Add support for the wasm32-unknown-emscripten target

Build is working with emscripten 3.1.1, but requires setting some
environment variables:

    EMCC_CFLAGS="-s ERROR_ON_UNDEFINED_SYMBOLS=0" EMSDK=~/.asdf/installs/emsdk/3.1.1 cargo build --target wasm32-unknown-emscripten

ERROR_ON_UNDEFINED_SYMBOLS is a workaround for:

    warning: ___gxx_personality_v0 may need to be added to EXPORTED_FUNCTIONS if it arrives from a system library

This error appeared with emscripten 2.0.10 and has yet to be fixed in
rust: rust-lang/rust#85821 (comment)

EMSDK is for building skia-bindings against emscripten's platform
includes.

This is adapted from https://github.com/tuxmark5/rust-skia.

Author: flupke

README.md

@@ -9,7 +9,7 @@ Skia Submodule Status: chrome/m97 ([upstream changes][skia-upstream], [our chang
 
 ## Goals
 
-This project provides _up to date_ safe bindings that bridge idiomatic Rust with Skia's C++ API on desktop and mobile platforms, including GPU rendering backends for [Vulkan](https://en.wikipedia.org/wiki/Vulkan_(API)), [Metal](https://en.wikipedia.org/wiki/Metal_(API)), [OpenGL](https://en.wikipedia.org/wiki/OpenGL), and [Direct3D](https://en.wikipedia.org/wiki/Direct3D).
+This project provides _up to date_ safe bindings that bridge idiomatic Rust with Skia's C++ API on desktop and mobile platforms, including GPU rendering backends for [Vulkan](<https://en.wikipedia.org/wiki/Vulkan_(API)>), [Metal](<https://en.wikipedia.org/wiki/Metal_(API)>), [OpenGL](https://en.wikipedia.org/wiki/OpenGL), and [Direct3D](https://en.wikipedia.org/wiki/Direct3D).
 
 ## Status
 
@@ -40,31 +40,29 @@ For other platforms, more information is available at the [OpenSSL crate documen
 
 ### Platform Support, Build Targets, and Prebuilt Binaries
 
-Because building Skia takes a lot of time and needs tools that may be missing, the skia-bindings crate's `build.rs` tries to download prebuilt binaries from [the skia-binaries repository](<https://github.com/rust-skia/skia-binaries/releases>).
+Because building Skia takes a lot of time and needs tools that may be missing, the skia-bindings crate's `build.rs` tries to download prebuilt binaries from [the skia-binaries repository](https://github.com/rust-skia/skia-binaries/releases).
 
-| Platform | Binaries |
-| -------- | -------- |
-|  Windows | `x86_64-pc-windows-msvc` |
-| Linux Ubuntu 16+<br />CentOS 7, 8 | `x86_64-unknown-linux-gnu` |
-| macOS    | `x86_64-apple-darwin` |
-| Android  | `aarch64-linux-android`<br/>`x86_64-linux-android` |
-| iOS      | `aarch64-apple-ios`<br/>`x86_64-apple-ios` |
-
-There no support for WebAssembly yet. If you'd like to help out, take a look at issue [#39](https://github.com/rust-skia/rust-skia/issues/39).
+| Platform                          | Binaries                                           |
+| --------------------------------- | -------------------------------------------------- |
+| Windows                           | `x86_64-pc-windows-msvc`                           |
+| Linux Ubuntu 16+<br />CentOS 7, 8 | `x86_64-unknown-linux-gnu`                         |
+| macOS                             | `x86_64-apple-darwin`                              |
+| Android                           | `aarch64-linux-android`<br/>`x86_64-linux-android` |
+| iOS                               | `aarch64-apple-ios`<br/>`x86_64-apple-ios`         |
 
 ### Wrappers & Codecs & Supported Features
 
 The supported wrappers, Skia codecs, and additional Skia features are documented in the [skia-safe package's readme](skia-safe/README.md). Prebuilt binaries are available for most feature combinations.
 
 ## Building
 
-If the target platform or feature configuration is not available as a prebuilt binary, skia-bindings' `build.rs` will try to build Skia and generate the Rust bindings. 
+If the target platform or feature configuration is not available as a prebuilt binary, skia-bindings' `build.rs` will try to build Skia and generate the Rust bindings.
 
 To prepare for that, **LLVM** and **Python 2** are needed:
 
 **LLVM**
 
-We recommend the version that comes preinstalled with your platform, or, if not available, the [latest official LLVM release](http://releases.llvm.org/download.html). To see which version of LLVM/Clang is installed on your system, use `clang --version`. 
+We recommend the version that comes preinstalled with your platform, or, if not available, the [latest official LLVM release](http://releases.llvm.org/download.html). To see which version of LLVM/Clang is installed on your system, use `clang --version`.
 
 **Python 2**
 
@@ -88,9 +86,9 @@ The build script probes for `python --version` and `python2 --version` and uses
   sudo open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg
   ```
 
-  If not installed, the Skia build _may_ fail to build `SkJpegUtility.cpp` and the binding generation _will_ fail with  `'TargetConditionals.h' file not found` . Also note that the Command Line Tools _and_ SDK headers _should_ be reinstalled after an update of XCode.
+  If not installed, the Skia build _may_ fail to build `SkJpegUtility.cpp` and the binding generation _will_ fail with `'TargetConditionals.h' file not found` . Also note that the Command Line Tools _and_ SDK headers _should_ be reinstalled after an update of XCode.
 
-- As an alternative to Apple's XCode LLVM, install LLVM via `brew install llvm` or `brew install llvm` and then set `PATH`, `CPPFLAGS`, and `LDFLAGS` like instructed. 
+- As an alternative to Apple's XCode LLVM, install LLVM via `brew install llvm` or `brew install llvm` and then set `PATH`, `CPPFLAGS`, and `LDFLAGS` like instructed.
 
   If the environment variables are not set, [bindgen](https://github.com/rust-lang/rust-bindgen) will most likely use the wrong `libclang.dylib` and cause confusing compilation errors (see #228).
 
@@ -103,15 +101,15 @@ The build script probes for `python --version` and `python2 --version` and uses
 - Install the [latest LLVM](http://releases.llvm.org/download.html) distribution.
 
   If the environment variable `LLVM_HOME` is not defined, the build script will look for LLVM installations located at `C:\Program Files\LLVM\`, `C:\LLVM\`, and `%USERPROFILE%\scoop\apps\llvm\current\`.
-  
+
 - [MSYS2](https://www.msys2.org/):
-  
+
   - Install Python2 with `pacman -S python2`.
-  
+
 - Windows Shell (`Cmd.exe`):
-  
+
   - Download and install Python version 2 from [python.org](https://www.python.org/downloads/release/python-2716/).
-  
+
 - Install and select the MSVC toolchain:
   ```bash
   rustup default stable-msvc
@@ -202,6 +200,7 @@ export CARGO_TARGET_AARCH64_LINUX_ANDROID_LINKER=aarch64-linux-android26-clang.c
 
 cargo build -vv --target aarch64-linux-android
 ```
+
 _Notes:_
 
 - The `CARGO_TARGET_${TARGET}_LINKER` environment variable name [needs to be all uppercase](https://github.com/rust-lang/cargo/issues/1109#issuecomment-386850387).
@@ -212,6 +211,51 @@ _Notes:_
 
 Compilation to iOS is supported on macOS targeting the iOS simulator (`--target x86_64-apple-ios`) and 64 bit ARM devices (`--target aarch64-apple-ios`). The ARM64**e** architecture is [not supported yet](https://github.com/rust-lang/rust/issues/73628).
 
+### For WebAssembly
+
+Install `emscripten` version 3.1.1 or superior.
+
+Build with the `wasm32-unknown-emscripten` target (`wasm32-unknown-unknown` is
+unsupported because it is [fundamentally incompatible with linking C code](https://github.com/rustwasm/team/issues/291#issuecomment-645482430):
+
+```bash
+export EMSDK=~/.asdf/installs/emsdk/3.1.1
+export EMCC_CFLAGS="-s ERROR_ON_UNDEFINED_SYMBOLS=0"
+
+cargo build --target wasm32-unknown-emscripten
+```
+
+The `EMSDK` environment variable must be set to the root of your `emscripten` SDK.
+
+In `EMCC_CFLAGS`, `-s ERROR_ON_UNDEFINED_SYMBOLS` is a
+[workaround](https://github.com/rust-lang/rust/issues/85821#issuecomment-969369677) to build with
+`emscripten > 2.0.9`.
+
+If you want to enable WebGL, you will also have to set `MAX_WEBGL_VERSION=2`:
+
+```bash
+export EMSDK=~/.asdf/installs/emsdk/3.1.1
+export EMCC_CFLAGS="-s ERROR_ON_UNDEFINED_SYMBOLS=0 -s MAX_WEBGL_VERSION=2"
+
+cargo build --target wasm32-unknown-emscripten --features gl
+```
+
+On MacOS there is a problem with the OS version of `ar` so you will have to install the GNU version from homebrew:
+
+```bash
+brew install binutils
+```
+
+Then prepend `binutils` path for the build:
+
+```bash
+export EMSDK=~/.asdf/installs/emsdk/3.1.1
+export EMCC_CFLAGS="-s ERROR_ON_UNDEFINED_SYMBOLS=0"
+export PATH="/opt/homebrew/opt/binutils/bin:$PATH"
+
+cargo build --target wasm32-unknown-emscripten
+```
+
 ### Skia
 
 For situations in which Skia does not build or needs to be configured differently, we support some customization support in `skia-bindings/build.rs`. For more details take a look at the [README of the skia-bindings package](skia-bindings/README.md).
@@ -249,7 +293,8 @@ cargo run -- [OUTPUT_DIR] --driver opengl
 ```
 
 And to show the drivers that are supported
-```bash 
+
+```bash
 cargo run -- --help
 ```
 
@@ -305,5 +350,3 @@ More details can be found at [CONTRIBUTING.md](https://github.com/rust-skia/rust
 ## License
 
 MIT
-
-  

skia-bindings/build_support/binaries_config.rs

@@ -110,6 +110,11 @@ impl BinariesConfiguration {
             (_, "apple", "ios", abi) => {
                 link_libraries.extend(ios::link_libraries(abi, features));
             }
+            ("wasm32", "unknown", "emscripten", _) => {
+                if features.gl {
+                    link_libraries.extend(["GL"]);
+                }
+            }
             _ => panic!("unsupported target: {:?}", cargo::target()),
         };
 

skia-bindings/build_support/skia/config.rs

@@ -223,6 +223,15 @@ impl FinalBuildConfiguration {
                     args.push(("target_cpu", quote(clang::target_arch(arch))));
                     ios::extra_skia_cflags(arch, abi, &mut cflags);
                 }
+                ("wasm32", "unknown", "emscripten", _) => {
+                    args.push(("cc", quote("emcc")));
+                    args.push(("cxx", quote("em++")));
+                    args.push(("skia_gl_standard", quote("webgl")));
+                    args.push(("skia_use_freetype", yes()));
+                    args.push(("skia_use_system_freetype2", no()));
+                    args.push(("skia_use_webgl", yes_if(features.gpu())));
+                    args.push(("target_cpu", quote("wasm")));
+                }
                 (arch, _, os, _) => {
                     let skia_target_os = match os {
                         "darwin" => {

skia-bindings/build_support/skia_bindgen.rs

@@ -243,6 +243,32 @@ pub fn generate_bindings(build: &FinalBuildConfiguration, output_directory: &Pat
                 builder = builder.clang_arg(arg);
             }
         }
+        ("wasm32", "unknown", "emscripten", _) => {
+            // visibility=default, otherwise some types may be missing:
+            // https://github.com/rust-lang/rust-bindgen/issues/751#issuecomment-555735577
+            builder = builder.clang_arg("-fvisibility=default");
+
+            let emsdk_base_dir = match std::env::var("EMSDK") {
+                Ok(val) => val,
+                Err(_e) => panic!("please set the EMSDK environment variable to the root of your Emscripten installation"),
+            };
+
+            // Add C++ includes (otherwise build will fail with <cmath> not found)
+            let add_sys_include = |builder: bindgen::Builder, path: &str| -> bindgen::Builder {
+                let cflag = format!(
+                    "-isystem{}/upstream/emscripten/system/{}",
+                    emsdk_base_dir, path
+                );
+                builder.clang_arg(&cflag)
+            };
+
+            builder = builder.clang_arg("-nobuiltininc");
+            builder = add_sys_include(builder, "lib/libc/musl/arch/emscripten");
+            builder = add_sys_include(builder, "lib/libc/musl/arch/generic");
+            builder = add_sys_include(builder, "lib/libcxx/include");
+            builder = add_sys_include(builder, "lib/libc/musl/include");
+            builder = add_sys_include(builder, "include");
+        }
         _ => {}
     }