Attribute Macro sp_version::runtime_version

#[runtime_version]

An attribute that accepts a version declaration of a runtime and generates a custom wasm section with the equivalent contents.

The custom section allows to read the version of the runtime without having to execute any code. Instead, the generated custom section can be relatively easily parsed from the wasm binary. The identifier of the custom section is “runtime_version”.

A shortcoming of this macro is that it is unable to embed information regarding supported APIs. This is supported by the construct_runtime! macro.

Usage

This macro accepts a const item like the following:

use sp_version::{create_runtime_str, RuntimeVersion};

#[sp_version::runtime_version]
pub const VERSION: RuntimeVersion = RuntimeVersion {
	spec_name: create_runtime_str!("test"),
	impl_name: create_runtime_str!("test"),
	authoring_version: 10,
	spec_version: 265,
	impl_version: 1,
	apis: RUNTIME_API_VERSIONS,
	transaction_version: 2,
};

It will pass it through and add code required for emitting a custom section. The information that will go into the custom section is parsed from the item declaration. Due to that, the macro is somewhat rigid in terms of the code it accepts. There are the following considerations:

• The spec_name and impl_name must be set by a macro-like expression. The name of the macro doesn’t matter though.

• authoring_version, spec_version, impl_version and transaction_version must be set by a literal. Literal must be an integer. No other expressions are allowed there. In particular, you can’t supply a constant variable.

• apis doesn’t have any specific constraints. This is because this information doesn’t get into the custom section and is not parsed.

Compilation Target & “std” feature

This macro assumes it will be used within a runtime. By convention, a runtime crate defines a feature named “std”. This feature is enabled when the runtime is compiled to native code and disabled when it is compiled to the wasm code.

The custom section can only be emitted while compiling to wasm. In order to detect the compilation target we use the “std” feature. This macro will emit the custom section only if the “std” feature is not enabled.

Including this macro in the context where there is no “std” feature and the code is not compiled to wasm can lead to cryptic linking errors.