fix: improve error msg on calling json on void function (#286)

examples/Cargo.toml

@@ -49,3 +49,7 @@ path = "src/croncat.rs"
 [[example]]
 name = "various_queries"
 path = "src/various_queries.rs"
+
+[[example]]
+name = "noop"
+path = "src/noop.rs"

examples/noop-contract/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "simple-contract"
+version = "0.1.0"
+publish = false
+edition = "2018"
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+near-sdk = "4.1.1"
+
+[profile.release]
+codegen-units = 1
+# Tell `rustc` to optimize for small code size.
+opt-level = "z"
+lto = true
+debug = false
+panic = "abort"
+# Opt into extra safety checks on arithmetic operations https://stackoverflow.com/a/64136471/249801
+overflow-checks = true

examples/noop-contract/src/lib.rs

@@ -0,0 +1,11 @@
+use near_sdk::borsh::{self, BorshDeserialize, BorshSerialize};
+use near_sdk::near_bindgen;
+
+#[near_bindgen]
+#[derive(Default, BorshDeserialize, BorshSerialize)]
+pub struct Noop;
+
+#[near_bindgen]
+impl Noop {
+    pub fn noop() {}
+}

examples/src/noop.rs

@@ -0,0 +1,19 @@
+/// This contract has only one method `noop` which does nothing and returns nothing.
+const NOOP_CONTRACT_WASM_FILEPATH: &str = "./examples/res/noop_contract.wasm";
+
+#[tokio::main]
+async fn main() -> anyhow::Result<()> {
+    let worker = workspaces::sandbox().await?;
+    let wasm = std::fs::read(NOOP_CONTRACT_WASM_FILEPATH)?;
+    let contract = worker.dev_deploy(&wasm).await?;
+
+    let res = contract.call("noop").transact().await?.json::<()>();
+
+    // Ok to error for call with no return value
+    assert_eq!(
+        *res.unwrap_err().kind(),
+        workspaces::error::ErrorKind::DataConversion,
+        "the function call returned an empty value, which cannot be parsed as JSON"
+    );
+    Ok(())
+}

workspaces/src/result.rs

@@ -229,7 +229,21 @@ impl ExecutionFinalResult {
     /// the internal state does not meet up with [`serde::de::DeserializeOwned`]'s
     /// requirements.
     pub fn json<T: serde::de::DeserializeOwned>(self) -> Result<T> {
-        self.into_result()?.json()
+        let val = self.into_result()?;
+        match val.json() {
+            Err(err) => {
+                // This catches the case: `EOF while parsing a value at line 1 column 0`
+                // for a function that doesn't return anything; this is a more descriptive error.
+                if *err.kind() == ErrorKind::DataConversion && val.value.repr.is_empty() {
+                    return Err(ErrorKind::DataConversion.custom(
+                        "the function call returned an empty value, which cannot be parsed as JSON",
+                    ));
+                }
+
+                Err(err)
+            }
+            ok => ok,
+        }
     }
 
     /// Deserialize an instance of type `T` from bytes sourced from the execution