C++: add a "std" feature to the slint-cpp crate

And make it no_std otherwise

This is a first step towards getting C++ on MCU (#2136)

api/cpp/CMakeLists.txt

@@ -137,6 +137,7 @@ set_property(
     TARGET slint-cpp
     PROPERTY CORROSION_FEATURES
     ${features}
+    std
 )
 set_property(
     TARGET slint-cpp

api/cpp/Cargo.toml

@@ -38,19 +38,21 @@ renderer-winit-software = ["i-slint-backend-selector/renderer-winit-software"]
 gettext = ["i-slint-core/gettext-rs"]
 accessibility = ["i-slint-backend-selector/accessibility"]
 
-experimental = ["i-slint-renderer-skia", "raw-window-handle"]
+experimental = ["i-slint-renderer-skia", "raw-window-handle", "std"]
 
-default = ["backend-winit", "renderer-winit-femtovg", "backend-qt", "experimental"]
+std = ["image", "i-slint-core/default", "i-slint-backend-selector"]
+
+default = ["std", "backend-winit", "renderer-winit-femtovg", "backend-qt", "experimental"]
 
 [dependencies]
-i-slint-backend-selector = { version = "=1.1.1", path="../../internal/backends/selector" }
+i-slint-backend-selector = { version = "=1.1.1", path="../../internal/backends/selector", optional = true }
 i-slint-backend-testing = { version = "=1.1.1", path="../../internal/backends/testing", optional = true }
 i-slint-renderer-skia = { version = "=1.1.1", path="../../internal/renderers/skia", optional = true, features = ["x11", "wayland"] }
-i-slint-core = { version = "=1.1.1", path="../../internal/core", features = ["ffi"] }
+i-slint-core = { version = "=1.1.1", path="../../internal/core", default-features = false, features = ["ffi"] }
 slint-interpreter = { version = "=1.1.1", path="../../internal/interpreter", default-features = false, features = ["ffi", "compat-1-0"], optional = true }
 raw-window-handle = { version = "0.5", optional = true }
 # Enable image-rs' default features to make all image formats to C++ users
-image = { version = "0.24.0" }
+image = { version = "0.24.0", optional = true }
 
 [build-dependencies]
 anyhow = "1.0"

api/cpp/lib.rs

@@ -3,13 +3,28 @@
 
 /*! This crate just expose the function used by the C++ integration */
 
+#![cfg_attr(not(feature = "std"), no_std)]
+extern crate alloc;
+
+use alloc::rc::Rc;
 use core::ffi::c_void;
 use i_slint_core::window::{ffi::WindowAdapterRcOpaque, WindowAdapter};
-use std::rc::Rc;
 
 #[cfg(feature = "experimental")]
 pub mod platform;
 
+#[cfg(feature = "i-slint-backend-selector")]
+use i_slint_backend_selector::with_platform;
+
+#[cfg(not(feature = "i-slint-backend-selector"))]
+pub fn with_platform<R>(
+    f: impl FnOnce(
+        &dyn i_slint_core::platform::Platform,
+    ) -> Result<R, i_slint_core::platform::PlatformError>,
+) -> Result<R, i_slint_core::platform::PlatformError> {
+    i_slint_core::with_platform(|| Err(i_slint_core::platform::PlatformError::NoPlatform), f)
+}
+
 /// One need to make sure something from the crate is exported,
 /// otherwise its symbols are not going to be in the final binary
 #[cfg(feature = "slint-interpreter")]
@@ -21,13 +36,13 @@ pub unsafe extern "C" fn slint_windowrc_init(out: *mut WindowAdapterRcOpaque) {
         core::mem::size_of::<Rc<dyn WindowAdapter>>(),
         core::mem::size_of::<WindowAdapterRcOpaque>()
     );
-    let win = i_slint_backend_selector::with_platform(|b| b.create_window_adapter()).unwrap();
+    let win = with_platform(|b| b.create_window_adapter()).unwrap();
     core::ptr::write(out as *mut Rc<dyn WindowAdapter>, win);
 }
 
 #[no_mangle]
 pub unsafe extern "C" fn slint_ensure_backend() {
-    i_slint_backend_selector::with_platform(|_b| {
+    with_platform(|_b| {
         // Nothing to do, just make sure a backend was created
         Ok(())
     })
@@ -36,7 +51,7 @@ pub unsafe extern "C" fn slint_ensure_backend() {
 
 #[no_mangle]
 pub unsafe extern "C" fn slint_run_event_loop() {
-    i_slint_backend_selector::with_platform(|b| b.run_event_loop()).unwrap();
+    with_platform(|b| b.run_event_loop()).unwrap();
 }
 
 /// Will execute the given functor in the main thread
@@ -72,6 +87,7 @@ pub unsafe extern "C" fn slint_quit_event_loop() {
     i_slint_core::api::quit_event_loop().unwrap();
 }
 
+#[cfg(feature = "std")]
 #[no_mangle]
 pub unsafe extern "C" fn slint_register_font_from_path(
     win: *const WindowAdapterRcOpaque,
@@ -89,6 +105,7 @@ pub unsafe extern "C" fn slint_register_font_from_path(
     )
 }
 
+#[cfg(feature = "std")]
 #[no_mangle]
 pub unsafe extern "C" fn slint_register_font_from_data(
     win: *const WindowAdapterRcOpaque,

api/cpp/platform.rs

@@ -1,6 +1,8 @@
 // Copyright © SixtyFPS GmbH <info@slint.dev>
 // SPDX-License-Identifier: GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.0 OR LicenseRef-Slint-commercial
 
+use alloc::boxed::Box;
+use alloc::rc::Rc;
 use core::ffi::c_void;
 use i_slint_core::api::{PhysicalSize, Window};
 use i_slint_core::graphics::{IntSize, Rgb8Pixel};
@@ -10,7 +12,6 @@ use i_slint_core::software_renderer::{RepaintBufferType, SoftwareRenderer};
 use i_slint_core::window::ffi::WindowAdapterRcOpaque;
 use i_slint_core::window::{WindowAdapter, WindowAdapterInternal};
 use raw_window_handle::{RawDisplayHandle, RawWindowHandle};
-use std::rc::Rc;
 
 type WindowAdapterUserData = *mut c_void;
 

internal/core/graphics.rs

@@ -194,5 +194,6 @@ pub(crate) mod ffi {
         y: f32,
     }
 
+    #[cfg(feature = "std")]
     pub use super::path::ffi::*;
 }

internal/core/graphics/image.rs

@@ -775,21 +775,23 @@ pub(crate) mod ffi {
         a: u8,
     }
 
+    #[cfg(feature = "image-decoders")]
     #[no_mangle]
     pub unsafe extern "C" fn slint_image_load_from_path(path: &SharedString, image: *mut Image) {
-        std::ptr::write(
+        core::ptr::write(
             image,
             Image::load_from_path(std::path::Path::new(path.as_str())).unwrap_or(Image::default()),
         )
     }
 
+    #[cfg(feature = "std")]
     #[no_mangle]
     pub unsafe extern "C" fn slint_image_load_from_embedded_data(
         data: Slice<'static, u8>,
         format: Slice<'static, u8>,
         image: *mut Image,
     ) {
-        std::ptr::write(image, super::load_image_from_embedded_data(data, format));
+        core::ptr::write(image, super::load_image_from_embedded_data(data, format));
     }
 
     #[no_mangle]

tests/driver/cpp/Cargo.toml

@@ -14,7 +14,7 @@ path = "main.rs"
 name = "test-driver-cpp"
 
 [dependencies]
-slint-cpp = { path = "../../../api/cpp", default-features = false, features = ["testing"] }
+slint-cpp = { path = "../../../api/cpp", default-features = false, features = ["testing", "std"] }
 
 [dev-dependencies]
 i-slint-compiler = { path = "../../../internal/compiler", features = ["cpp", "display-diagnostics"] }