Add Swift function call ABI

Adds an unstable `extern "Swift"` ABI behind the `abi_swift` feature
gate, mapping to LLVM's `swiftcc` calling convention. Cranelift and
GCC backends fall back to the platform default since they have no
equivalent.

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 @@ pub fn is_rustic_abi(self) -> bool {
             CanonAbi::Rust | CanonAbi::RustCold | CanonAbi::RustPreserveNone => true,
             CanonAbi::C
             | CanonAbi::Custom
+            | CanonAbi::Swift
             | CanonAbi::Arm(_)
             | CanonAbi::GpuKernel
             | CanonAbi::Interrupt(_)
@@ -77,6 +82,7 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
             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 @@ fn from_str(s: &str) -> Result<$e_name, Self::Err> {
             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 @@ pub fn supports_guaranteed_tail_call(self) -> bool {
             | 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 @@ fn check_extern_fn_signature(&self, abi: ExternAbi, ctxt: FnCtxt, ident: &Ident,
                     | 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 @@ pub fn internal(&self, feature: Symbol) -> bool {
     (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 @@ pub(crate) fn check_call_abi(&self, abi: ExternAbi, span: Span) {
             | CanonAbi::Rust
             | CanonAbi::RustCold
             | CanonAbi::RustPreserveNone
+            | CanonAbi::Swift
             | CanonAbi::Arm(_)
             | CanonAbi::X86(_) => {}
         }

compiler/rustc_middle/src/ty/layout.rs

@@ -1291,6 +1291,7 @@ pub fn fn_can_unwind(tcx: TyCtxt<'_>, fn_def_id: Option<DefId>, abi: ExternAbi)
         | RiscvInterruptM
         | RiscvInterruptS
         | RustInvalid
+        | Swift
         | Unadjusted => false,
         Rust | RustCall | RustCold | RustPreserveNone => tcx.sess.panic_strategy().unwinds(),
     }

compiler/rustc_public/src/abi.rs

@@ -458,6 +458,8 @@ pub enum CallConvention {
 
     Custom,
 
+    Swift,
+
     // Target-specific calling conventions.
     ArmAapcs,
     CCmseNonSecureCall,

compiler/rustc_public/src/ty.rs

@@ -1169,6 +1169,7 @@ pub enum Abi {
     RustPreserveNone,
     RustInvalid,
     Custom,
+    Swift,
 }
 
 /// A binder represents a possibly generic type and its bound vars.

compiler/rustc_public/src/unstable/convert/internal.rs

@@ -619,6 +619,7 @@ fn internal<'tcx>(
             Abi::RiscvInterruptS => rustc_abi::ExternAbi::RiscvInterruptS,
             Abi::RustPreserveNone => rustc_abi::ExternAbi::RustPreserveNone,
             Abi::Custom => rustc_abi::ExternAbi::Custom,
+            Abi::Swift => rustc_abi::ExternAbi::Swift,
         }
     }
 }

compiler/rustc_public/src/unstable/convert/stable/abi.rs

@@ -126,6 +126,7 @@ fn stable(&self, _: &mut Tables<'_, BridgeTys>, _: &CompilerCtxt<'_, BridgeTys>)
             CanonAbi::RustCold => CallConvention::Cold,
             CanonAbi::RustPreserveNone => CallConvention::PreserveNone,
             CanonAbi::Custom => CallConvention::Custom,
+            CanonAbi::Swift => CallConvention::Swift,
             CanonAbi::Arm(arm_call) => match arm_call {
                 ArmCall::Aapcs => CallConvention::ArmAapcs,
                 ArmCall::CCmseNonSecureCall => CallConvention::CCmseNonSecureCall,

compiler/rustc_public/src/unstable/convert/stable/ty.rs

@@ -1050,6 +1050,7 @@ fn stable(&self, _: &mut Tables<'_, BridgeTys>, _: &CompilerCtxt<'_, BridgeTys>)
             ExternAbi::RiscvInterruptM => Abi::RiscvInterruptM,
             ExternAbi::RiscvInterruptS => Abi::RiscvInterruptS,
             ExternAbi::Custom => Abi::Custom,
+            ExternAbi::Swift => Abi::Swift,
         }
     }
 }

compiler/rustc_span/src/symbol.rs

@@ -355,6 +355,7 @@
         abi_msp430_interrupt,
         abi_ptx,
         abi_riscv_interrupt,
+        abi_swift,
         abi_sysv64,
         abi_thiscall,
         abi_unadjusted,

compiler/rustc_target/src/spec/abi_map.rs

@@ -65,7 +65,9 @@ pub fn from_target(target: &Target) -> Self {
             _ => ArchKind::Other,
         };
 
-        let os = if target.is_like_windows {
+        let os = if target.is_like_darwin {
+            OsKind::Apple
+        } else if target.is_like_windows {
             OsKind::Windows
         } else if target.is_like_vexos {
             OsKind::VEXos
@@ -80,6 +82,15 @@ pub fn from_target(target: &Target) -> Self {
     pub fn canonize_abi(&self, extern_abi: ExternAbi, has_c_varargs: bool) -> AbiMapping {
         let AbiMap { os, arch } = *self;
 
+        if extern_abi == ExternAbi::Swift {
+            // Per https://www.swift.org/blog/abi-stability-and-more/, Swift's ABI
+            // is only stable on Apple platforms, so we reject it elsewhere.
+            match os {
+                OsKind::Apple => {}
+                _ => return AbiMapping::Invalid,
+            }
+        }
+
         let canon_abi = match (extern_abi, arch) {
             // infallible lowerings
             (ExternAbi::C { .. }, _) => CanonAbi::C,
@@ -92,6 +103,8 @@ pub fn canonize_abi(&self, extern_abi: ExternAbi, has_c_varargs: bool) -> AbiMap
 
             (ExternAbi::Custom, _) => CanonAbi::Custom,
 
+            (ExternAbi::Swift, _) => CanonAbi::Swift,
+
             (ExternAbi::System { .. }, ArchKind::X86)
                 if os == OsKind::Windows && !has_c_varargs =>
             {
@@ -213,6 +226,7 @@ enum ArchKind {
 
 #[derive(Debug, PartialEq, Copy, Clone)]
 enum OsKind {
+    Apple,
     Windows,
     VEXos,
     Other,

tests/ui/abi/swift-abi-non-apple.rs

@@ -0,0 +1,33 @@
+//@ add-minicore
+//@ needs-llvm-components: x86
+//@ compile-flags: --target=x86_64-unknown-linux-gnu --crate-type=rlib
+//@ ignore-backends: gcc
+#![no_core]
+#![feature(no_core, lang_items, abi_swift)]
+
+extern crate minicore;
+use minicore::*;
+
+// The Swift ABI is only stable on Apple platforms, so it must be rejected
+// on other targets even when the `abi_swift` feature gate is enabled.
+
+extern "Swift" fn f() {} //~ ERROR is not a supported ABI
+
+trait T {
+    extern "Swift" fn m(); //~ ERROR is not a supported ABI
+
+    extern "Swift" fn dm() {} //~ ERROR is not a supported ABI
+}
+
+struct S;
+impl T for S {
+    extern "Swift" fn m() {} //~ ERROR is not a supported ABI
+}
+
+impl S {
+    extern "Swift" fn im() {} //~ ERROR is not a supported ABI
+}
+
+type TA = extern "Swift" fn(); //~ ERROR is not a supported ABI
+
+extern "Swift" {} //~ ERROR is not a supported ABI

tests/ui/abi/swift-abi-non-apple.stderr

@@ -0,0 +1,45 @@
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:14:8
+   |
+LL | extern "Swift" fn f() {}
+   |        ^^^^^^^
+
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:17:12
+   |
+LL |     extern "Swift" fn m();
+   |            ^^^^^^^
+
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:19:12
+   |
+LL |     extern "Swift" fn dm() {}
+   |            ^^^^^^^
+
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:24:12
+   |
+LL |     extern "Swift" fn m() {}
+   |            ^^^^^^^
+
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:28:12
+   |
+LL |     extern "Swift" fn im() {}
+   |            ^^^^^^^
+
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:31:18
+   |
+LL | type TA = extern "Swift" fn();
+   |                  ^^^^^^^
+
+error[E0570]: "Swift" is not a supported ABI for the current target
+  --> $DIR/swift-abi-non-apple.rs:33:8
+   |
+LL | extern "Swift" {}
+   |        ^^^^^^^
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0570`.

tests/ui/feature-gates/feature-gate-abi_swift.rs

@@ -0,0 +1,33 @@
+//@ add-minicore
+//@ needs-llvm-components: aarch64
+//@ compile-flags: --target=aarch64-apple-darwin --crate-type=rlib
+//@ ignore-backends: gcc
+#![no_core]
+#![feature(no_core, lang_items)]
+
+extern crate minicore;
+use minicore::*;
+
+// Test that the "Swift" ABI is feature-gated, and cannot be used when
+// the `abi_swift` feature gate is not used.
+
+extern "Swift" fn f() {} //~ ERROR "Swift" ABI is experimental
+
+trait T {
+    extern "Swift" fn m(); //~ ERROR "Swift" ABI is experimental
+
+    extern "Swift" fn dm() {} //~ ERROR "Swift" ABI is experimental
+}
+
+struct S;
+impl T for S {
+    extern "Swift" fn m() {} //~ ERROR "Swift" ABI is experimental
+}
+
+impl S {
+    extern "Swift" fn im() {} //~ ERROR "Swift" ABI is experimental
+}
+
+type TA = extern "Swift" fn(); //~ ERROR "Swift" ABI is experimental
+
+extern "Swift" {} //~ ERROR "Swift" ABI is experimental

tests/ui/feature-gates/feature-gate-abi_swift.stderr

@@ -0,0 +1,73 @@
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:14:8
+   |
+LL | extern "Swift" fn f() {}
+   |        ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:17:12
+   |
+LL |     extern "Swift" fn m();
+   |            ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:19:12
+   |
+LL |     extern "Swift" fn dm() {}
+   |            ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:24:12
+   |
+LL |     extern "Swift" fn m() {}
+   |            ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:28:12
+   |
+LL |     extern "Swift" fn im() {}
+   |            ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:31:18
+   |
+LL | type TA = extern "Swift" fn();
+   |                  ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: the extern "Swift" ABI is experimental and subject to change
+  --> $DIR/feature-gate-abi_swift.rs:33:8
+   |
+LL | extern "Swift" {}
+   |        ^^^^^^^
+   |
+   = note: see issue #156481 <https://github.com/rust-lang/rust/issues/156481> for more information
+   = help: add `#![feature(abi_swift)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/print-request/print-calling-conventions.stdout

@@ -1,6 +1,7 @@
 C
 C-unwind
 Rust
+Swift
 aapcs
 aapcs-unwind
 avr-interrupt