Enable emscripten builds to compile slang.dll to WebAssembly. (#5131)

* Compile fixes for Wasm

The issues are all are due to 'long' types being 32 bits on WASM.

- class members redeclared errors
- << with StringBuilder and unsigned long is ambiguous

This helps to address issue #5115.

* Use the host executable suffix for generators

Since the generators are run at build-time, we should not use CMAKE_EXECUTABLE_SUFFIX,
which is the suffix for the target platform.
Instead, define CMAKE_HOST_EXECUTABLE_SUFFIX as appropriate, and use that suffix instead.

This helps to address issue #5115.

* Add support for Wasm as a platform

This helps to address issue #5115.

* Add emscripten build

This closes #5115.

1 files changed, 14 insertions, 0 deletions

diff --git a/docs/building.md b/docs/building.md
index d86b80205..8bc20acb3 100644
--- a/docs/building.md
+++ b/docs/building.md
@@ -47,6 +47,20 @@ cmake --build --preset release # to build from the CLI
 
 The `vs2022-dev` preset turns on features that makes debugging easy.
 
+### WebAssembly build
+
+First do a native build of the `all-generators` target, so that you get the generator executables in a path like `build\generators\Debug\bin` under the
+Slang source tree.
+
+For an [emscripten](https://emscripten.org) -based build targeting [WebAssembly](https://webassembly.org), run:
+```bash
+emcmake cmake -DSLANG_GENERATORS_PATH=%SLANG_SRC_PATH%\build\generators\Debug\bin --preset emscripten -G "Ninja"
+cmake --build --preset emscripten --target slang
+```
+
+**Note:** `emcmake` is a `cmake` wrapper that comes with [`emsdk`](https://emscripten.org/docs/getting_started/downloads.html), it will wrap your `cmake` calls with setup options that add support for emscripten.
+**Note:** If this fails, try running the command that `emcmake` outputs, directly.
+
 ## Testing
 
 ```bash