Rollup merge of rust-lang#155815 - djc:swift-cc, r=jieyouxu

Add Swift function call ABI

Adds an unstable `extern "Swift"` ABI behind the `abi_swift` feature gate, mapping to LLVM's `swiftcc` calling convention. This is only allowed (a) for `is_darwin_like` targets, since the [ABI is only stable for those platforms](https://www.swift.org/blog/abi-stability-and-more/) and (b) with the LLVM backend, since the other backends don't support it.

Current approaches to interoperability with Swift lower to Objective-C (or require a Swift stub exposing a C ABI), but that is an optional mapping on the Swift side that some newer Apple frameworks omit. It would be great to be able to more directly/natively be able to call into Swift code directly via its stable API (on Apple platforms at least).

Reimplements rust-lang#64582 on top of current main. The main objection to the previous PR seemed to be that it needed an RFC, but there was pushback (which seems sensible to me) that an RFC could be deferred until stabilization.

I think this needs a tracking issue? Would be happy to write one up if/when there is a consensus that this will be merged.

Author: JonathanBrouwer

compiler/rustc_abi/src/canon_abi.rs

@@ -32,6 +32,10 @@ pub enum CanonAbi {
     /// An ABI that rustc does not know how to call or define.
     Custom,
 
+    /// Swift calling convention, exposed via LLVM's `swiftcc`. Cross-platform
+    /// and not tied to a specific target architecture.
+    Swift,
+
     /// ABIs relevant to 32-bit Arm targets
     Arm(ArmCall),
     /// ABI relevant to GPUs: the entry point for a GPU kernel
@@ -58,6 +62,7 @@ impl CanonAbi {
             CanonAbi::Rust | CanonAbi::RustCold | CanonAbi::RustPreserveNone => true,
             CanonAbi::C
             | CanonAbi::Custom
+            | CanonAbi::Swift
             | CanonAbi::Arm(_)
             | CanonAbi::GpuKernel
             | CanonAbi::Interrupt(_)
@@ -77,6 +82,7 @@ impl fmt::Display for CanonAbi {
             CanonAbi::RustCold => ExternAbi::RustCold,
             CanonAbi::RustPreserveNone => ExternAbi::RustPreserveNone,
             CanonAbi::Custom => ExternAbi::Custom,
+            CanonAbi::Swift => ExternAbi::Swift,
             CanonAbi::Arm(arm_call) => match arm_call {
                 ArmCall::Aapcs => ExternAbi::Aapcs { unwind: false },
                 ArmCall::CCmseNonSecureCall => ExternAbi::CmseNonSecureCall,

compiler/rustc_abi/src/extern_abi.rs

@@ -62,6 +62,10 @@ pub enum ExternAbi {
     /// and only valid on platforms that have a UEFI standard
     EfiApi,
 
+    /// Swift's calling convention, used to interoperate with Swift code without
+    /// going through C as an intermediary.
+    Swift,
+
     /* arm */
     /// Arm Architecture Procedure Call Standard, sometimes `ExternAbi::C` is an alias for this
     Aapcs {
@@ -173,6 +177,7 @@ abi_impls! {
             C { unwind: false } =><= "C",
             C { unwind: true } =><= "C-unwind",
             Rust =><= "Rust",
+            Swift =><= "Swift",
             Aapcs { unwind: false } =><= "aapcs",
             Aapcs { unwind: true } =><= "aapcs-unwind",
             AvrInterrupt =><= "avr-interrupt",
@@ -348,7 +353,8 @@ impl ExternAbi {
             | Self::Vectorcall { .. }
             | Self::SysV64 { .. }
             | Self::Win64 { .. }
-            | Self::RustPreserveNone => true,
+            | Self::RustPreserveNone
+            | Self::Swift => true,
         }
     }
 }

compiler/rustc_ast_lowering/src/stability.rs

@@ -144,5 +144,8 @@ pub fn extern_abi_stability(abi: ExternAbi) -> Result<(), UnstableAbi> {
         ExternAbi::Custom => {
             Err(UnstableAbi { abi, feature: sym::abi_custom, explain: GateReason::Experimental })
         }
+        ExternAbi::Swift => {
+            Err(UnstableAbi { abi, feature: sym::abi_swift, explain: GateReason::Experimental })
+        }
     }
 }

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -424,6 +424,7 @@ impl<'a> AstValidator<'a> {
                     | CanonAbi::Rust
                     | CanonAbi::RustCold
                     | CanonAbi::RustPreserveNone
+                    | CanonAbi::Swift
                     | CanonAbi::Arm(_)
                     | CanonAbi::X86(_) => { /* nothing to check */ }
 

compiler/rustc_codegen_cranelift/src/abi/mod.rs

@@ -70,7 +70,7 @@ pub(crate) fn conv_to_call_conv(
             _ => default_call_conv,
         },
 
-        CanonAbi::Interrupt(_) | CanonAbi::Arm(_) => {
+        CanonAbi::Interrupt(_) | CanonAbi::Arm(_) | CanonAbi::Swift => {
             sess.dcx().fatal("call conv {c:?} is not yet implemented")
         }
         CanonAbi::GpuKernel => {

compiler/rustc_codegen_gcc/src/abi.rs

@@ -245,6 +245,8 @@ pub fn conv_to_fn_attribute<'gcc>(conv: CanonAbi, arch: &Arch) -> Option<FnAttri
         // possible to declare an `extern "custom"` block, so the backend still needs a calling
         // convention for declaring foreign functions.
         CanonAbi::Custom => return None,
+        // gcc/gccjit does not have anything for Swift's calling convention.
+        CanonAbi::Swift => panic!("gcc/gccjit backend does not support Swift calling convention"),
         CanonAbi::Arm(arm_call) => match arm_call {
             ArmCall::CCmseNonSecureCall => FnAttribute::ArmCmseNonsecureCall,
             ArmCall::CCmseNonSecureEntry => FnAttribute::ArmCmseNonsecureEntry,

compiler/rustc_codegen_llvm/src/abi.rs

@@ -722,6 +722,7 @@ pub(crate) fn to_llvm_calling_convention(sess: &Session, abi: CanonAbi) -> llvm:
         // possible to declare an `extern "custom"` block, so the backend still needs a calling
         // convention for declaring foreign functions.
         CanonAbi::Custom => llvm::CCallConv,
+        CanonAbi::Swift => llvm::SwiftCallConv,
         CanonAbi::GpuKernel => match &sess.target.arch {
             Arch::AmdGpu => llvm::AmdgpuKernel,
             Arch::Nvptx64 => llvm::PtxKernel,

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -166,6 +166,7 @@ pub(crate) enum CallConv {
     ColdCallConv = 9,
     PreserveMost = 14,
     PreserveAll = 15,
+    SwiftCallConv = 16,
     Tail = 18,
     PreserveNone = 21,
     X86StdcallCallConv = 64,

compiler/rustc_feature/src/unstable.rs

@@ -380,6 +380,8 @@ declare_features! (
     (unstable, abi_ptx, "1.15.0", Some(38788)),
     /// Allows `extern "riscv-interrupt-m" fn()` and `extern "riscv-interrupt-s" fn()`.
     (unstable, abi_riscv_interrupt, "1.73.0", Some(111889)),
+    /// Allows `extern "Swift" fn()`.
+    (unstable, abi_swift, "CURRENT_RUSTC_VERSION", Some(156481)),
     /// Allows `extern "x86-interrupt" fn()`.
     (unstable, abi_x86_interrupt, "1.17.0", Some(40180)),
     /// Allows additional const parameter types, such as `[u8; 10]` or user defined types

compiler/rustc_hir_typeck/src/callee.rs

@@ -202,6 +202,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             | CanonAbi::Rust
             | CanonAbi::RustCold
             | CanonAbi::RustPreserveNone
+            | CanonAbi::Swift
             | CanonAbi::Arm(_)
             | CanonAbi::X86(_) => {}
         }