better error trait design

src/lib.rs

@@ -1,18 +1,30 @@
 pub mod test {
+    // FIXME: parameterize macro on what ctx type is used here
     generate::from_witx!("test.witx");
 
-    pub struct WasiCtx {} // FIXME: parameterize macro on what ctx type is used here
+    pub struct WasiCtx {
+        mem_errors: Vec<::memory::MemoryError>,
+        value_errors: Vec<::memory::GuestValueError>,
+    }
 
-    impl types::WitxErrorConversion for WasiCtx {
-        fn success_to_errno(&mut self) -> types::Errno {
-            types::Errno::Ok
+    // Errno is used as a first return value in the functions above, therefore
+    // it must implement GuestError with type Context = WasiCtx.
+    // The context type should let you do logging or debugging or whatever you need
+    // with these errors. We just push them to vecs.
+    impl ::memory::GuestError for types::Errno {
+        type Context = WasiCtx;
+        fn is_success(&self) -> bool {
+            match self {
+                types::Errno::Ok => true,
+                _ => false,
+            }
         }
-        fn memory_error_to_errno(&mut self, e: ::memory::MemoryError) -> types::Errno {
-            eprintln!("memory error: {:?}", e);
+        fn from_memory_error(e: ::memory::MemoryError, ctx: &mut WasiCtx) -> types::Errno {
+            ctx.mem_errors.push(e);
             types::Errno::InvalidArg
         }
-        fn value_error_to_errno(&mut self, e: ::memory::GuestValueError) -> types::Errno {
-            eprintln!("guest value error: {:?}", e);
+        fn from_value_error(e: ::memory::GuestValueError, ctx: &mut WasiCtx) -> types::Errno {
+            ctx.value_errors.push(e);
             types::Errno::InvalidArg
         }
     }