Merge #858

858: [fix 857] panic when target module don't have exported _start function r=syrusakbary a=pventuzelo

# Description

Fix #857 
* replace `expect` by `map_err` for `loader`
* replace `expect` by `map_err` in other part of `wasmer.rs`

# Review

- [x] Add a short description of the the change to the CHANGELOG.md file


Co-authored-by: Patrick Ventuzelo <ventuzelo.patrick@gmail.com>
Co-authored-by: Patrick Ventuzelo <9038181+pventuzelo@users.noreply.github.com>

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ## **[Unreleased]**
 
+- [#858](https://github.com/wasmerio/wasmer/pull/858) Minor panic fix when wasmer binary with `loader` option run a module without exported `_start` function.
 - [#1056](https://github.com/wasmerio/wasmer/pull/1056) Improved `--invoke` args parsing (supporting `i32`, `i64`, `f32` and `f32`) in Wasmer CLI
 - [#1054](https://github.com/wasmerio/wasmer/pull/1054) Improve `--invoke` output in Wasmer CLI
 - [#1053](https://github.com/wasmerio/wasmer/pull/1053) For RuntimeError and breakpoints, use Box<Any + Send> instead of Box<Any>.

src/bin/wasmer.rs

@@ -430,7 +430,7 @@ fn execute_wasi(
                     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
@@ -751,20 +751,21 @@ 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));