Add a component type accessor for Component (#8078)

* Add a component type accessor for `Component`

All the bits are already in place so all that's needed is to touch up
some docs and fixup a minor case uncovered through testing. Otherwise
this enables runtime reflection over the imports and exports of an
uninstantiated component.

* Run test through miri

crates/wasmtime/src/runtime/component/component.rs

@@ -1,3 +1,5 @@
+use crate::component::matching::InstanceType;
+use crate::component::types;
 use crate::{
     code::CodeObject, code_memory::CodeMemory, instantiate::MmapVecWrapper,
     type_registry::TypeCollection, Engine, Module, ResourcesRequired,
@@ -284,6 +286,141 @@ impl Component {
         Component::from_parts(engine, code, None)
     }
 
+    /// Returns the type of this component as a [`types::Component`].
+    ///
+    /// This method enables runtime introspection of the type of a component
+    /// before instantiation, if necessary.
+    ///
+    /// ## Component types and Resources
+    ///
+    /// An important point to note here is that the precise type of imports and
+    /// exports of a component change when it is instantiated with respect to
+    /// resources. For example a [`Component`] represents an un-instantiated
+    /// component meaning that its imported resources are represeted as abstract
+    /// resource types. These abstract types are not equal to any other
+    /// component's types.
+    ///
+    /// For example:
+    ///
+    /// ```
+    /// # use wasmtime::Engine;
+    /// # use wasmtime::component::Component;
+    /// # use wasmtime::component::types::ComponentItem;
+    /// # fn main() -> wasmtime::Result<()> {
+    /// # let engine = Engine::default();
+    /// let a = Component::new(&engine, r#"
+    ///     (component (import "x" (type (sub resource))))
+    /// "#)?;
+    /// let b = Component::new(&engine, r#"
+    ///     (component (import "x" (type (sub resource))))
+    /// "#)?;
+    ///
+    /// let (_, a_ty) = a.component_type().imports(&engine).next().unwrap();
+    /// let (_, b_ty) = b.component_type().imports(&engine).next().unwrap();
+    ///
+    /// let a_ty = match a_ty {
+    ///     ComponentItem::Resource(ty) => ty,
+    ///     _ => unreachable!(),
+    /// };
+    /// let b_ty = match b_ty {
+    ///     ComponentItem::Resource(ty) => ty,
+    ///     _ => unreachable!(),
+    /// };
+    /// assert!(a_ty != b_ty);
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// Additionally, however, these abstract types are "substituted" during
+    /// instantiation meaning that a component type will appear to have changed
+    /// once it is instantiated.
+    ///
+    /// ```
+    /// # use wasmtime::{Engine, Store};
+    /// # use wasmtime::component::{Component, Linker, ResourceType};
+    /// # use wasmtime::component::types::ComponentItem;
+    /// # fn main() -> wasmtime::Result<()> {
+    /// # let engine = Engine::default();
+    /// // Here this component imports a resource and then exports it as-is
+    /// // which means that the export is equal to the import.
+    /// let a = Component::new(&engine, r#"
+    ///     (component
+    ///         (import "x" (type $x (sub resource)))
+    ///         (export "x" (type $x))
+    ///     )
+    /// "#)?;
+    ///
+    /// let (_, import) = a.component_type().imports(&engine).next().unwrap();
+    /// let (_, export) = a.component_type().exports(&engine).next().unwrap();
+    ///
+    /// let import = match import {
+    ///     ComponentItem::Resource(ty) => ty,
+    ///     _ => unreachable!(),
+    /// };
+    /// let export = match export {
+    ///     ComponentItem::Resource(ty) => ty,
+    ///     _ => unreachable!(),
+    /// };
+    /// assert_eq!(import, export);
+    ///
+    /// // However after instantiation the resource type "changes"
+    /// let mut store = Store::new(&engine, ());
+    /// let mut linker = Linker::new(&engine);
+    /// linker.root().resource("x", ResourceType::host::<()>(), |_, _| Ok(()))?;
+    /// let instance = linker.instantiate(&mut store, &a)?;
+    /// let instance_ty = instance.exports(&mut store).root().resource("x").unwrap();
+    ///
+    /// // Here `instance_ty` is not the same as either `import` or `export`,
+    /// // but it is equal to what we provided as an import.
+    /// assert!(instance_ty != import);
+    /// assert!(instance_ty != export);
+    /// assert!(instance_ty == ResourceType::host::<()>());
+    /// # Ok(())
+    /// # }
+    /// ```
+    ///
+    /// Finally, each instantiation of an exported resource from a component is
+    /// considered "fresh" for all instantiations meaning that different
+    /// instantiations will have different exported resource types:
+    ///
+    /// ```
+    /// # use wasmtime::{Engine, Store};
+    /// # use wasmtime::component::{Component, Linker};
+    /// # fn main() -> wasmtime::Result<()> {
+    /// # let engine = Engine::default();
+    /// let a = Component::new(&engine, r#"
+    ///     (component
+    ///         (type $x (resource (rep i32)))
+    ///         (export "x" (type $x))
+    ///     )
+    /// "#)?;
+    ///
+    /// let mut store = Store::new(&engine, ());
+    /// let linker = Linker::new(&engine);
+    /// let instance1 = linker.instantiate(&mut store, &a)?;
+    /// let instance2 = linker.instantiate(&mut store, &a)?;
+    ///
+    /// let x1 = instance1.exports(&mut store).root().resource("x").unwrap();
+    /// let x2 = instance2.exports(&mut store).root().resource("x").unwrap();
+    ///
+    /// // Despite these two resources being the same export of the same
+    /// // component they come from two different instances meaning that their
+    /// // types will be unique.
+    /// assert!(x1 != x2);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn component_type(&self) -> types::Component {
+        let resources = Arc::new(PrimaryMap::new());
+        types::Component::from(
+            self.inner.ty,
+            &InstanceType {
+                types: self.types(),
+                resources: &resources,
+            },
+        )
+    }
+
     /// Final assembly step for a component from its in-memory representation.
     ///
     /// If the `artifacts` are specified as `None` here then they will be

crates/wasmtime/src/runtime/component/matching.rs

@@ -195,7 +195,10 @@ impl<'a> InstanceType<'a> {
 
     pub fn resource_type(&self, index: TypeResourceTableIndex) -> ResourceType {
         let index = self.types[index].ty;
-        self.resources[index]
+        self.resources
+            .get(index)
+            .copied()
+            .unwrap_or_else(|| ResourceType::uninstantiated(&self.types, index))
     }
 }
 

tests/all/component_model/aot.rs

@@ -1,5 +1,6 @@
 use anyhow::Result;
-use wasmtime::component::{Component, Linker};
+use wasmtime::component::types::ComponentItem;
+use wasmtime::component::{Component, Linker, Type};
 use wasmtime::{Module, Precompiled, Store};
 
 #[test]
@@ -133,3 +134,33 @@ fn detect_precompiled() -> Result<()> {
     );
     Ok(())
 }
+
+#[test]
+fn reflect_resource_import() -> Result<()> {
+    let engine = super::engine();
+    let c = Component::new(
+        &engine,
+        r#"
+        (component
+            (import "x" (type $x (sub resource)))
+            (import "y" (func (result (own $x))))
+        )
+        "#,
+    )?;
+    let ty = c.component_type();
+    let mut imports = ty.imports(&engine);
+    let (_, x) = imports.next().unwrap();
+    let (_, y) = imports.next().unwrap();
+    let x = match x {
+        ComponentItem::Resource(t) => t,
+        _ => unreachable!(),
+    };
+    let y = match y {
+        ComponentItem::ComponentFunc(t) => t,
+        _ => unreachable!(),
+    };
+    let result = y.results().next().unwrap();
+    assert_eq!(result, Type::Own(x));
+
+    Ok(())
+}