Rollup merge of rust-lang#145660 - jbatez:darwin_objc, r=jdonszelmann,madsmtm,tmandry

initial implementation of the darwin_objc unstable feature

Tracking issue: rust-lang#145496

This feature makes it possible to reference Objective-C classes and selectors using the same ABI used by native Objective-C on Apple/Darwin platforms. Without it, Rust code interacting with Objective-C must resort to loading classes and selectors using costly string-based lookups at runtime. With it, these references can be loaded efficiently at dynamic load time.

r? ```@tmandry```

try-job: `*apple*`
try-job: `x86_64-gnu-nopt`

Author: Zalathar

core/src/lib.rs

@@ -313,6 +313,7 @@ pub mod io;
 pub mod iter;
 pub mod net;
 pub mod option;
+pub mod os;
 pub mod panic;
 pub mod panicking;
 #[unstable(feature = "pattern_type_macro", issue = "123646")]

core/src/os/darwin/mod.rs

@@ -0,0 +1,19 @@
+//! Platform-specific extensions to `core` for Darwin / Apple platforms.
+//!
+//! This is available on the following operating systems:
+//! - macOS
+//! - iOS
+//! - tvOS
+//! - watchOS
+//! - visionOS
+//!
+//! Note: This module is called "Darwin" as that's the name of the underlying
+//! core OS of the above operating systems, but it should not be confused with
+//! the `-darwin` suffix in the `x86_64-apple-darwin` and
+//! `aarch64-apple-darwin` target names, which are mostly named that way for
+//! legacy reasons.
+
+#![unstable(feature = "darwin_objc", issue = "145496")]
+#![doc(cfg(target_vendor = "apple"))]
+
+pub mod objc;

core/src/os/darwin/objc.rs

@@ -0,0 +1,113 @@
+//! Defines types and macros for Objective-C interoperability.
+
+#![unstable(feature = "darwin_objc", issue = "145496")]
+#![allow(nonstandard_style)]
+
+use crate::fmt;
+
+/// Equivalent to Objective-C’s `struct objc_class` type.
+#[cfg_attr(not(doc), repr(u8))] // An implementation detail we don't want to show up in rustdoc
+pub enum objc_class {
+    #[unstable(
+        feature = "objc_class_variant",
+        reason = "temporary implementation detail",
+        issue = "none"
+    )]
+    #[doc(hidden)]
+    __variant1,
+    #[unstable(
+        feature = "objc_class_variant",
+        reason = "temporary implementation detail",
+        issue = "none"
+    )]
+    #[doc(hidden)]
+    __variant2,
+}
+
+impl fmt::Debug for objc_class {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("objc_class").finish()
+    }
+}
+
+/// Equivalent to Objective-C’s `struct objc_selector` type.
+#[cfg_attr(not(doc), repr(u8))] // An implementation detail we don't want to show up in rustdoc
+pub enum objc_selector {
+    #[unstable(
+        feature = "objc_selector_variant",
+        reason = "temporary implementation detail",
+        issue = "none"
+    )]
+    #[doc(hidden)]
+    __variant1,
+    #[unstable(
+        feature = "objc_selector_variant",
+        reason = "temporary implementation detail",
+        issue = "none"
+    )]
+    #[doc(hidden)]
+    __variant2,
+}
+
+impl fmt::Debug for objc_selector {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("objc_selector").finish()
+    }
+}
+
+/// Equivalent to Objective-C’s `Class` type.
+pub type Class = *mut objc_class;
+
+/// Equivalent to Objective-C’s `SEL` type.
+pub type SEL = *mut objc_selector;
+
+/// Gets a reference to an Objective-C class.
+///
+/// This macro will yield an expression of type [`Class`] for the given class name string literal.
+///
+/// # Example
+///
+/// ```no_run
+/// #![feature(darwin_objc)]
+/// use core::os::darwin::objc;
+///
+/// let string_class = objc::class!("NSString");
+/// ```
+#[allow_internal_unstable(rustc_attrs)]
+pub macro class($classname:expr) {{
+    // Since static Objective-C class references actually end up with multiple definitions
+    // across dylib boundaries, we only expose the value of the static and don't provide a way to
+    // get the address of or a reference to the static.
+    unsafe extern "C" {
+        #[rustc_objc_class = $classname]
+        safe static VAL: $crate::os::darwin::objc::Class;
+    }
+    VAL
+}}
+
+/// Gets a reference to an Objective-C selector.
+///
+/// This macro will yield an expression of type [`SEL`] for the given method name string literal.
+///
+/// It is similar to Objective-C’s `@selector` directive.
+///
+/// # Examples
+///
+/// ```no_run
+/// #![feature(darwin_objc)]
+/// use core::os::darwin::objc;
+///
+/// let alloc_sel = objc::selector!("alloc");
+/// let init_sel = objc::selector!("initWithCString:encoding:");
+/// ```
+#[allow_internal_unstable(rustc_attrs)]
+pub macro selector($methname:expr) {{
+    // Since static Objective-C selector references actually end up with multiple definitions
+    // across dylib boundaries, we only expose the value of the static and don't provide a way to
+    // get the address of or a reference to the static.
+    unsafe extern "C" {
+        #[rustc_objc_selector = $methname]
+        safe static VAL: $crate::os::darwin::objc::SEL;
+    }
+    VAL
+}}

core/src/os/mod.rs

@@ -0,0 +1,24 @@
+//! OS-specific functionality.
+
+#![unstable(feature = "darwin_objc", issue = "145496")]
+
+#[cfg(all(
+    doc,
+    any(
+        all(target_arch = "wasm32", not(target_os = "wasi")),
+        all(target_vendor = "fortanix", target_env = "sgx")
+    )
+))]
+#[unstable(issue = "none", feature = "std_internals")]
+pub mod darwin {}
+
+// darwin
+#[cfg(not(all(
+    doc,
+    any(
+        all(target_arch = "wasm32", not(target_os = "wasi")),
+        all(target_vendor = "fortanix", target_env = "sgx")
+    )
+)))]
+#[cfg(any(target_vendor = "apple", doc))]
+pub mod darwin;

std/src/os/darwin/mod.rs

@@ -17,6 +17,8 @@
 #![doc(cfg(target_vendor = "apple"))]
 
 pub mod fs;
+pub mod objc;
+
 // deprecated, but used for public reexport under `std::os::unix::raw`, as
 // well as `std::os::macos`/`std::os::ios`, because those modules precede the
 // decision to remove these.

std/src/os/darwin/objc.rs

@@ -0,0 +1,13 @@
+//! Defines types and macros for Objective-C interoperability.
+//!
+//! This module re-exports all the items in [`core::os::darwin::objc`].
+//!
+//! [`core::os::darwin::objc`]: ../../../../core/os/darwin/objc/index.html "mod core::os::darwin::objc"
+
+#![unstable(feature = "darwin_objc", issue = "145496")]
+
+// We can't generate an intra-doc link for this automatically since `core::os::darwin` isn't
+// compiled into `core` on every platform even though it's documented on every platform.
+// We just link to it directly in the module documentation above instead.
+#[cfg(not(doc))]
+pub use core::os::darwin::objc::*;