Keep Config options for disabling compiled-out features

This patch keeps config options for disabling default-enabled wasmtime
features, even when those features are disabled via crate feature flags.
That way, these wasm features can be disabled by libraries even if the
crate-level features are later enabled by some other crate in the
build-tree.

Specifically, the following methods are available on the
`wasmtime::Config` type, regardless of the enabled crate features:

1. `Config::wasm_threads`.
2. `Config::wasm_reference_types`.
3. `Config::wasm_component_model`.
4. `Config::parallel_compilation`.

Of these, enabling any of the first three while the corresponding crate
feature is disabled will lead to a runtime error when constructing the
`wasmtime::Engine`. Enabling the last one, parallel compilation, will
simply do nothing.

fixes #10454

Author: Stebalien

crates/wasmtime/src/config.rs

@@ -423,6 +423,8 @@ impl Config {
     /// WebAssembly will execute and what compute resources will be allotted to
     /// it. If Wasmtime doesn't support exactly what you'd like just yet, please
     /// feel free to open an issue!
+    ///
+    /// This feature is `false` by default.
     #[cfg(feature = "async")]
     pub fn async_support(&mut self, enable: bool) -> &mut Self {
         self.async_support = enable;
@@ -837,13 +839,11 @@ impl Config {
     /// Embeddings of Wasmtime are able to build their own custom threading
     /// scheme on top of the core wasm threads proposal, however.
     ///
-    /// The default value for this option is whether the `threads`
-    /// crate feature of Wasmtime is enabled or not. By default this crate
-    /// feature is enabled.
+    /// This feature is enabled by default as long as the `threads` crate feature
+    /// (enabled by default) is also enabled.
     ///
     /// [threads]: https://github.com/webassembly/threads
     /// [wasi-threads]: https://github.com/webassembly/wasi-threads
-    #[cfg(feature = "threads")]
     pub fn wasm_threads(&mut self, enable: bool) -> &mut Self {
         self.wasm_feature(WasmFeatures::THREADS, enable);
         self
@@ -857,15 +857,14 @@ impl Config {
     ///
     /// Note that the reference types proposal depends on the bulk memory proposal.
     ///
-    /// This feature is `true` by default.
+    /// This feature is enabled by default.
     ///
     /// # Errors
     ///
     /// The validation of this feature are deferred until the engine is being built,
     /// and thus may cause `Engine::new` fail if the `bulk_memory` feature is disabled.
     ///
     /// [proposal]: https://github.com/webassembly/reference-types
-    #[cfg(feature = "gc")]
     pub fn wasm_reference_types(&mut self, enable: bool) -> &mut Self {
         self.wasm_feature(WasmFeatures::REFERENCE_TYPES, enable);
         self
@@ -1100,12 +1099,10 @@ impl Config {
     /// [`Component`](crate::component::Component) instead of
     /// [`Module`](crate::Module) for example anyway.
     ///
-    /// The default value for this option is whether the `component-model`
-    /// crate feature of Wasmtime is enabled or not. By default this crate
-    /// feature is enabled.
+    /// This feature is enabled by default as long as the `component-model` crate
+    /// feature (enabled by default) is also enabled.
     ///
     /// [proposal]: https://github.com/webassembly/component-model
-    #[cfg(feature = "component-model")]
     pub fn wasm_component_model(&mut self, enable: bool) -> &mut Self {
         self.wasm_feature(WasmFeatures::COMPONENT_MODEL, enable);
         self
@@ -1837,8 +1834,8 @@ impl Config {
     /// Disabling this will result in a single thread being used to compile
     /// the wasm bytecode.
     ///
-    /// By default parallel compilation is enabled.
-    #[cfg(feature = "parallel-compilation")]
+    /// This feature is enabled by default as long as the `parallel-compilation`
+    /// crate feature (enabled by default) is also enabled.
     pub fn parallel_compilation(&mut self, parallel: bool) -> &mut Self {
         self.parallel_compilation = parallel;
         self
@@ -2165,6 +2162,21 @@ impl Config {
             bail!("wmemcheck (memory checker) was requested but is not enabled in this build");
         }
 
+        #[cfg(not(feature = "component-model"))]
+        if features.component_model() {
+            bail!("component-model support was requested but is not enabled in this build (requires the `component-model` crate feature)");
+        }
+
+        #[cfg(not(feature = "gc"))]
+        if features.gc_types() {
+            bail!("gc types support was requested but is not enabled in this build (requires the `gc` crate feature)");
+        }
+
+        #[cfg(not(feature = "threads"))]
+        if features.threads() {
+            bail!("threads support was requested but is not enabled in this build (requires the `threads` crate feature)");
+        }
+
         let mut tunables = Tunables::default_for_target(&self.compiler_target())?;
 
         // If no target is explicitly specified then further refine `tunables`