Add a test to help track wasm features over time

compiler/rustc_target/src/target_features.rs

@@ -427,6 +427,13 @@ const WASM_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] = &[
     ("sign-ext", Stable, &[]),
     ("simd128", Stable, &[]),
     // tidy-alphabetical-end
+    //
+    // Note that if a new feature is added above please add it to the test at
+    // tests/ui/wasm/default-enabled-features.rs to ensure that its
+    // on-by-default state is tracked over time. If you'd also be so kind as to
+    // update the reference, such as in
+    // https://github.com/rust-lang/reference/pull/1638, it would also be much
+    // appreciated.
 ];
 
 const BPF_ALLOWED_FEATURES: &[(&str, Stability, ImpliedFeatures)] =

tests/ui/wasm/default-enabled-features.rs

@@ -0,0 +1,41 @@
+//@ only-wasm32
+//@ build-pass
+
+#![feature(wasm_target_feature)]
+
+fn main() {
+    #[cfg(any(
+        // on-by-default features
+        not(target_feature = "multivalue"),
+        not(target_feature = "mutable-globals"),
+        not(target_feature = "reference-types"),
+        not(target_feature = "sign-ext"),
+
+        // off-by-default features
+        target_feature = "atomics",
+        target_feature = "bulk-memory",
+        target_feature = "exception-handling",
+        target_feature = "extended-const",
+        target_feature = "nontrapping-fptoint",
+        target_feature = "simd128",
+        target_feature = "relaxed-simd",
+    ))]
+    compile_error!(
+        "\
+If this test fails to compile then it means that the default set of features
+active on WebAssembly targets is no longer what it used to be. This is likely
+due to LLVM being updated and changing the definition of the `generic` CPU in
+wasm.
+
+If you'd be so obliged please update the feature listings above this error
+message. Additionally please update this file too:
+
+* src/doc/rustc/src/platform-support/wasm32-unknown-unknown.md
+
+Specifically the section about \"Enabled WebAssembly features\". If you'd also
+be so kind as to ping wasm maintainers and/or the release team in the PR that
+updates this test it would also be much appreciated to ensure that this change
+is communicated out to users.
+"
+    );
+}