Add example of execution limits using fuel consumption

examples/fuel.rs

@@ -0,0 +1,28 @@
+//! Example of limiting a WebAssembly function's runtime using "fuel consumption".
+
+// You can execute this example with `cargo run --example fuel`
+
+use anyhow::Result;
+use wasmtime::*;
+
+fn main() -> Result<()> {
+    let mut config = Config::new();
+    config.consume_fuel(true);
+    let engine = Engine::new(&config)?;
+    let store = Store::new(&engine);
+    store.add_fuel(10_000)?;
+    let module = Module::from_file(store.engine(), "examples/fuel.wat")?;
+    let instance = Instance::new(&store, &module, &[])?;
+
+    // Invoke `fibonacci` export with higher and higher numbers until we exhaust our fuel.
+    let fibonacci = instance.get_typed_func::<i32, i32>("fibonacci")?;
+    for n in 1.. {
+        let fuel_before = store.fuel_consumed().unwrap();
+        let output = fibonacci.call(n)?;
+        let fuel_consumed = store.fuel_consumed().unwrap() - fuel_before;
+
+        println!("fib({}) = {} [consumed {} fuel]", n, output, fuel_consumed);
+        store.add_fuel(fuel_consumed)?;
+    }
+    Ok(())
+}

examples/fuel.wat

@@ -0,0 +1,36 @@
+(module
+  (func $fibonacci (param i32) (result i32)
+    (local i32)
+    local.get 0
+    i32.const 2
+    i32.ge_s
+    if (result i32)  ;; label = @1
+      local.get 0
+      i32.const 1
+      i32.add
+      local.set 0
+      loop  ;; label = @2
+        local.get 0
+        i32.const -3
+        i32.add
+        call 0
+        local.get 1
+        i32.add
+        local.set 1
+        local.get 0
+        i32.const -1
+        i32.add
+        local.tee 0
+        i32.const 2
+        i32.gt_s
+        br_if 0 (;@2;)
+      end
+      i32.const 1
+    else
+      local.get 0
+    end
+    local.get 1
+    i32.add
+  )
+  (export "fibonacci" (func $fibonacci))
+)