Merge pull request #1892 from tylerjenningsw/fix/windows-gnu-import-libs

Windows: document the GNU toolchain's import-library requirements (#1886)

CONTRIBUTING.md

@@ -37,6 +37,45 @@ debugger session use the `dev-full` profile, which is the same build with full d
 cargo run --profile dev-full -p effectcraft -- --demo
 ```
 
+### Windows
+
+The MSVC toolchain, rustup's default on Windows, is what the releases use: with the Visual Studio
+C++ build tools installed, the commands above work as they are.
+
+The GNU toolchain (`x86_64-pc-windows-gnu`) needs a current mingw-w64 GCC and binutils first on
+`PATH`. They provide the linker and the Windows import libraries, such as `libshlwapi.a` for
+opening links from the app. With MSYS2:
+
+```powershell
+C:\msys64\usr\bin\pacman.exe -S --needed mingw-w64-x86_64-gcc
+$env:PATH = "C:\msys64\mingw64\bin;$env:PATH"
+cargo +stable-x86_64-pc-windows-gnu run -p effectcraft -- --demo
+```
+
+An older MinGW earlier on `PATH` can break the build: CodeBlocks' GCC 8.1 fails to link
+`boa_engine` ([#1882](https://github.com/storytold/effectcraft/issues/1882)).
+
+Rust's bundled LLD and self-contained libraries don't include every import library EffectCraft
+needs, so on their own they stop with `unable to find library` for `-lshlwapi`, `-lktmw32`,
+`-lpropsys` and `-lversion` ([#1886](https://github.com/storytold/effectcraft/issues/1886)). To
+link that way, copy those four from mingw-w64 into a folder of its own and add that folder to the
+search path in your own Cargo config (`%USERPROFILE%\.cargo\config.toml`). Don't add all of
+mingw-w64's `lib` folder: its C runtime clashes with Rust's bundled one
+(`undefined symbol: DllEntryPoint`).
+
+```powershell
+New-Item -ItemType Directory -Force C:\gnu-import-libs
+foreach ($lib in "shlwapi", "ktmw32", "propsys", "version") {
+    Copy-Item "C:\msys64\mingw64\lib\lib$lib.a" C:\gnu-import-libs\
+}
+```
+
+```toml
+[target.x86_64-pc-windows-gnu]
+linker = "rust-lld"
+rustflags = ["-C", "link-self-contained=yes", "-L", "native=C:/gnu-import-libs"]
+```
+
 ## Ground rules
 
 - **Clean room.** Work from After Effects' public documentation and its observable behaviour only.