Enable experimental wasm support

examples/dodge-the-creeps/godot/DodgeTheCreeps.gdextension

@@ -11,3 +11,5 @@ macos.debug = "res://../../../target/debug/libdodge_the_creeps.dylib"
 macos.release = "res://../../../target/release/libdodge_the_creeps.dylib"
 macos.debug.arm64 = "res://../../../target/debug/libdodge_the_creeps.dylib"
 macos.release.arm64 = "res://../../../target/release/libdodge_the_creeps.dylib"
+web.debug.wasm32 = "res://../../../target/wasm32-unknown-emscripten/debug/dodge_the_creeps.wasm"
+web.release.wasm32 = "res://../../../target/wasm32-unknown-emscripten/release/dodge_the_creeps.wasm"

examples/dodge-the-creeps/rust/.cargo/config

@@ -0,0 +1,9 @@
+# The cargo flag "-Zbuild-std" is also required but this cannot yet be specified for specific
+# targets: https://github.com/rust-lang/cargo/issues/8733
+[target.wasm32-unknown-emscripten]
+rustflags = [
+    "-C", "link-args=-sSIDE_MODULE=2",
+    "-C", "link-args=-sUSE_PTHREADS=1",
+    "-C", "target-feature=+atomics,+bulk-memory,+mutable-globals",
+    "-Zlink-native-libraries=no",
+]

examples/dodge-the-creeps/rust/Cargo.toml

@@ -9,5 +9,5 @@ publish = false
 crate-type = ["cdylib"]
 
 [dependencies]
-godot = { path = "../../../godot", default-features = false }
+godot = { path = "../../../godot", default-features = false, features = ["experimental-wasm"] }
 rand = "0.8"

examples/dodge-the-creeps/rust/build-wasm.sh

@@ -0,0 +1,8 @@
+#!/bin/sh
+
+# Must be in dodge-the-creep's rust directory in order to pick up the .cargo/config
+cd `dirname "$0"`
+
+# We build the host gdextension first so that the godot editor doesn't complain.
+cargo +nightly build --package dodge-the-creeps &&
+cargo +nightly build --package dodge-the-creeps --target wasm32-unknown-emscripten -Zbuild-std $@

godot-ffi/Cargo.toml

@@ -17,6 +17,9 @@ trace = []
 [dependencies]
 paste = "1"
 
+[target.'cfg(target_family = "wasm")'.dependencies]
+gensym = "0.1.1"
+
 [build-dependencies]
 godot-bindings = { path = "../godot-bindings" }
 godot-codegen = { path = "../godot-codegen" }

godot-ffi/src/compat/compat_4_1.rs

@@ -16,6 +16,7 @@ use crate::compat::BindingCompat;
 
 pub type InitCompat = sys::GDExtensionInterfaceGetProcAddress;
 
+#[cfg(not(target_family = "wasm"))]
 #[repr(C)]
 struct LegacyLayout {
     version_major: u32,
@@ -25,6 +26,13 @@ struct LegacyLayout {
 }
 
 impl BindingCompat for sys::GDExtensionInterfaceGetProcAddress {
+    // Fundamentally in wasm function references and data pointers live in different memory
+    // spaces so trying to read the "memory" at a function pointer (an index into a table) to
+    // heuristically determine which API we have (as is done below) is not quite going to work.
+    #[cfg(target_family = "wasm")]
+    fn ensure_static_runtime_compatibility(&self) {}
+
+    #[cfg(not(target_family = "wasm"))]
     fn ensure_static_runtime_compatibility(&self) {
         // In Godot 4.0.x, before the new GetProcAddress mechanism, the init function looked as follows.
         // In place of the `get_proc_address` function pointer, the `p_interface` data pointer was passed.

godot-ffi/src/lib.rs

@@ -47,6 +47,10 @@ use std::ffi::CStr;
 #[doc(hidden)]
 pub use paste;
 
+#[doc(hidden)]
+#[cfg(target_family = "wasm")]
+pub use gensym::gensym;
+
 pub use crate::godot_ffi::{
     from_sys_init_or_init_default, GodotFfi, GodotNullableFfi, PrimitiveConversionError,
     PtrcallType,

godot-ffi/src/plugins.rs

@@ -26,6 +26,28 @@ macro_rules! plugin_registry {
     };
 }
 
+#[doc(hidden)]
+#[macro_export]
+#[allow(clippy::deprecated_cfg_attr)]
+#[cfg_attr(rustfmt, rustfmt::skip)]
+// ^ skip: paste's [< >] syntax chokes fmt
+//   cfg_attr: workaround for https://github.com/rust-lang/rust/pull/52234#issuecomment-976702997
+macro_rules! plugin_add_inner_wasm {
+    ($gensym:ident,) => {
+        // Rust presently requires that statics with a custom `#[link_section]` must be a simple
+        // list of bytes on the wasm target (with no extra levels of indirection such as references).
+        //
+        // As such, instead we export a fn with a random name of predictable format to be used
+        // by the embedder.
+        $crate::paste::paste! {
+            #[no_mangle]
+            extern "C" fn [< rust_gdext_registrant_ $gensym >] () {
+                __init();
+            }
+        }
+    };
+}
+
 #[doc(hidden)]
 #[macro_export]
 #[allow(clippy::deprecated_cfg_attr)]
@@ -60,6 +82,9 @@ macro_rules! plugin_add_inner {
                 }
                 __inner_init
             };
+
+            #[cfg(target_family = "wasm")]
+            $crate::gensym! { $crate::plugin_add_inner_wasm!() }
         };
     };
 }

godot-macros/src/gdextension.rs

@@ -36,12 +36,57 @@ pub fn attribute_gdextension(decl: Declaration) -> ParseResult<TokenStream> {
     Ok(quote! {
         #impl_decl
 
+        // This cfg cannot be checked from the outer proc-macro since its 'target' is the build
+        // host. See: https://github.com/rust-lang/rust/issues/42587
+        #[cfg(target_os = "emscripten")]
+        fn emscripten_preregistration() {
+            // Module is documented here[1] by emscripten so perhaps we can consider it a part
+            // of its public API? In any case for now we mutate global state directly in order
+            // to get things working.
+            // [1] https://emscripten.org/docs/api_reference/module.html
+            //
+            // Warning: It may be possible that in the process of executing the code leading up
+            // to `emscripten_run_script` that we might trigger usage of one of the symbols we
+            // wish to monkey patch? It seems fairly unlikely, especially as long as no i64 are
+            // involved, but I don't know what guarantees we have here.
+            //
+            // We should keep an eye out for these sorts of failures!
+            let script = std::ffi::CString::new(concat!(
+                "var pkgName = '", env!("CARGO_PKG_NAME"), "';", r#"
+                var libName = pkgName.replaceAll('-', '_') + '.wasm';
+                var dso = LDSO.loadedLibsByName[libName]["module"];
+                var registrants = [];
+                for (sym in dso) {
+                    if (sym.startsWith("dynCall_")) {
+                        if (!(sym in Module)) {
+                            console.log(`Patching Module with ${sym}`);
+                            Module[sym] = dso[sym];
+                        }
+                    } else if (sym.startsWith("rust_gdext_registrant_")) {
+                        registrants.push(sym);
+                    }
+                }
+                for (sym of registrants) {
+                    console.log(`Running registrant ${sym}`);
+                    dso[sym]();
+                }
+                console.log("Added",  registrants.length, "plugins to registry!");
+            "#)).expect("Unable to create CString from script");
+
+            extern "C" { fn emscripten_run_script(script: *const std::ffi::c_char); }
+            unsafe { emscripten_run_script(script.as_ptr()); }
+        }
+
         #[no_mangle]
         unsafe extern "C" fn #entry_point(
             interface_or_get_proc_address: ::godot::sys::InitCompat,
             library: ::godot::sys::GDExtensionClassLibraryPtr,
             init: *mut ::godot::sys::GDExtensionInitialization,
         ) -> ::godot::sys::GDExtensionBool {
+            // Required due to the lack of a constructor facility such as .init_array in rust wasm
+            #[cfg(target_os = "emscripten")]
+            emscripten_preregistration();
+
             ::godot::init::__gdext_load_library::<#impl_ty>(
                 interface_or_get_proc_address,
                 library,

godot/Cargo.toml

@@ -16,6 +16,7 @@ serde = ["godot-core/serde"]
 lazy-function-tables = ["godot-core/codegen-lazy-fptrs"]
 experimental-threads = ["godot-core/experimental-threads"]
 experimental-godot-api = ["godot-core/experimental-godot-api"]
+experimental-wasm = []
 
 # Private features, they are under no stability guarantee
 codegen-full = ["godot-core/codegen-full"]

godot/src/lib.rs

@@ -146,6 +146,11 @@
 //!   Access to `godot::engine` APIs that Godot marks "experimental". These are under heavy development and may change at any time.
 //!   If you opt in to this feature, expect breaking changes at compile and runtime.
 //!
+//! * **`experimental-wasm`**
+//!
+//!   Support for WebAssembly exports is still a work-in-progress and is not yet well tested. This feature is in place for users
+//!   to explicitly opt-in to any instabilities or rough edges that may result.
+//!
 //! * **`lazy-function-tables`**
 //!
 //!   Instead of loading all engine function pointers at startup, load them lazily on first use. This reduces startup time and RAM usage, but
@@ -178,6 +183,9 @@ pub use godot_core::sys;
 #[cfg(all(feature = "lazy-function-tables", feature = "experimental-threads"))]
 compile_error!("Thread safety for lazy function pointers is not yet implemented.");
 
+#[cfg(all(target_family = "wasm", not(feature = "experimental-wasm")))]
+compile_error!("Must opt-in using `experimental-wasm` Cargo feature; keep in mind that this is work in progress");
+
 pub mod init {
     pub use godot_core::init::*;