Add HTML5 export guide

Author: derivator

src/SUMMARY.md

@@ -39,6 +39,7 @@
     - [Android](./exporting/android.md)
     - [(TODO) iOS](./exporting/ios.md)
     - [Mac OS X](./exporting/macosx.md)
+    - [HTML5](./exporting/html5.md)
 - [Advanced Guides](./advanced-guides.md)
     - [Using custom Godot versions](./advanced-guides/custom-godot.md)
     - [Migrating from godot-rust 0.8](advanced-guides/migrating-0-8.md)

src/exporting/html5.md

@@ -0,0 +1,92 @@
+# HTML5
+
+Exporting to HTML5 works just like exporting to other platforms, however there are some things that can make the process a bit tricky and require extra attention.
+
+## General considerations
+
+The Godot HTML5 export templates are built with [Emscripten](https://emscripten.org/), so you need to build your code for the `wasm32-unknown-emscripten` target, `wasm32-unknown-unknown` will not work. Furthermore, you need to use a version of emscripten that is compatible with both your version of Rust and the version of emscripten used to build the export template.  
+In practice, this means you might have to [build the HTML5 export template yourself](https://docs.godotengine.org/en/stable/development/compiling/compiling_for_web.html) and/or use a specific version of Rust. You can check which versions you are using like this:
+
+```bash
+emcc -v
+rustc --version --verbose
+```
+
+Also note that `wasm32-unknown-emscripten` is a 32-bit target, which can cause problems with crates incorrectly assuming that `usize` is 64 bits wide.
+
+## Godot 3.4.4
+
+**Disclaimer**: _Currently, the following steps are only tested and confirmed to work on Linux._
+
+Godot 3.4.4's export template is built with a very old Emscripten version that doesn't play nice with Rust. We will have to build it ourselves with a more recent version.  
+We also need to build the `std` crate ourselves, so we will have to use a nightly Rust build.
+
+Confirmed working versions are:
+* rustc 1.63.0-nightly (dc80ca78b 2022-06-21)
+* Emscripten 3.1.15-git (4100960b42619b28f19baf4254d5db2097234b32)
+
+
+### Getting Emscripten
+
+We will be using Emscripten tip-of-tree:
+
+```bash
+# Get the emsdk repo
+git clone https://github.com/emscripten-core/emsdk.git
+
+# Enter that directory
+cd emsdk
+
+# Download and install the tip-of-tree SDK tools.
+./emsdk install tot
+
+# Make the "tot" SDK "active" for the current user. (writes .emscripten file)
+./emsdk activate tot
+
+# Activate PATH and other environment variables in the current terminal
+source ./emsdk_env.sh
+```
+
+### Building Godot
+
+Build the Godot Export template according to the [instructions](https://docs.godotengine.org/en/stable/development/compiling/compiling_for_web.html):
+
+```bash
+source "/path/to/emsdk-portable/emsdk_env.sh"
+scons platform=javascript tools=no gdnative_enabled=yes target=release
+mv bin/godot.javascript.opt.gdnative.zip bin/webassembly_gdnative_release.zip
+```
+
+Set the newly built export template as a custom template in Godot and be sure to set the export type as GDNative. When exporting, uncheck "Export With Debug".
+
+### Building your Rust code
+
+In your project's `config.toml`, add the following:
+
+```toml
+[target.wasm32-unknown-emscripten]
+rustflags = [
+	"-Clink-arg=-sSIDE_MODULE=2", # build a side module that Godot can load
+	"-Crelocation-model=pic", # needed to prevent linker errors
+	"-Cpanic=abort", # panic unwinding is currently broken without -sWASM_BIGINT, see below
+	#"-Clink-arg=-sWASM_BIGINT", # alternative to panic=abort, requires building godot with -sWASM_BIGINT also
+]
+```
+
+Build like this:
+
+```bash
+source "/path/to/emsdk-portable/emsdk_env.sh"
+export C_INCLUDE_PATH=$EMSDK/upstream/emscripten/cache/sysroot/include
+	
+cargo +nightly build --target=wasm32-unknown-emscripten --release -Zbuild-std=core,std,alloc,panic_abort
+```
+
+### Further reading
+
+In the future, some/all of will probably not be needed, for more info see:
+* [Tracking issue for godot-rust wasm support](https://github.com/godot-rust/godot-rust/issues/647)
+* [Rust PR that obsoletes -Clink-arg=-sSIDE_MODULE=2](https://github.com/rust-lang/rust/pull/98358)
+* [Rust PR that obsoletes -Crelocation-model=pic and -Zbuild-std](https://github.com/rust-lang/rust/pull/98149)
+* [Godot PR that updates Emscripten version](https://github.com/godotengine/godot/pull/61989)
+* [Godot PR that would obsolete -Cpanic=abort in favor of -sWASM_BIGINT](https://github.com/godotengine/godot/pull/62397)