Make use of `cfg(doc)` in `sp-runtime-interface` (#4673)

By using `cfg(doc)`, we can generate docs for modules which are only
available from `no_std`. This drastically improves the documentation for
the developers.

substrate/primitives/runtime-interface/proc-macro/src/runtime_interface/bare_function_interface.rs

@@ -21,10 +21,10 @@
 //! functions will prepare the parameters for the FFI boundary, call the external host function
 //! exported into wasm and convert back the result.
 //!
-//! [`generate`](bare_function_interface::generate) is the entry point for generating for each
+//! [`generate`] is the entry point for generating for each
 //! trait method one bare function.
 //!
-//! [`function_for_method`](bare_function_interface::function_for_method) generates the bare
+//! [`function_for_method`] generates the bare
 //! function per trait method. Each bare function contains both implementations. The implementations
 //! are feature-gated, so that one is compiled for the native and the other for the wasm side.
 

substrate/primitives/runtime-interface/src/host.rs

@@ -49,6 +49,9 @@ pub trait IntoPreallocatedFFIValue: RIType {
 }
 
 /// Something that can be created from a ffi value.
+/// Implementations are safe to assume that the `arg` given to `from_ffi_value`
+/// is only generated by the corresponding [`wasm::IntoFFIValue`](crate::wasm::IntoFFIValue)
+/// implementation.
 pub trait FromFFIValue: RIType {
 	/// As `Self` can be an unsized type, it needs to be represented by a sized type at the host.
 	/// This `SelfInstance` is the sized type.

substrate/primitives/runtime-interface/src/lib.rs

@@ -25,10 +25,10 @@
 //! # Using a type in a runtime interface
 //!
 //! Any type that should be used in a runtime interface as argument or return value needs to
-//! implement [`RIType`]. The associated type `FFIType` is the type that is used in the FFI
-//! function to represent the actual type. For example `[T]` is represented by an `u64`. The slice
-//! pointer and the length will be mapped to an `u64` value. For more information, see the
-//! implementation of [`RIType`] for `T`. The FFI function definition is used when calling from
+//! implement [`RIType`]. The associated type [`FFIType`](RIType::FFIType) is the type that is used
+//! in the FFI function to represent the actual type. For example `[T]` is represented by an `u64`.
+//! The slice pointer and the length will be mapped to an `u64` value. For more information see
+//! this [table](#ffi-type-and-conversion). The FFI function definition is used when calling from
 //! the wasm runtime into the node.
 //!
 //! Traits are used to convert from a type to the corresponding [`RIType::FFIType`].
@@ -265,7 +265,7 @@ pub use codec;
 pub(crate) mod impls;
 #[cfg(feature = "std")]
 pub mod host;
-#[cfg(not(feature = "std"))]
+#[cfg(any(not(feature = "std"), doc))]
 pub mod wasm;
 pub mod pass_by;
 

substrate/primitives/runtime-interface/src/wasm.rs

@@ -26,7 +26,8 @@ use sp_std::cell::Cell;
 ///
 /// It is unsafe behavior to call `Something::into_ffi_value().get()` and take this as input for
 /// `from_ffi_value`. Implementations are safe to assume that the `arg` given to `from_ffi_value`
-/// is only generated by the corresponding `host::IntoFFIValue` implementation.
+/// is only generated by the corresponding [`host::IntoFFIValue`](crate::host::IntoFFIValue)
+/// implementation.
 pub trait FromFFIValue: Sized + RIType {
 	/// Create `Self` from the given ffi value.
 	fn from_ffi_value(arg: Self::FFIType) -> Self;