win32: load DPI-related function pointers

These DPI-related functions will fail to link on older versions of
Windows, so we load them dynamically at runtime and implement a fallback
strategy if they're not available.

Cargo.toml

@@ -16,7 +16,9 @@ features = [
     "Win32_Foundation",
     "Win32_UI_WindowsAndMessaging",
     "Win32_UI_Input_KeyboardAndMouse",
+    "Win32_UI_HiDpi",
     "Win32_Graphics_Gdi",
+    "Win32_System_LibraryLoader",
     "Win32_System_SystemServices",
 ]
 

src/backend/win32/app.rs

@@ -12,13 +12,15 @@ use windows_sys::Win32::UI::WindowsAndMessaging::{
     RegisterClassW, TranslateMessage, UnregisterClassW, MSG, WNDCLASSW,
 };
 
+use super::dpi::DpiFns;
 use super::timer::{TimerHandleInner, Timers};
 use super::window::wnd_proc;
 use super::{class_name, hinstance, to_wstring, OsError};
 use crate::{App, AppContext, AppOptions, Error, IntoInnerError, Result};
 
 pub struct AppState {
     pub class: u16,
+    pub dpi: DpiFns,
     pub timers: Timers,
     pub data: RefCell<Option<Box<dyn Any>>>,
 }
@@ -68,10 +70,13 @@ impl<T: 'static> AppInner<T> {
             class
         };
 
+        let dpi = DpiFns::load();
+
         let timers = Timers::new()?;
 
         let state = Rc::new(AppState {
             class,
+            dpi,
             timers,
             data: RefCell::new(None),
         });

src/backend/win32/dpi.rs

@@ -0,0 +1,73 @@
+use std::mem;
+
+use windows_sys::core::HRESULT;
+use windows_sys::Win32::Foundation::{BOOL, HWND, RECT};
+use windows_sys::Win32::Graphics::Gdi::HMONITOR;
+use windows_sys::Win32::System::LibraryLoader::{GetProcAddress, LoadLibraryA};
+use windows_sys::Win32::UI::HiDpi::{
+    DPI_AWARENESS_CONTEXT, MONITOR_DPI_TYPE, PROCESS_DPI_AWARENESS,
+};
+use windows_sys::Win32::UI::WindowsAndMessaging::{WINDOW_EX_STYLE, WINDOW_STYLE};
+
+macro_rules! c_str {
+    ($str:literal) => {
+        concat!($str, "\0").as_ptr()
+    };
+}
+
+#[allow(non_snake_case)]
+pub struct DpiFns {
+    pub SetProcessDPIAware: Option<unsafe extern "system" fn() -> BOOL>,
+    pub SetProcessDpiAwareness:
+        Option<unsafe extern "system" fn(value: PROCESS_DPI_AWARENESS) -> HRESULT>,
+    pub SetProcessDpiAwarenessContext:
+        Option<unsafe extern "system" fn(value: DPI_AWARENESS_CONTEXT) -> BOOL>,
+    pub GetDpiForMonitor: Option<
+        unsafe extern "system" fn(
+            hmonitor: HMONITOR,
+            dpitype: MONITOR_DPI_TYPE,
+            dpix: *mut u32,
+            dpiy: *mut u32,
+        ) -> HRESULT,
+    >,
+    pub GetDpiForWindow: Option<unsafe extern "system" fn(hwnd: HWND) -> u32>,
+    pub EnableNonClientDpiScaling: Option<unsafe extern "system" fn(hwnd: HWND) -> BOOL>,
+    pub AdjustWindowRectExForDpi: Option<
+        unsafe extern "system" fn(
+            lprect: *mut RECT,
+            dwstyle: WINDOW_STYLE,
+            bmenu: BOOL,
+            dwexstyle: WINDOW_EX_STYLE,
+            dpi: u32,
+        ) -> BOOL,
+    >,
+}
+
+impl DpiFns {
+    pub fn load() -> DpiFns {
+        macro_rules! load {
+            ($lib:expr, $symbol:literal) => {
+                if $lib != 0 {
+                    mem::transmute(GetProcAddress($lib, c_str!($symbol)))
+                } else {
+                    None
+                }
+            };
+        }
+
+        unsafe {
+            let user32 = LoadLibraryA(c_str!("user32.dll"));
+            let shcore = LoadLibraryA(c_str!("shcore.dll"));
+
+            DpiFns {
+                SetProcessDPIAware: load!(user32, "SetProcessDPIAware"),
+                SetProcessDpiAwareness: load!(shcore, "SetProcessDpiAwareness"),
+                SetProcessDpiAwarenessContext: load!(user32, "SetProcessDpiAwarenessContext"),
+                GetDpiForMonitor: load!(shcore, "GetDpiForMonitor"),
+                GetDpiForWindow: load!(user32, "GetDpiForWindow"),
+                EnableNonClientDpiScaling: load!(user32, "EnableNonClientDpiScaling"),
+                AdjustWindowRectExForDpi: load!(user32, "AdjustWindowRectExForDpi"),
+            }
+        }
+    }
+}

src/backend/win32/mod.rs

@@ -6,6 +6,7 @@ use windows_sys::Win32::Foundation::{HMODULE, WIN32_ERROR};
 use windows_sys::Win32::System::SystemServices::IMAGE_DOS_HEADER;
 
 mod app;
+mod dpi;
 mod timer;
 mod window;