode

• For structs where all fields (if any) implement SolEncode and SolDecode respectively, including support for generic types
• For enums where all variants are either unit-only or field-less (see notes below for the rationale for this limitation)

use ink_macro::{SolDecode, SolEncode};

#[derive(SolDecode, SolEncode)]
struct UnitStruct;

#[derive(SolDecode, SolEncode)]
struct TupleStruct(bool, u8, String);

#[derive(SolDecode, SolEncode)]
struct FieldStruct {
    status: bool,
    count: u8,
    reason: String,
}

#[derive(SolDecode, SolEncode)]
enum SimpleEnum {
    One,
    Two,
    Three,
}

#[derive(SolDecode, SolEncode)]
struct NestedStruct {
    unit: UnitStruct,
    tuple: TupleStruct,
    fields: FieldStruct,
    enumerate: SimpleEnum,
}

#[derive(SolDecode, SolEncode)]
struct GenericStruct<T> {
    concrete: u8,
    generic: T,
}

note

Solidity has no semantic equivalent for Rust/ink! enums with fields (i.e. Solidity enums can only express the equivalent of Rust unit-only or field-less enums).

So mapping complex Rust enums (i.e. enums with fields) to "equivalent" Solidity representations typically yields complex structures based on tuples (at Solidity ABI encoding level) and structs (at Solidity language level).

Because of this, the Derive macros for SolEncode and SolDecode do NOT generate implementations for enums with fields.

However, you can define custom representations for these types by manually implementing the SolEncode and SolDecode (see linked rustdoc for instructions).

note

Rust's coherence/orphan rules mean that you can only implement the SolEncode and SolDecode traits for local types.

Solidity Tooling

You can deploy and interact with ink! smart contracts using popular Solidity tools like Hardhat and Foundry thanks to the Solidity-compatible ABI output.

Full Tutorial: Use Solidity Tooling with ink! Contracts