Add wasmtime fuel metering for Rust WASM modules

Author: cargopete

runtime/wasm/src/mapping.rs

@@ -338,6 +338,12 @@ impl ValidModule {
         config.max_wasm_stack(ENV_VARS.mappings.max_stack_size);
         config.async_support(true);
 
+        // Enable wasmtime fuel metering for Rust modules.
+        // AS modules use parity_wasm-injected gas() calls instead.
+        if is_rust_module {
+            config.consume_fuel(true);
+        }
+
         let engine = &wasmtime::Engine::new(&config)?;
         let module = wasmtime::Module::from_binary(engine, &raw_module)?;
 

runtime/wasm/src/module/instance.rs

@@ -246,6 +246,17 @@ impl WasmInstance {
                         .as_secs()
                 ))));
             }
+            Err(trap)
+                if trap
+                    .chain()
+                    .any(|e| e.downcast_ref::<Trap>() == Some(&Trap::OutOfFuel)) =>
+            {
+                // Fuel exhaustion is deterministic — same code always burns the same fuel.
+                Some(anyhow::anyhow!(
+                    "Handler '{}' exceeded fuel limit (possible infinite loop)",
+                    handler
+                ))
+            }
             Err(trap) => {
                 let trap_is_deterministic = is_trap_deterministic(&trap)
                     || self.instance_ctx().as_ref().deterministic_host_trap;
@@ -851,6 +862,13 @@ impl WasmInstance {
         // See also: runtime-timeouts
         store.set_epoch_deadline(2);
 
+        // For Rust modules, set initial fuel for wasmtime's built-in fuel metering.
+        // This replaces the parity_wasm gas injection which can't parse modern WASM features.
+        // 10 billion fuel units is generous for any reasonable handler but catches infinite loops.
+        if valid_module.language == crate::rust_abi::MappingLanguage::Rust {
+            store.set_fuel(10_000_000_000)?;
+        }
+
         let instance = valid_module
             .instance_pre
             .instantiate_async(store.as_context_mut())