Implement the post-return attribute (#4297)

This commit implements the `post-return` feature of the canonical ABI in
the component model. This attribute is an optionally-specified function
which is to be executed after the return value has been processed by the
caller to optionally clean-up the return value. This enables, for
example, returning an allocated string and the host then knows how to
clean it up to prevent memory leaks in the original module.

The API exposed in this PR changes the prior `TypedFunc::call` API in
behavior but not in its signature. Previously the `TypedFunc::call`
method would set the `may_enter` flag on the way out, but now that
operation is deferred until a new `TypedFunc::post_return` method is
called. This means that once a method on an instance is invoked then
nothing else can be done on the instance until the `post_return` method
is called. Note that the method must be called irrespective of whether
the `post-return` canonical ABI option was specified or not. Internally
wasm will be invoked if necessary.

This is a pretty wonky and unergonomic API to work with. For now I
couldn't think of a better alternative that improved on the ergonomics.
In the theory that the raw Wasmtime bindings for a component may not be
used all that heavily (instead `wit-bindgen` would largely be used) I'm
hoping that this isn't too much of an issue in the future.

cc #4185

crates/environ/src/component/info.rs

@@ -129,6 +129,9 @@ pub struct Component {
     /// `VMComponentContext`.
     pub num_runtime_reallocs: u32,
 
+    /// Same as `num_runtime_reallocs`, but for post-return functions.
+    pub num_runtime_post_returns: u32,
+
     /// The number of lowered host functions (maximum `LoweredIndex`) needed to
     /// instantiate this component.
     pub num_lowerings: u32,
@@ -180,6 +183,10 @@ pub enum GlobalInitializer {
     /// used as a `realloc` function.
     ExtractRealloc(ExtractRealloc),
 
+    /// Same as `ExtractMemory`, except it's extracting a function pointer to be
+    /// used as a `post-return` function.
+    ExtractPostReturn(ExtractPostReturn),
+
     /// The `module` specified is saved into the runtime state at the next
     /// `RuntimeModuleIndex`, referred to later by `Export` definitions.
     SaveStaticModule(StaticModuleIndex),
@@ -207,6 +214,15 @@ pub struct ExtractRealloc {
     pub def: CoreDef,
 }
 
+/// Same as `ExtractMemory` but for the `post-return` canonical option.
+#[derive(Debug, Serialize, Deserialize)]
+pub struct ExtractPostReturn {
+    /// The index of the post-return being defined.
+    pub index: RuntimePostReturnIndex,
+    /// Where this post-return is being extracted from.
+    pub def: CoreDef,
+}
+
 /// Different methods of instantiating a core wasm module.
 #[derive(Debug, Serialize, Deserialize)]
 pub enum InstantiateModule {
@@ -361,7 +377,9 @@ pub struct CanonicalOptions {
 
     /// The realloc function used by these options, if specified.
     pub realloc: Option<RuntimeReallocIndex>,
-    // TODO: need to represent post-return here as well
+
+    /// The post-return function used by these options, if specified.
+    pub post_return: Option<RuntimePostReturnIndex>,
 }
 
 impl Default for CanonicalOptions {
@@ -370,6 +388,7 @@ impl Default for CanonicalOptions {
             string_encoding: StringEncoding::Utf8,
             memory: None,
             realloc: None,
+            post_return: None,
         }
     }
 }