[fix 857] panic when target module don't have exported _start function

CHANGELOG.md

@@ -6,6 +6,7 @@ Blocks of changes will separated by version increments.
 
 ## **[Unreleased]**
 
+- [#858](https://github.com/wasmerio/wasmer/pull/858) Fix panic when `loader` option associated with wasm module without exported `_start` function
 - [#852](https://github.com/wasmerio/wasmer/pull/852) Make minor grammar/capitalization fixes to README.md
 - [#841](https://github.com/wasmerio/wasmer/pull/841) Slightly improve rustdoc documentation and small updates to outdated info in readme files
 - [#835](https://github.com/wasmerio/wasmer/pull/836) Update Cranelift fork version to `0.44.0`

src/bin/wasmer.rs

@@ -525,20 +525,21 @@ fn execute_wasm(options: &Run) -> Result<(), String> {
             args.push(Value::I32(x));
         }
 
-        let index = instance
-            .resolve_func("_start")
-            .expect("The loader requires a _start function to be present in the module");
+        let index = instance.resolve_func("_start").map_err(|_| {
+            format!("The loader requires a _start function to be present in the module")
+        })?;
+
         let mut ins: Box<dyn LoadedInstance<Error = String>> = match loader {
             LoaderName::Local => Box::new(
                 instance
                     .load(LocalLoader)
-                    .expect("Can't use the local loader"),
+                    .map_err(|e| format!("Can't use the local loader: {:?}", e))?,
             ),
             #[cfg(feature = "loader-kernel")]
             LoaderName::Kernel => Box::new(
                 instance
                     .load(::wasmer_kernel_loader::KernelLoader)
-                    .expect("Can't use the kernel loader"),
+                    .map_err(|e| format!("Can't use the local loader: {:?}", e))?,
             ),
         };
         println!("{:?}", ins.call(index, &args));
@@ -610,7 +611,7 @@ fn execute_wasm(options: &Run) -> Result<(), String> {
                             f.read_to_end(&mut out).unwrap();
                             Some(
                                 wasmer_runtime_core::state::InstanceImage::from_bytes(&out)
-                                    .expect("failed to decode image"),
+                                    .map_err(|_| format!("failed to decode image"))?,
                             )
                         } else {
                             None