Adds function to_substrate_wasm_fn_return_value (#3905)

* Adds function `to_substrate_wasm_fn_return_value`

Instead of replicating this piece of code over and over again, just move
it to a function that does it.

* Feedback

* Comment

substrate/core/primitives/src/lib.rs

@@ -287,3 +287,25 @@ impl From<LogLevel> for log::Level {
 		}
 	}
 }
+
+/// Encodes the given value into a buffer and returns the pointer and the length as a single `u64`.
+///
+/// When Substrate calls into Wasm it expects a fixed signature for functions exported
+/// from the Wasm blob. The return value of this signature is always a `u64`.
+/// This `u64` stores the pointer to the encoded return value and the length of this encoded value.
+/// The low `32bits` are reserved for the pointer, followed by `32bit` for the length.
+#[cfg(not(feature = "std"))]
+pub fn to_substrate_wasm_fn_return_value(value: &impl Encode) -> u64 {
+	let encoded = value.encode();
+
+	let ptr = encoded.as_ptr() as u64;
+	let length = encoded.len() as u64;
+	let res = ptr | (length << 32);
+
+	// Leak the output vector to avoid it being freed.
+	// This is fine in a WASM context since the heap
+	// will be discarded after the call.
+	rstd::mem::forget(encoded);
+
+	res
+}

substrate/core/primitives/src/testing.rs

@@ -196,15 +196,7 @@ macro_rules! wasm_export_functions {
 				$( $fn_impl )*
 			}
 
-			// We need to return *something*
-			let output = Vec::<u8>::new();
-			let res = output.as_ptr() as u64 + ((output.len() as u64) << 32);
-
-			// Leak the output vector to avoid it being freed.
-			// This is fine in a WASM context since the heap
-			// will be discarded after the call.
-			$crate::rstd::mem::forget(output);
-			res
+			$crate::to_substrate_wasm_fn_return_value(&())
 		}
 	};
 	(@IMPL
@@ -232,14 +224,7 @@ macro_rules! wasm_export_functions {
 				$( $fn_impl )*
 			};
 
-			let output = $crate::Encode::encode(&output);
-			let res = output.as_ptr() as u64 + ((output.len() as u64) << 32);
-
-			// Leak the output vector to avoid it being freed.
-			// This is fine in a WASM context since the heap
-			// will be discarded after the call.
-			$crate::rstd::mem::forget(output);
-			res
+			$crate::to_substrate_wasm_fn_return_value(&output)
 		}
 	};
 }