start: Make wWinMain nCmdShow setting match Windows better

Author: squeek502

lib/std/start.zig

@@ -603,14 +603,29 @@ pub fn callMain() u8 {
 }
 
 pub fn call_wWinMain() std.os.windows.INT {
+    const peb = std.os.windows.peb();
     const MAIN_HINSTANCE = @typeInfo(@TypeOf(root.wWinMain)).Fn.params[0].type.?;
     const hInstance = @as(MAIN_HINSTANCE, @ptrCast(std.os.windows.kernel32.GetModuleHandleW(null).?));
     const lpCmdLine = std.os.windows.kernel32.GetCommandLineW();
 
-    // There's no (documented) way to get the nCmdShow parameter, so we're
-    // using this fairly standard default.
-    const nCmdShow = 5;
+    // This makes Zig mostly match the nCmdShow behavior of a C program with a WinMain symbol:
+    // - With STARTF_USESHOWWINDOW set in STARTUPINFO.dwFlags of the CreateProcess call:
+    //   - Compiled with subsystem:console -> nCmdShow is always SW_SHOWDEFAULT
+    //   - Compiled with subsystem:windows -> nCmdShow is STARTUPINFO.wShowWindow from
+    //     the parent CreateProcess call
+    // - With STARTF_USESHOWWINDOW unset:
+    //   - nCmdShow is always SW_SHOWDEFAULT
+    //
+    // TODO: Figure out how to check the subsystem and use it to match the above
+    //       behavior, or intentionally avoid matching the above behavior with
+    //       regards to the subsystem.
+    const SW_SHOWDEFAULT = 10;
+    const STARTF_USESHOWWINDOW = 1;
+    var nCmdShow: std.os.windows.ULONG = SW_SHOWDEFAULT;
+    if (peb.ProcessParameters.dwFlags & STARTF_USESHOWWINDOW != 0) {
+        nCmdShow = peb.ProcessParameters.dwShowWindow;
+    }
 
     // second parameter hPrevInstance, MSDN: "This parameter is always NULL"
-    return root.wWinMain(hInstance, null, lpCmdLine, nCmdShow);
+    return root.wWinMain(hInstance, null, lpCmdLine, @intCast(nCmdShow));
 }