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925: feat(runtime-core) Support closures with a captured environment as host functions r=Hywan a=Hywan Reboot of #882 and #219. For the moment, host functions (aka imported functions) can be regular function pointers, or (as a side-effect) closures without a captured environment. This PR extends the support of host functions to closures with a captured environment. This is required for many other features (incl. the Python integration, the Ruby integration, WebAssembly Interface Types [see #787], and so on). This PR is the culmination of previous ones, notably #915, #916 and #917. ### General idea The user-defined host function is wrapped inside a `wrap` function. This wrapper function initially receives a `vm::Ctx` as its first argument, which is passed to the host function when necessary. The patch keeps this behavior but it comes from `vm::FuncCtx`, which is a new structure. A `vm::FuncCtx` is held by `vm::ImportedFunc` such as: ```rust #[repr(C)] pub struct ImportedFunc { pub(crate) func: *const Func, pub(crate) func_ctx: NonNull<FuncCtx>, } ``` where `vm::FuncCtx` is: ```rust #[repr(C)] pub struct FuncCtx { pub(crate) vmctx: NonNull<Ctx>, pub(crate) func_env: Option<NonNull<FuncEnv>>, } ``` where `vm::FuncEnv` is: ```rust #[repr(transparent)] pub struct FuncEnv(pub(self) *mut c_void); ``` i.e. a raw opaque pointer. So the wrapper function of a host function receives a `vm::Ctx`, which is used to find out the associated `FuncCtx` (by using the import backing), which holds `vm::FuncEnv`. It holds a pointer to the closure captured environment. ### Implementation details #### How to get a pointer to a closure captured environment A closure with a captured environment has a memory size greater than zero. This is how we detect it: ```rust if mem::size_of::<Self>() != 0 { … } ``` To get a pointer to its captured environment, we use this statement: ```rust NonNull::new(Box::into_raw(Box::new(self))).map(NonNull::cast) ``` (in `typed_func.rs`, in the `wrap` functions). To reconstruct the closure based on the pointer, we use this statement: ```rust let func: &FN = { let func: NonNull<FN> = func_env.cast(); &*func.as_ptr() }; ``` That's basically how it works. And that's the core idea of this patch. As a side effect, we have removed an undefined behavior (UB) in 2 places: The `mem::transmute(&())` has been removed (it was used to get the function pointer of `FN`). The transmute is replaced by `FuncEnv`, which provides a unified API, erasing the difference between host functions as closures with a captured environment, and host functions as function pointer. For a reason I ignore, the UB wasn't showing himself until this PR and a modification in the Singlepass backend. But now it's fixed. #### Impact on `Backing` After the modification on the `typed_func` and the `vm` modules, this is the other core idea of this patch: Updating the `backing` module so that `vm::ImportedFunc` replaces `vm::Ctx` by `vm::FuncCtx`. When creating `vm::ImportedFunc`, a new `vm::FuncCtx` is created and its pointer is used. We are purposely leaking `vm::FuncCtx` so that the pointer is always valid. Hence the specific `Drop` implementation on `ImportBacking` to dereference the pointer, and to drop it properly. #### Impact on the backends Since the structure of `vm::ImportedFunc` has changed, backends must be updated. We must deref `FuncCtx` to reach its `vmctx` field. #### Impact on `Instance` Because `vm::ImportedFunc` has changed, it has a minor impact on the `instance` module, nothing crazy though. Co-authored-by: Ivan Enderlin <ivan.enderlin@hoa-project.net>
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