Add RecGroupBuilder embedder API for defining recursion groups

Adds `RecGroupBuilder`, which lets embedders declare kind-typed labels
(`PendingStructId`/`PendingArrayId`/`PendingFuncId`), use them as forward
references while defining other types via a small build-time "template"
family, and register the whole group at once with `build()`. This makes
it possible to construct self-referential and mutually-recursive
struct/array/func types directly from the embedder API, which previously
required plucking such types out of a module's imports/exports.

The builder lowers its members to module-canonical `WasmSubType`s (using
0-based `Module` indices for intra-group references and `Engine` indices
for already-registered types) and reuses the existing rec-group
registration path, so hash-consing, runtime canonicalization, supertype
lists, and GC layouts all come for free.

Implements the embedder API requested in #10176.

Author: somdoron

RELEASES.md

@@ -4,8 +4,18 @@ Unreleased.
 
 ### Added
 
+* Added `RecGroupBuilder`, an embedder API for defining recursion groups of Wasm
+  types, including types that reference themselves or each other (e.g. linked
+  lists and mutually-recursive structs).
+  [#10176](https://github.com/bytecodealliance/wasmtime/issues/10176)
+
 ### Changed
 
+### Fixed
+
+* Fixed `StorageType::is_val_type` to report whether the storage type is a value
+  type; it previously returned whether it was an `i16`.
+
 --------------------------------------------------------------------------------
 
 Release notes for previous releases of Wasmtime can be found on the respective

crates/wasmtime/src/runtime.rs

@@ -53,6 +53,7 @@ pub(crate) mod memory;
 pub(crate) mod module;
 #[cfg(feature = "debug-builtins")]
 pub(crate) mod native_debug;
+pub(crate) mod rec_group;
 pub(crate) mod resources;
 pub(crate) mod store;
 pub(crate) mod trampoline;
@@ -97,6 +98,7 @@ pub use limits::*;
 pub use linker::*;
 pub use memory::*;
 pub use module::{Module, ModuleExport, ModuleFunction};
+pub use rec_group::*;
 pub use resources::*;
 #[cfg(all(feature = "async", feature = "call-hook"))]
 pub use store::CallHookHandler;