feat(c-api) No longer expand with cbindgen: drop dependency to Rust nightly.

So. Let's explain a dirty hack. `cbindgen` reads the code and collects
symbols. What symbols do we need? None of the one declared in
`wasm.h`, but for non-standard API, we need to collect all of
them. The problem is that `wasm_named_extern_t` is the only
non-standard type where extra symbols are generated by a macro
(`wasm_declare_boxed_vec!`). If we want those macro-generated symbols
to be collected by `cbindgen`, we need to _expand_ the crate
(i.e. running something like `rustc -- -Zunstable-options
--pretty=expanded`). Expanding code is unstable and available only on
nightly compiler. We _don't want_ to use a nightly compiler only for
that. So how can we help `cbindgen` to _see_ those symbols?

First solution: We write the C code directly in a file, which is then
included in the generated header file with the `cbindgen`
API. Problem, it's super easy to get it outdated, and it makes the
build process more complex.

Second solution: We write those symbols in a custom module, that is
just here for `cbindgen`, never used by our Rust code (otherwise it's
duplicated code), with no particular implementation.

And that's why we have the following `cbindgen_hack` module.

But this module must not be compiled by `rustc`. How to force `rustc`
to ignore a module? With conditional compilation. Because `cbindgen`
does not support conditional compilation, it will always _ignore_ the
`#[cfg]` attribute, and will always read the content of the module.

Sorry.

lib/c-api/src/wasm_c_api/macros.rs

@@ -403,7 +403,7 @@ int main() {
             #[no_mangle]
             pub unsafe extern "C" fn [<wasm_ $name _vec_copy>](
                 out_ptr: &mut [<wasm_ $name _vec_t>],
-                in_ptr: & [<wasm _$name _vec_t>])
+                in_ptr: & [<wasm_ $name _vec_t>])
             {
                 *out_ptr = in_ptr.clone();
             }

lib/c-api/src/wasm_c_api/wasi/mod.rs

@@ -361,6 +361,88 @@ pub struct wasm_named_extern_t {
 
 wasm_declare_boxed_vec!(named_extern);
 
+/// So. Let's explain a dirty hack. `cbindgen` reads the code and
+/// collects symbols. What symbols do we need? None of the one
+/// declared in `wasm.h`, but for non-standard API, we need to collect
+/// all of them. The problem is that `wasm_named_extern_t` is the only
+/// non-standard type where extra symbols are generated by a macro
+/// (`wasm_declare_boxed_vec!`). If we want those macro-generated
+/// symbols to be collected by `cbindgen`, we need to _expand_ the
+/// crate (i.e. running something like `rustc -- -Zunstable-options
+/// --pretty=expanded`). Expanding code is unstable and available only
+/// on nightly compiler. We _don't want_ to use a nightly compiler
+/// only for that. So how can we help `cbindgen` to _see_ those
+/// symbols?
+///
+/// First solution: We write the C code directly in a file, which is
+/// then included in the generated header file with the `cbindgen`
+/// API. Problem, it's super easy to get it outdated, and it makes the
+/// build process more complex.
+///
+/// Second solution: We write those symbols in a custom module, that
+/// is just here for `cbindgen`, never used by our Rust code
+/// (otherwise it's duplicated code), with no particular
+/// implementation.
+///
+/// And that's why we have the following `cbindgen_hack`
+/// module.
+///
+/// But this module must not be compiled by `rustc`. How to force
+/// `rustc` to ignore a module? With conditional compilation. Because
+/// `cbindgen` does not support conditional compilation, it will
+/// always _ignore_ the `#[cfg]` attribute, and will always read the
+/// content of the module.
+///
+/// Sorry.
+#[doc(hidden)]
+#[cfg(__cbindgen_hack__ = "yes")]
+mod __cbindgen_hack__ {
+    use super::*;
+
+    #[repr(C)]
+    pub struct wasm_named_extern_vec_t {
+        pub size: usize,
+        pub data: *mut *mut wasm_named_extern_t,
+    }
+
+    #[no_mangle]
+    pub unsafe extern "C" fn wasm_named_extern_vec_new(
+        out: *mut wasm_named_extern_vec_t,
+        length: usize,
+        init: *const *mut wasm_named_extern_t,
+    ) {
+        unimplemented!()
+    }
+
+    #[no_mangle]
+    pub unsafe extern "C" fn wasm_named_extern_vec_new_uninitialized(
+        out: *mut wasm_named_extern_vec_t,
+        length: usize,
+    ) {
+        unimplemented!()
+    }
+
+    #[no_mangle]
+    pub unsafe extern "C" fn wasm_named_extern_vec_copy(
+        out_ptr: &mut wasm_named_extern_vec_t,
+        in_ptr: &wasm_named_extern_vec_t,
+    ) {
+        unimplemented!()
+    }
+
+    #[no_mangle]
+    pub unsafe extern "C" fn wasm_named_extern_vec_delete(
+        ptr: Option<&mut wasm_named_extern_vec_t>,
+    ) {
+        unimplemented!()
+    }
+
+    #[no_mangle]
+    pub unsafe extern "C" fn wasm_named_extern_vec_new_empty(out: *mut wasm_named_extern_vec_t) {
+        unimplemented!()
+    }
+}
+
 /// Non-standard function to get the module name of a
 /// `wasm_named_extern_t`.
 ///