wiggle: emit a metadata module containing witx document (#1387)

* wiggle: emit a metadata module containing witx document

* wiggle: put metadata module behind a wiggle_metadata feature

* wasi-common: add wiggle_metadata feature and optional witx dep

* refactor according to alex's advice

* wasi-common: make snapshots pub

* wasi-common: i do need a wiggle_metadata feature to be available

* Tweak features and such

* wiggle: fix tests by passing metadata flag to wiggle-runtime

* wiggle: need to move wiggle-runtime to a non-dev dependency

so that the feature resolves for external users of the crates

Co-authored-by: Alex Crichton <alex@alexcrichton.com>

crates/wiggle/tests/wasi.rs

@@ -1,11 +1,26 @@
 use wiggle_runtime::{GuestBorrows, GuestError, GuestErrorType, GuestPtr};
 use wiggle_test::WasiCtx;
 
+// This test file exists to make sure that the entire `wasi.witx` file can be
+// handled by wiggle, producing code that compiles correctly.
+// The trait impls here are never executed, and just exist to validate that the
+// witx is exposed with the type signatures that we expect.
+
 wiggle::from_witx!({
     witx: ["tests/wasi.witx"],
     ctx: WasiCtx,
 });
 
+// The only test in this file is to verify that the witx document provided by the
+// proc macro in the `metadata` module is equal to the document on the disk.
+#[test]
+fn document_equivelant() {
+    let macro_doc = metadata::document();
+    let disk_doc = witx::load(&["tests/wasi.witx"]).expect("load wasi.witx from disk");
+
+    assert_eq!(macro_doc, disk_doc);
+}
+
 type Result<T> = std::result::Result<T, types::Errno>;
 
 impl<'a> GuestErrorType<'a> for types::Errno {