web: Detect and log whether WebAssembly extensions are actually used

web/packages/core/src/ruffle-player.ts

@@ -427,7 +427,11 @@ export class RufflePlayer extends HTMLElement {
             this,
             config
         );
-        console.log("New Ruffle instance created.");
+        console.log(
+            "New Ruffle instance created (WebAssembly extensions: " +
+                (ruffleConstructor.is_wasm_simd_used() ? "ON" : "OFF") +
+                ")"
+        );
 
         // In Firefox, AudioContext.state is always "suspended" when the object has just been created.
         // It may change by itself to "running" some milliseconds later. So we need to wait a little

web/src/lib.rs

@@ -454,6 +454,15 @@ impl Ruffle {
         })
         .unwrap_or_default()
     }
+
+    /// Returns whether the `simd128` target feature was enabled at build time.
+    /// This is intended to discriminate between the two WebAssembly module
+    /// versions, one of which uses WebAssembly extensions, and the other one
+    /// being "vanilla". `simd128` is used as proxy for most extensions, since
+    /// no other WebAssembly target feature is exposed to `cfg!`.
+    pub fn is_wasm_simd_used() -> bool {
+        cfg!(target_feature = "simd128")
+    }
 }
 
 impl Ruffle {