Add embind example to Bazel docs (#910)

* Add embind example to Bazel docs

* address feedback

bazel/README.md

@@ -7,33 +7,26 @@ In `WORKSPACE` file, put:
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")
 http_archive(
     name = "emsdk",
-    strip_prefix = "emsdk-c1589b55641787d55d53e883852035beea9aec3f/bazel",
-    url = "https://github.com/emscripten-core/emsdk/archive/c1589b55641787d55d53e883852035beea9aec3f.tar.gz",
-    sha256 = "7a58a9996b113d3e0675df30b5f17e28aa47de2e684a844f05394fe2f6f12e8e",
+    sha256 = "d55e3c73fc4f8d1fecb7aabe548de86bdb55080fe6b12ce593d63b8bade54567",
+    strip_prefix = "emsdk-3891e7b04bf8cbb3bc62758e9c575ae096a9a518/bazel",
+    url = "https://github.com/emscripten-core/emsdk/archive/3891e7b04bf8cbb3bc62758e9c575ae096a9a518.tar.gz",
 )
 
 load("@emsdk//:deps.bzl", emsdk_deps = "deps")
 emsdk_deps()
 
 load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
-emsdk_emscripten_deps()
+emsdk_emscripten_deps(emscripten_version = "2.0.31")
 ```
+The SHA1 hash in the above `strip_prefix` and `url` parameters correspond to the git revision of
+[emsdk 2.0.31](https://github.com/emscripten-core/emsdk/releases/tag/2.0.31). To get access to
+newer versions, you'll need to update those. To make use of older versions, change the
+parameter of `emsdk_emscripten_deps()`. Supported versions are listed in `revisions.bzl`
+
 
 ## Building
 
-### Using --config=wasm
-
-Put the following lines into your `.bazelrc`:
-```
-build:wasm --crosstool_top=@emsdk//emscripten_toolchain:everything
-build:wasm --cpu=wasm
-build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
-```
-
-Simply pass `--config=wasm` when building a normal `cc_binary`. The result of
-this build will be a tar archive containing any files produced by emscripten.
-
-### Using wasm_cc_binary
+### Using wasm_cc_binary (preferred)
 First, write a new rule wrapping your `cc_binary`.
 
 ```
@@ -59,3 +52,19 @@ rules.
 `wasm_cc_binary` uses transition to use emscripten toolchain on `cc_target`
 and all of its dependencies, and does not require amending `.bazelrc`. This
 is the preferred way, since it also unpacks the resulting tarball.
+
+See `test_external/` for an example using [embind](https://emscripten.org/docs/porting/connecting_cpp_and_javascript/embind.html).
+
+### Using --config=wasm
+
+Put the following lines into your `.bazelrc`:
+```
+build:wasm --crosstool_top=@emsdk//emscripten_toolchain:everything
+build:wasm --cpu=wasm
+build:wasm --host_crosstool_top=@bazel_tools//tools/cpp:toolchain
+```
+
+Simply pass `--config=wasm` when building a normal `cc_binary`. The result of
+this build will be a tar archive containing any files produced by emscripten.
+See the [Bazel documentation](https://docs.bazel.build/versions/main/tutorial/cc-toolchain-config.html)
+for more details

bazel/test_external/.gitignore

@@ -0,0 +1,4 @@
+bazel-bin
+bazel-out
+bazel-test_external
+bazel-testlogs

bazel/test_external/BUILD

@@ -10,3 +10,22 @@ wasm_cc_binary(
     cc_target = ":hello-world",
 )
 
+cc_binary(
+    name = "hello-embind",
+    srcs = ["hello-embind.cc"],
+    # It's probably a good idea to use select() and config_setting
+    # to add different linkopts to BASE_LINKOPTS depending on things
+    # like a "opt" or "dbg" compilation mode.
+    linkopts = [
+        "--bind",       # Enable embind
+        "-sMODULARIZE",
+    ],
+    # This target won't build successfully on its own because of missing emscripten
+    # headers etc. Therefore, we hide it from wildcards.
+    tags = ["manual"],
+)
+
+wasm_cc_binary(
+    name = "hello-embind-wasm",
+    cc_target = ":hello-embind",
+)

bazel/test_external/hello-embind.cc

@@ -0,0 +1,16 @@
+#include <emscripten/bind.h>
+
+using namespace emscripten;
+
+class HelloClass {
+    public:
+    static std::string SayHello(const std::string &name) {
+        return "Yo! " + name;
+    };
+};
+
+EMSCRIPTEN_BINDINGS(Hello) {
+    emscripten::class_<HelloClass>("HelloClass")
+        .constructor<>()
+        .class_function("SayHello", &HelloClass::SayHello);
+}

test/test_bazel.sh

@@ -26,3 +26,4 @@ bazel build //hello-world:hello-world-wasm-simd
 
 cd test_external
 bazel build //:hello-world-wasm
+bazel build //:hello-embind-wasm