Migrate the rust_xcrate test to wit-bindgen test (#1236)

* Migrate the `rust_xcrate` test to `wit-bindgen test`

This was a tricky test to migrate. Doing this required implementing a
number of new features:

* Rust now has a `[lang]`-specific configuration `externs = [...]` which
  is a list of files to compile as external crates. This tests the
  cross-crate behavior of the test.
* Tests now support more than just runner/test composed components, now
  they also support multiple components being composed. This is required
  because the test was specifically testing behavior where a component
  simultaneously imports and exports.

This last point was particularly tricky. Namely this required
abstracting the definition of a test to a runner plus a list of tests,
not just one test. These are still modeled as a list of `world` items in
the original WIT document. Additionally I was having trouble getting
`wasm-compose` working so I ended up switching to `wac`. This means that
tests can now support custom `wac` composition scripts to configure how
exactly the composition is created, and that's used in this new test to
insert a component as a sandwich between two others.

* Review comments

Author: alexcrichton

Cargo.lock

@@ -65,6 +65,7 @@ wit-bindgen-csharp = { workspace = true, features = ['clap'], optional = true }
 wit-bindgen-test = { workspace = true }
 wit-component = { workspace = true }
 wasm-encoder = { workspace = true }
+env_logger = "0.11.7"
 
 [features]
 default = [

Cargo.toml

@@ -6,7 +6,6 @@ publish = false
 
 [dependencies]
 wit-bindgen = { path = "../guest-rust" }
-rust-xcrate-test = { path = './rust-xcrate-test' }
 
 [lib]
 test = false
@@ -71,7 +70,3 @@ test = false
 [[bin]]
 name = "resource_borrow_simple"
 test = false
-
-[[bin]]
-name = "rust_xcrate"
-test = false

crates/test-rust-wasm/Cargo.toml

@@ -24,7 +24,10 @@ regex = "1.11.1"
 serde = { version = "1.0.218", features = ["derive"] }
 toml = "0.8.20"
 wasi-preview1-component-adapter-provider = "30.0.2"
-wasm-compose = { workspace = true }
+wac-parser = "0.6.1"
+wac-types = "0.6.1"
+wac-graph = "0.6.1"
+indexmap = { workspace = true }
 wasm-encoder = { workspace = true }
 wasmparser = { workspace = true, features = ["features"] }
 wat = { workspace = true }

crates/test-rust-wasm/rust-xcrate-test/Cargo.toml

@@ -208,6 +208,67 @@ have `runner-std.rs` and `runner-nostd.rs` to test with and without the
 bindings generator flags it's expected that the original `runner` or `test`
 worlds are still adhered to.
 
+#### Test Configuration: World Names
+
+By default `runner` and `test` worlds are expected, but this can be configured
+with:
+
+```wit
+//@ runner = "other-runner"
+//@ dependencies = ["other-test"]
+
+package foo:bar;
+
+world other-runner {
+    // ...
+}
+
+world other-test {
+    // ...
+}
+```
+
+This will then expect `other-runner.rs` for example as a test file, so test
+files are still named after their worlds.
+
+#### Test Configuration: Fancy Compositions
+
+The `wac` tooling is available for composing components together. This can be
+configured with `dependencies` and `wac` keys:
+
+```wit
+//@ dependencies = ["intermediate", "leaf"]
+//@ wac = "./compose.wac"
+
+package foo:bar;
+
+world runner {
+    // ...
+}
+
+world intermediate {
+    // ...
+}
+
+world leaf {
+    // ...
+}
+```
+
+This would then require a `compose.wac` file in the test directory. Components
+named `test:{world}` are made available to the script to perform compositions
+with:
+
+```wac
+package example:composition;
+
+let leaf = new test:leaf { ... };
+let intermediate = new test:intermediate { ...leaf, ... };
+let runner = new test:runner { ...intermediate, ... };
+
+export runner...;
+```
+
 ## Language Support
 
 Currently the `wit-bindgen test` CLI comes with built-in language support for a
@@ -236,3 +297,26 @@ This would recognize the `rs` file extension and use the
 `./wit-bindgen-rust-runner` script or binary to execute tests. The exact
 interface to the tests is documented as part of `wit-bindgen test --help` for
 the `--custom` argument.
+
+#### Configuration: Rust
+
+Rust configuration supports a few keys at the top of files in addition to the
+default `args` option for bindings generator options
+
+```rust
+//@ [lang]
+//@ rustflags = '-O'
+//@ externs = ['./other.rs']
+```
+
+Here the crate will be compiled with `-O` and `./other.rs` will be compiled as a
+separate crate and passed as `--extern`
+
+#### Configuration: C
+
+C/C++ configuration supports configuring compilation flags at this time:
+
+```rust
+//@ [lang]
+//@ cflags = '-O'
+```

crates/test-rust-wasm/src/bin/rust_xcrate.rs

@@ -75,7 +75,7 @@ pub struct RuntimeTestConfig<T = HashMap<String, toml::Value>> {
     pub lang: Option<T>,
 }
 
-#[derive(Deserialize)]
+#[derive(Deserialize, Clone, Debug)]
 #[serde(untagged)]
 pub enum StringList {
     String(String),
@@ -114,6 +114,32 @@ pub struct WitConfig {
     /// arguments. For example with Rust it avoids passing `--generate-all` by
     /// default to bindings generation.
     pub default_bindgen_args: Option<bool>,
+
+    /// Name of the world for the "runner" component, and note that this affects
+    /// filenames as well.
+    pub runner: Option<String>,
+
+    /// List of worlds for "test" components. This affects filenames and these
+    /// are all available to import to the "runner".
+    pub dependencies: Option<StringList>,
+
+    /// Path to a `*.wac` file to specify how composition is done.
+    pub wac: Option<String>,
+}
+
+impl WitConfig {
+    /// Returns the name of the "runner" world
+    pub fn runner_world(&self) -> &str {
+        self.runner.as_deref().unwrap_or("runner")
+    }
+
+    /// Returns the list of dependency worlds that this configuration uses.
+    pub fn dependency_worlds(&self) -> Vec<String> {
+        match self.dependencies.clone() {
+            Some(list) => list.into(),
+            None => vec!["test".to_string()],
+        }
+    }
 }
 
 /// Parses the configuration `T` from `contents` in comments at the start of the