Auto merge of #76570 - cratelyn:implement-rfc-2945-c-unwind-abi, r=Amanieu

Implement RFC 2945: "C-unwind" ABI

## Implement RFC 2945: "C-unwind" ABI

This branch implements [RFC 2945]. The tracking issue for this RFC is #74990.

The feature gate for the issue is `#![feature(c_unwind)]`.

This RFC was created as part of the ffi-unwind project group tracked at rust-lang/lang-team#19.

### Changes

Further details will be provided in commit messages, but a high-level overview
of the changes follows:

* A boolean `unwind` payload is added to the `C`, `System`, `Stdcall`,
and `Thiscall` variants, marking whether unwinding across FFI boundaries is
acceptable. The cases where each of these variants' `unwind` member is true
correspond with the `C-unwind`, `system-unwind`, `stdcall-unwind`, and
`thiscall-unwind` ABI strings introduced in RFC 2945 [3].

* This commit adds a `c_unwind` feature gate for the new ABI strings.
Tests for this feature gate are included in `src/test/ui/c-unwind/`, which
ensure that this feature gate works correctly for each of the new ABIs.
A new language features entry in the unstable book is added as well.

* We adjust the `rustc_middle::ty::layout::fn_can_unwind` function,
used to compute whether or not a `FnAbi` object represents a function that
should be able to unwind when `panic=unwind` is in use.

* Changes are also made to
`rustc_mir_build::build::should_abort_on_panic` so that the function ABI is
used to determind whether it should abort, assuming that the `panic=unwind`
strategy is being used, and no explicit unwind attribute was provided.

[RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md

43 files changed, 661 insertions, 70 deletions

diff --git a/compiler/rustc_ast_lowering/src/item.rs b/compiler/rustc_ast_lowering/src/item.rs
index 777d3a5060b..f656325f681 100644
--- a/compiler/rustc_ast_lowering/src/item.rs
+++ b/compiler/rustc_ast_lowering/src/item.rs
@@ -319,10 +319,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
             },
             ItemKind::ForeignMod(ref fm) => {
                 if fm.abi.is_none() {
-                    self.maybe_lint_missing_abi(span, id, abi::Abi::C);
+                    self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
                 }
                 hir::ItemKind::ForeignMod {
-                    abi: fm.abi.map_or(abi::Abi::C, |abi| self.lower_abi(abi)),
+                    abi: fm.abi.map_or(abi::Abi::C { unwind: false }, |abi| self.lower_abi(abi)),
                     items: self
                         .arena
                         .alloc_from_iter(fm.items.iter().map(|x| self.lower_foreign_item_ref(x))),
@@ -1334,8 +1334,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
         match ext {
             Extern::None => abi::Abi::Rust,
             Extern::Implicit => {
-                self.maybe_lint_missing_abi(span, id, abi::Abi::C);
-                abi::Abi::C
+                self.maybe_lint_missing_abi(span, id, abi::Abi::C { unwind: false });
+                abi::Abi::C { unwind: false }
             }
             Extern::Explicit(abi) => self.lower_abi(abi),
         }
diff --git a/compiler/rustc_ast_passes/src/feature_gate.rs b/compiler/rustc_ast_passes/src/feature_gate.rs
index 053fa5b7897..8a17ac90a02 100644
--- a/compiler/rustc_ast_passes/src/feature_gate.rs
+++ b/compiler/rustc_ast_passes/src/feature_gate.rs
@@ -164,6 +164,38 @@ impl<'a> PostExpansionVisitor<'a> {
                     "C-cmse-nonsecure-call ABI is experimental and subject to change"
                 );
             }
+            "C-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "C-unwind ABI is experimental and subject to change"
+                );
+            }
+            "stdcall-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "stdcall-unwind ABI is experimental and subject to change"
+                );
+            }
+            "system-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "system-unwind ABI is experimental and subject to change"
+                );
+            }
+            "thiscall-unwind" => {
+                gate_feature_post!(
+                    &self,
+                    c_unwind,
+                    span,
+                    "thiscall-unwind ABI is experimental and subject to change"
+                );
+            }
             abi => self
                 .sess
                 .parse_sess
diff --git a/compiler/rustc_codegen_cranelift/src/abi/mod.rs b/compiler/rustc_codegen_cranelift/src/abi/mod.rs
index c79889f8ca1..b158d73f3a1 100644
--- a/compiler/rustc_codegen_cranelift/src/abi/mod.rs
+++ b/compiler/rustc_codegen_cranelift/src/abi/mod.rs
@@ -476,8 +476,11 @@ pub(crate) fn codegen_terminator_call<'tcx>(
 
     // FIXME find a cleaner way to support varargs
     if fn_sig.c_variadic {
-        if fn_sig.abi != Abi::C {
-            fx.tcx.sess.span_fatal(span, &format!("Variadic call for non-C abi {:?}", fn_sig.abi));
+        if !matches!(fn_sig.abi, Abi::C { .. }) {
+            fx.tcx.sess.span_fatal(
+                span,
+                &format!("Variadic call for non-C abi {:?}", fn_sig.abi),
+            );
         }
         let sig_ref = fx.bcx.func.dfg.call_signature(call_inst).unwrap();
         let abi_params = call_args
diff --git a/compiler/rustc_feature/src/active.rs b/compiler/rustc_feature/src/active.rs
index 9cb06684c0f..ee2fce34b0b 100644
--- a/compiler/rustc_feature/src/active.rs
+++ b/compiler/rustc_feature/src/active.rs
@@ -641,6 +641,9 @@ declare_features! (
     /// Allows associated types in inherent impls.
     (active, inherent_associated_types, "1.52.0", Some(8995), None),
 
+    /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries.
+    (active, c_unwind, "1.52.0", Some(74990), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/compiler/rustc_middle/src/ty/layout.rs b/compiler/rustc_middle/src/ty/layout.rs
index 6d2ab0e5f5a..814581a6cf1 100644
--- a/compiler/rustc_middle/src/ty/layout.rs
+++ b/compiler/rustc_middle/src/ty/layout.rs
@@ -2562,6 +2562,7 @@ fn fn_can_unwind(
     panic_strategy: PanicStrategy,
     codegen_fn_attr_flags: CodegenFnAttrFlags,
     call_conv: Conv,
+    abi: SpecAbi,
 ) -> bool {
     if panic_strategy != PanicStrategy::Unwind {
         // In panic=abort mode we assume nothing can unwind anywhere, so
@@ -2586,17 +2587,34 @@ fn fn_can_unwind(
             //
             //  2. A Rust item using a non-Rust ABI (like `extern "C" fn foo() { ... }`).
             //
-            // Foreign items (case 1) are assumed to not unwind; it is
-            // UB otherwise. (At least for now; see also
-            // rust-lang/rust#63909 and Rust RFC 2753.)
-            //
-            // Items defined in Rust with non-Rust ABIs (case 2) are also
-            // not supposed to unwind. Whether this should be enforced
-            // (versus stating it is UB) and *how* it would be enforced
-            // is currently under discussion; see rust-lang/rust#58794.
-            //
-            // In either case, we mark item as explicitly nounwind.
-            false
+            // In both of these cases, we should refer to the ABI to determine whether or not we
+            // should unwind. See Rust RFC 2945 for more information on this behavior, here:
+            // https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
+            use SpecAbi::*;
+            match abi {
+                C { unwind } | Stdcall { unwind } | System { unwind } | Thiscall { unwind } => {
+                    unwind
+                }
+                Cdecl
+                | Fastcall
+                | Vectorcall
+                | Aapcs
+                | Win64
+                | SysV64
+                | PtxKernel
+                | Msp430Interrupt
+                | X86Interrupt
+                | AmdGpuKernel
+                | EfiApi
+                | AvrInterrupt
+                | AvrNonBlockingInterrupt
+                | CCmseNonSecureCall
+                | RustIntrinsic
+                | PlatformIntrinsic
+                | Unadjusted => false,
+                // In the `if` above, we checked for functions with the Rust calling convention.
+                Rust | RustCall => unreachable!(),
+            }
         }
     }
 }
@@ -2654,14 +2672,14 @@ where
             RustIntrinsic | PlatformIntrinsic | Rust | RustCall => Conv::Rust,
 
             // It's the ABI's job to select this, not ours.
-            System => bug!("system abi should be selected elsewhere"),
+            System { .. } => bug!("system abi should be selected elsewhere"),
             EfiApi => bug!("eficall abi should be selected elsewhere"),
 
-            Stdcall => Conv::X86Stdcall,
+            Stdcall { .. } => Conv::X86Stdcall,
             Fastcall => Conv::X86Fastcall,
             Vectorcall => Conv::X86VectorCall,
-            Thiscall => Conv::X86ThisCall,
-            C => Conv::C,
+            Thiscall { .. } => Conv::X86ThisCall,
+            C { .. } => Conv::C,
             Unadjusted => Conv::C,
             Win64 => Conv::X86_64Win64,
             SysV64 => Conv::X86_64SysV,
@@ -2823,7 +2841,12 @@ where
             c_variadic: sig.c_variadic,
             fixed_count: inputs.len(),
             conv,
-            can_unwind: fn_can_unwind(cx.tcx().sess.panic_strategy(), codegen_fn_attr_flags, conv),
+            can_unwind: fn_can_unwind(
+                cx.tcx().sess.panic_strategy(),
+                codegen_fn_attr_flags,
+                conv,
+                sig.abi,
+            ),
         };
         fn_abi.adjust_for_abi(cx, sig.abi);
         debug!("FnAbi::new_internal = {:?}", fn_abi);
diff --git a/compiler/rustc_mir/src/interpret/terminator.rs b/compiler/rustc_mir/src/interpret/terminator.rs
index 0807949a2d9..4aa1360d535 100644
--- a/compiler/rustc_mir/src/interpret/terminator.rs
+++ b/compiler/rustc_mir/src/interpret/terminator.rs
@@ -248,9 +248,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             };
             if normalize_abi(caller_abi) != normalize_abi(callee_abi) {
                 throw_ub_format!(
-                    "calling a function with ABI {:?} using caller ABI {:?}",
-                    callee_abi,
-                    caller_abi
+                    "calling a function with ABI {} using caller ABI {}",
+                    callee_abi.name(),
+                    caller_abi.name()
                 )
             }
         }
diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/build/mod.rs
index b928458df8e..3ac9c631d03 100644
--- a/compiler/rustc_mir_build/src/build/mod.rs
+++ b/compiler/rustc_mir_build/src/build/mod.rs
@@ -548,7 +548,7 @@ macro_rules! unpack {
     }};
 }
 
-fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, _abi: Abi) -> bool {
+fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, abi: Abi) -> bool {
     // Validate `#[unwind]` syntax regardless of platform-specific panic strategy.
     let attrs = &tcx.get_attrs(fn_def_id.to_def_id());
     let unwind_attr = attr::find_unwind_attr(&tcx.sess, attrs);
@@ -558,12 +558,42 @@ fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, _abi: Abi) -> b
         return false;
     }
 
-    // This is a special case: some functions have a C abi but are meant to
-    // unwind anyway. Don't stop them.
     match unwind_attr {
-        None => false, // FIXME(#58794); should be `!(abi == Abi::Rust || abi == Abi::RustCall)`
+        // If an `#[unwind]` attribute was found, we should adhere to it.
         Some(UnwindAttr::Allowed) => false,
         Some(UnwindAttr::Aborts) => true,
+        // If no attribute was found and the panic strategy is `unwind`, then we should examine
+        // the function's ABI string to determine whether it should abort upon panic.
+        None => {
+            use Abi::*;
+            match abi {
+                // In the case of ABI's that have an `-unwind` equivalent, check whether the ABI
+                // permits unwinding. If so, we should not abort. Otherwise, we should.
+                C { unwind } | Stdcall { unwind } | System { unwind } | Thiscall { unwind } => {
+                    !unwind
+                }
+                // Rust and `rust-call` functions are allowed to unwind, and should not abort.
+                Rust | RustCall => false,
+                // Other ABI's should abort.
+                Cdecl
+                | Fastcall
+                | Vectorcall
+                | Aapcs
+                | Win64
+                | SysV64
+                | PtxKernel
+                | Msp430Interrupt
+                | X86Interrupt
+                | AmdGpuKernel
+                | EfiApi
+                | AvrInterrupt
+                | AvrNonBlockingInterrupt
+                | CCmseNonSecureCall
+                | RustIntrinsic
+                | PlatformIntrinsic
+                | Unadjusted => true,
+            }
+        }
     }
 }
 
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 507eb1e1cbe..90c5fd422cd 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -330,6 +330,7 @@ symbols! {
         bridge,
         bswap,
         c_str,
+        c_unwind,
         c_variadic,
         call,
         call_mut,
diff --git a/compiler/rustc_symbol_mangling/src/v0.rs b/compiler/rustc_symbol_mangling/src/v0.rs
index bbf7ecc39cf..12c0a147990 100644
--- a/compiler/rustc_symbol_mangling/src/v0.rs
+++ b/compiler/rustc_symbol_mangling/src/v0.rs
@@ -440,7 +440,7 @@ impl Printer<'tcx> for SymbolMangler<'tcx> {
                     }
                     match sig.abi {
                         Abi::Rust => {}
-                        Abi::C => cx.push("KC"),
+                        Abi::C { unwind: false } => cx.push("KC"),
                         abi => {
                             cx.push("K");
                             let name = abi.name();
diff --git a/compiler/rustc_target/src/spec/abi.rs b/compiler/rustc_target/src/spec/abi.rs
index 65e8a4e8db2..17eb33b8f2e 100644
--- a/compiler/rustc_target/src/spec/abi.rs
+++ b/compiler/rustc_target/src/spec/abi.rs
@@ -8,24 +8,21 @@ mod tests;
 #[derive(PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Debug)]
 #[derive(HashStable_Generic, Encodable, Decodable)]
 pub enum Abi {
-    // N.B., this ordering MUST match the AbiDatas array below.
-    // (This is ensured by the test indices_are_correct().)
-
     // Multiplatform / generic ABIs
     //
     // These ABIs come first because every time we add a new ABI, we
     // have to re-bless all the hashing tests. These are used in many
     // places, so giving them stable values reduces test churn. The
     // specific values are meaningless.
-    Rust = 0,
-    C = 1,
+    Rust,
+    C { unwind: bool },
 
     // Single platform ABIs
     Cdecl,
-    Stdcall,
+    Stdcall { unwind: bool },
     Fastcall,
     Vectorcall,
-    Thiscall,
+    Thiscall { unwind: bool },
     Aapcs,
     Win64,
     SysV64,
@@ -39,7 +36,7 @@ pub enum Abi {
     CCmseNonSecureCall,
 
     // Multiplatform / generic ABIs
-    System,
+    System { unwind: bool },
     RustIntrinsic,
     RustCall,
     PlatformIntrinsic,
@@ -61,13 +58,16 @@ pub struct AbiData {
 const AbiDatas: &[AbiData] = &[
     // Cross-platform ABIs
     AbiData { abi: Abi::Rust, name: "Rust", generic: true },
-    AbiData { abi: Abi::C, name: "C", generic: true },
+    AbiData { abi: Abi::C { unwind: false }, name: "C", generic: true },
+    AbiData { abi: Abi::C { unwind: true }, name: "C-unwind", generic: true },
     // Platform-specific ABIs
     AbiData { abi: Abi::Cdecl, name: "cdecl", generic: false },
-    AbiData { abi: Abi::Stdcall, name: "stdcall", generic: false },
+    AbiData { abi: Abi::Stdcall { unwind: false }, name: "stdcall", generic: false },
+    AbiData { abi: Abi::Stdcall { unwind: true }, name: "stdcall-unwind", generic: false },
     AbiData { abi: Abi::Fastcall, name: "fastcall", generic: false },
     AbiData { abi: Abi::Vectorcall, name: "vectorcall", generic: false },
-    AbiData { abi: Abi::Thiscall, name: "thiscall", generic: false },
+    AbiData { abi: Abi::Thiscall { unwind: false }, name: "thiscall", generic: false },
+    AbiData { abi: Abi::Thiscall { unwind: true }, name: "thiscall-unwind", generic: false },
     AbiData { abi: Abi::Aapcs, name: "aapcs", generic: false },
     AbiData { abi: Abi::Win64, name: "win64", generic: false },
     AbiData { abi: Abi::SysV64, name: "sysv64", generic: false },
@@ -84,7 +84,8 @@ const AbiDatas: &[AbiData] = &[
     },
     AbiData { abi: Abi::CCmseNonSecureCall, name: "C-cmse-nonsecure-call", generic: false },
     // Cross-platform ABIs
-    AbiData { abi: Abi::System, name: "system", generic: true },
+    AbiData { abi: Abi::System { unwind: false }, name: "system", generic: true },
+    AbiData { abi: Abi::System { unwind: true }, name: "system-unwind", generic: true },
     AbiData { abi: Abi::RustIntrinsic, name: "rust-intrinsic", generic: true },
     AbiData { abi: Abi::RustCall, name: "rust-call", generic: true },
     AbiData { abi: Abi::PlatformIntrinsic, name: "platform-intrinsic", generic: true },
@@ -103,7 +104,52 @@ pub fn all_names() -> Vec<&'static str> {
 impl Abi {
     #[inline]
     pub fn index(self) -> usize {
-        self as usize
+        // N.B., this ordering MUST match the AbiDatas array above.
+        // (This is ensured by the test indices_are_correct().)
+        use Abi::*;
+        let i = match self {
+            // Cross-platform ABIs
+            Rust => 0,
+            C { unwind: false } => 1,
+            C { unwind: true } => 2,
+            // Platform-specific ABIs
+            Cdecl => 3,
+            Stdcall { unwind: false } => 4,
+            Stdcall { unwind: true } => 5,
+            Fastcall => 6,
+            Vectorcall => 7,
+            Thiscall { unwind: false } => 8,
+            Thiscall { unwind: true } => 9,
+            Aapcs => 10,
+            Win64 => 11,
+            SysV64 => 12,
+            PtxKernel => 13,
+            Msp430Interrupt => 14,
+            X86Interrupt => 15,
+            AmdGpuKernel => 16,
+            EfiApi => 17,
+            AvrInterrupt => 18,
+            AvrNonBlockingInterrupt => 19,
+            CCmseNonSecureCall => 20,
+            // Cross-platform ABIs
+            System { unwind: false } => 21,
+            System { unwind: true } => 22,
+            RustIntrinsic => 23,
+            RustCall => 24,
+            PlatformIntrinsic => 25,
+            Unadjusted => 26,
+        };
+        debug_assert!(
+            AbiDatas
+                .iter()
+                .enumerate()
+                .find(|(_, AbiData { abi, .. })| *abi == self)
+                .map(|(index, _)| index)
+                .expect("abi variant has associated data")
+                == i,
+            "Abi index did not match `AbiDatas` ordering"
+        );
+        i
     }
 
     #[inline]
@@ -122,6 +168,8 @@ impl Abi {
 
 impl fmt::Display for Abi {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "\"{}\"", self.name())
+        match self {
+            abi => write!(f, "\"{}\"", abi.name()),
+        }
     }
 }
diff --git a/compiler/rustc_target/src/spec/arm_base.rs b/compiler/rustc_target/src/spec/arm_base.rs
index b74d80dc6bb..01f573313c9 100644
--- a/compiler/rustc_target/src/spec/arm_base.rs
+++ b/compiler/rustc_target/src/spec/arm_base.rs
@@ -2,5 +2,14 @@ use crate::spec::abi::Abi;
 
 // All the calling conventions trigger an assertion(Unsupported calling convention) in llvm on arm
 pub fn unsupported_abis() -> Vec<Abi> {
-    vec![Abi::Stdcall, Abi::Fastcall, Abi::Vectorcall, Abi::Thiscall, Abi::Win64, Abi::SysV64]
+    vec![
+        Abi::Stdcall { unwind: false },
+        Abi::Stdcall { unwind: true },
+        Abi::Fastcall,
+        Abi::Vectorcall,
+        Abi::Thiscall { unwind: false },
+        Abi::Thiscall { unwind: true },
+        Abi::Win64,
+        Abi::SysV64,
+    ]
 }
diff --git a/compiler/rustc_target/src/spec/mipsel_unknown_none.rs b/compiler/rustc_target/src/spec/mipsel_unknown_none.rs
index 0f9d3c3de15..110c8dd80ea 100644
--- a/compiler/rustc_target/src/spec/mipsel_unknown_none.rs
+++ b/compiler/rustc_target/src/spec/mipsel_unknown_none.rs
@@ -23,10 +23,12 @@ pub fn target() -> Target {
             panic_strategy: PanicStrategy::Abort,
             relocation_model: RelocModel::Static,
             unsupported_abis: vec![
-                Abi::Stdcall,
+                Abi::Stdcall { unwind: false },
+                Abi::Stdcall { unwind: true },
                 Abi::Fastcall,
                 Abi::Vectorcall,
-                Abi::Thiscall,
+                Abi::Thiscall { unwind: false },
+                Abi::Thiscall { unwind: true },
                 Abi::Win64,
                 Abi::SysV64,
             ],
diff --git a/compiler/rustc_target/src/spec/mod.rs b/compiler/rustc_target/src/spec/mod.rs
index 09400a1d130..c9fffd213d7 100644
--- a/compiler/rustc_target/src/spec/mod.rs
+++ b/compiler/rustc_target/src/spec/mod.rs
@@ -1284,24 +1284,31 @@ impl Target {
     /// Given a function ABI, turn it into the correct ABI for this target.
     pub fn adjust_abi(&self, abi: Abi) -> Abi {
         match abi {
-            Abi::System => {
+            Abi::System { unwind } => {
                 if self.is_like_windows && self.arch == "x86" {
-                    Abi::Stdcall
+                    Abi::Stdcall { unwind }
                 } else {
-                    Abi::C
+                    Abi::C { unwind }
                 }
             }
             // These ABI kinds are ignored on non-x86 Windows targets.
             // See https://docs.microsoft.com/en-us/cpp/cpp/argument-passing-and-naming-conventions
             // and the individual pages for __stdcall et al.
-            Abi::Stdcall | Abi::Fastcall | Abi::Vectorcall | Abi::Thiscall => {
-                if self.is_like_windows && self.arch != "x86" { Abi::C } else { abi }
+            Abi::Stdcall { unwind } | Abi::Thiscall { unwind } => {
+                if self.is_like_windows && self.arch != "x86" { Abi::C { unwind } } else { abi }
+            }
+            Abi::Fastcall | Abi::Vectorcall => {
+                if self.is_like_windows && self.arch != "x86" {
+                    Abi::C { unwind: false }
+                } else {
+                    abi
+                }
             }
             Abi::EfiApi => {
                 if self.arch == "x86_64" {
                     Abi::Win64
                 } else {
-                    Abi::C
+                    Abi::C { unwind: false }
                 }
             }
             abi => abi,
diff --git a/compiler/rustc_target/src/spec/nvptx64_nvidia_cuda.rs b/compiler/rustc_target/src/spec/nvptx64_nvidia_cuda.rs
index 3c9c7d578fb..15d8e4843f9 100644
--- a/compiler/rustc_target/src/spec/nvptx64_nvidia_cuda.rs
+++ b/compiler/rustc_target/src/spec/nvptx64_nvidia_cuda.rs
@@ -49,10 +49,12 @@ pub fn target() -> Target {
             // create the tests for this.
             unsupported_abis: vec![
                 Abi::Cdecl,
-                Abi::Stdcall,
+                Abi::Stdcall { unwind: false },
+                Abi::Stdcall { unwind: true },
                 Abi::Fastcall,
                 Abi::Vectorcall,
-                Abi::Thiscall,
+                Abi::Thiscall { unwind: false },
+                Abi::Thiscall { unwind: true },
                 Abi::Aapcs,
                 Abi::Win64,
                 Abi::SysV64,
diff --git a/compiler/rustc_target/src/spec/riscv_base.rs b/compiler/rustc_target/src/spec/riscv_base.rs
index 64cf890037e..5bcbb2e621b 100644
--- a/compiler/rustc_target/src/spec/riscv_base.rs
+++ b/compiler/rustc_target/src/spec/riscv_base.rs
@@ -5,10 +5,12 @@ use crate::spec::abi::Abi;
 pub fn unsupported_abis() -> Vec<Abi> {
     vec![
         Abi::Cdecl,
-        Abi::Stdcall,
+        Abi::Stdcall { unwind: false },
+        Abi::Stdcall { unwind: true },
         Abi::Fastcall,
         Abi::Vectorcall,
-        Abi::Thiscall,
+        Abi::Thiscall { unwind: false },
+        Abi::Thiscall { unwind: true },
         Abi::Aapcs,
         Abi::Win64,
         Abi::SysV64,
diff --git a/compiler/rustc_typeck/src/collect.rs b/compiler/rustc_typeck/src/collect.rs
index 3881d55ef91..a175da32706 100644
--- a/compiler/rustc_typeck/src/collect.rs
+++ b/compiler/rustc_typeck/src/collect.rs
@@ -2666,7 +2666,7 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, id: DefId) -> CodegenFnAttrs {
         } else if tcx.sess.check_name(attr, sym::used) {
             codegen_fn_attrs.flags |= CodegenFnAttrFlags::USED;
         } else if tcx.sess.check_name(attr, sym::cmse_nonsecure_entry) {
-            if tcx.fn_sig(id).abi() != abi::Abi::C {
+            if !matches!(tcx.fn_sig(id).abi(), abi::Abi::C { .. }) {
                 struct_span_err!(
                     tcx.sess,
                     attr.span,
diff --git a/compiler/rustc_typeck/src/lib.rs b/compiler/rustc_typeck/src/lib.rs
index 0fc49c4ae2f..29d9df61906 100644
--- a/compiler/rustc_typeck/src/lib.rs
+++ b/compiler/rustc_typeck/src/lib.rs
@@ -118,14 +118,19 @@ use astconv::AstConv;
 use bounds::Bounds;
 
 fn require_c_abi_if_c_variadic(tcx: TyCtxt<'_>, decl: &hir::FnDecl<'_>, abi: Abi, span: Span) {
-    if decl.c_variadic && !(abi == Abi::C || abi == Abi::Cdecl) {
-        let mut err = struct_span_err!(
-            tcx.sess,
-            span,
-            E0045,
-            "C-variadic function must have C or cdecl calling convention"
-        );
-        err.span_label(span, "C-variadics require C or cdecl calling convention").emit();
+    match (decl.c_variadic, abi) {
+        // The function has the correct calling convention, or isn't a "C-variadic" function.
+        (false, _) | (true, Abi::C { .. }) | (true, Abi::Cdecl) => {}
+        // The function is a "C-variadic" function with an incorrect calling convention.
+        (true, _) => {
+            let mut err = struct_span_err!(
+                tcx.sess,
+                span,
+                E0045,
+                "C-variadic function must have C or cdecl calling convention"
+            );
+            err.span_label(span, "C-variadics require C or cdecl calling convention").emit();
+        }
     }
 }
 
diff --git a/src/doc/unstable-book/src/language-features/c-unwind.md b/src/doc/unstable-book/src/language-features/c-unwind.md
new file mode 100644
index 00000000000..2801d9b5e77
--- /dev/null
+++ b/src/doc/unstable-book/src/language-features/c-unwind.md
@@ -0,0 +1,15 @@
+# `c_unwind`
+
+The tracking issue for this feature is: [#74990]
+
+[#74990]: https://github.com/rust-lang/rust/issues/74990
+
+------------------------
+
+Introduces four new ABI strings: "C-unwind", "stdcall-unwind",
+"thiscall-unwind", and "system-unwind". These enable unwinding from other
+languages (such as C++) into Rust frames and from Rust into other languages.
+
+See [RFC 2945] for more information.
+
+[RFC 2945]: https://github.com/rust-lang/rfcs/blob/master/text/2945-c-unwind-abi.md
diff --git a/src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs b/src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs
new file mode 100644
index 00000000000..afd65ff6741
--- /dev/null
+++ b/src/test/codegen/unwind-abis/c-unwind-abi-panic-abort.rs
@@ -0,0 +1,18 @@
+// compile-flags: -C panic=abort -C opt-level=0
+
+// Test that `nounwind` atributes are applied to `C-unwind` extern functions when the
+// code is compiled with `panic=abort`.  We disable optimizations above to prevent LLVM from
+// inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "C-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make sure that the LLVM attributes for this functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
diff --git a/src/test/codegen/unwind-abis/c-unwind-abi.rs b/src/test/codegen/unwind-abis/c-unwind-abi.rs
new file mode 100644
index 00000000000..f1576536753
--- /dev/null
+++ b/src/test/codegen/unwind-abis/c-unwind-abi.rs
@@ -0,0 +1,29 @@
+// compile-flags: -C opt-level=0
+
+// Test that `nounwind` atributes are correctly applied to exported `C` and `C-unwind` extern
+// functions. `C-unwind` functions MUST NOT have this attribute. We disable optimizations above
+// to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "C" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "C-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/codegen/unwind-abis/stdcall-unwind-abi.rs b/src/test/codegen/unwind-abis/stdcall-unwind-abi.rs
new file mode 100644
index 00000000000..ed804ca278d
--- /dev/null
+++ b/src/test/codegen/unwind-abis/stdcall-unwind-abi.rs
@@ -0,0 +1,32 @@
+// compile-flags: -C opt-level=0
+// ignore-arm stdcall isn't supported
+// ignore-aarch64 stdcall isn't supported
+// ignore-riscv64 stdcall isn't supported
+
+// Test that `nounwind` atributes are correctly applied to exported `stdcall` and `stdcall-unwind`
+// extern functions. `stdcall-unwind` functions MUST NOT have this attribute. We disable
+// optimizations above to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "stdcall" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "stdcall-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/codegen/unwind-abis/system-unwind-abi.rs b/src/test/codegen/unwind-abis/system-unwind-abi.rs
new file mode 100644
index 00000000000..c4d51328352
--- /dev/null
+++ b/src/test/codegen/unwind-abis/system-unwind-abi.rs
@@ -0,0 +1,29 @@
+// compile-flags: -C opt-level=0
+
+// Test that `nounwind` atributes are correctly applied to exported `system` and `system-unwind`
+// extern functions. `system-unwind` functions MUST NOT have this attribute. We disable
+// optimizations above to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "system" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "system-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/codegen/unwind-abis/thiscall-unwind-abi.rs b/src/test/codegen/unwind-abis/thiscall-unwind-abi.rs
new file mode 100644
index 00000000000..aaa63ae55c3
--- /dev/null
+++ b/src/test/codegen/unwind-abis/thiscall-unwind-abi.rs
@@ -0,0 +1,33 @@
+// compile-flags: -C opt-level=0
+// ignore-arm thiscall isn't supported
+// ignore-aarch64 thiscall isn't supported
+// ignore-riscv64 thiscall isn't supported
+
+// Test that `nounwind` atributes are correctly applied to exported `thiscall` and
+// `thiscall-unwind` extern functions. `thiscall-unwind` functions MUST NOT have this attribute. We
+// disable optimizations above to prevent LLVM from inferring the attribute.
+
+#![crate_type = "lib"]
+#![feature(abi_thiscall)]
+#![feature(c_unwind)]
+
+// CHECK: @rust_item_that_cannot_unwind() unnamed_addr #0 {
+#[no_mangle]
+pub extern "thiscall" fn rust_item_that_cannot_unwind() {
+}
+
+// CHECK: @rust_item_that_can_unwind() unnamed_addr #1 {
+#[no_mangle]
+pub extern "thiscall-unwind" fn rust_item_that_can_unwind() {
+}
+
+// Now, make some assertions that the LLVM attributes for these functions are correct.  First, make
+// sure that the first item is correctly marked with the `nounwind` attribute:
+//
+// CHECK: attributes #0 = { {{.*}}nounwind{{.*}} }
+//
+// Next, let's assert that the second item, which CAN unwind, does not have this attribute.
+//
+// CHECK: attributes #1 = {
+// CHECK-NOT: nounwind
+// CHECK: }
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/Makefile b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/Makefile
new file mode 100644
index 00000000000..a8515c533af
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/Makefile
@@ -0,0 +1,30 @@
+-include ../tools.mk
+
+all: archive
+	# Compile `main.rs`, which will link into our library, and run it.
+	$(RUSTC) main.rs
+	$(call RUN,main)
+
+ifdef IS_MSVC
+archive: add.o panic.o
+	# Now, create an archive using these two objects.
+	$(AR) crus $(TMPDIR)/add.lib $(TMPDIR)/add.o $(TMPDIR)/panic.o
+else
+archive: add.o panic.o
+	# Now, create an archive using these two objects.
+	$(AR) crus $(TMPDIR)/libadd.a $(TMPDIR)/add.o $(TMPDIR)/panic.o
+endif
+
+# Compile `panic.rs` into an object file.
+#
+# Note that we invoke `rustc` directly, so we may emit an object rather
+# than an archive. We'll do that later.
+panic.o:
+	$(BARE_RUSTC) $(RUSTFLAGS)  \
+		--out-dir $(TMPDIR) \
+		--emit=obj panic.rs
+
+# Compile `add.c` into an object file.
+add.o:
+	$(call COMPILE_OBJ,$(TMPDIR)/add.o,add.c)
+
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/add.c b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/add.c
new file mode 100644
index 00000000000..444359451f6
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/add.c
@@ -0,0 +1,12 @@
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+
+// An external function, defined in Rust.
+extern void panic_if_greater_than_10(unsigned x);
+
+unsigned add_small_numbers(unsigned a, unsigned b) {
+    unsigned c = a + b;
+    panic_if_greater_than_10(c);
+    return c;
+}
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/main.rs b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/main.rs
new file mode 100644
index 00000000000..78a71219c78
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/main.rs
@@ -0,0 +1,35 @@
+//! A test for calling `C-unwind` functions across foreign function boundaries.
+//!
+//! This test triggers a panic in a Rust library that our foreign function invokes. This shows
+//! that we can unwind through the C code in that library, and catch the underlying panic.
+#![feature(c_unwind)]
+
+use std::panic::{catch_unwind, AssertUnwindSafe};
+
+fn main() {
+    // Call `add_small_numbers`, passing arguments that will NOT trigger a panic.
+    let (a, b) = (9, 1);
+    let c = unsafe { add_small_numbers(a, b) };
+    assert_eq!(c, 10);
+
+    // Call `add_small_numbers`, passing arguments that will trigger a panic, and catch it.
+    let caught_unwind = catch_unwind(AssertUnwindSafe(|| {
+        let (a, b) = (10, 1);
+        let _c = unsafe { add_small_numbers(a, b) };
+        unreachable!("should have unwound instead of returned");
+    }));
+
+    // Assert that we did indeed panic, then unwrap and downcast the panic into the sum.
+    assert!(caught_unwind.is_err());
+    let panic_obj = caught_unwind.unwrap_err();
+    let msg = panic_obj.downcast_ref::<String>().unwrap();
+    assert_eq!(msg, "11");
+}
+
+#[link(name = "add", kind = "static")]
+extern "C-unwind" {
+    /// An external function, defined in C.
+    ///
+    /// Returns the sum of two numbers, or panics if the sum is greater than 10.
+    fn add_small_numbers(a: u32, b: u32) -> u32;
+}
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/panic.rs b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/panic.rs
new file mode 100644
index 00000000000..a99a04d5c6f
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-lib-panic/panic.rs
@@ -0,0 +1,12 @@
+#![crate_type = "staticlib"]
+#![feature(c_unwind)]
+
+/// This function will panic if `x` is greater than 10.
+///
+/// This function is called by `add_small_numbers`.
+#[no_mangle]
+pub extern "C-unwind" fn panic_if_greater_than_10(x: u32) {
+    if x > 10 {
+        panic!("{}", x); // That is too big!
+    }
+}
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/Makefile b/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/Makefile
new file mode 100644
index 00000000000..9553b7aeeb9
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/Makefile
@@ -0,0 +1,5 @@
+-include ../tools.mk
+
+all: $(call NATIVE_STATICLIB,add)
+	$(RUSTC) main.rs
+	$(call RUN,main) || exit 1
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/add.c b/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/add.c
new file mode 100644
index 00000000000..444359451f6
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/add.c
@@ -0,0 +1,12 @@
+#ifdef _WIN32
+__declspec(dllexport)
+#endif
+
+// An external function, defined in Rust.
+extern void panic_if_greater_than_10(unsigned x);
+
+unsigned add_small_numbers(unsigned a, unsigned b) {
+    unsigned c = a + b;
+    panic_if_greater_than_10(c);
+    return c;
+}
diff --git a/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/main.rs b/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/main.rs
new file mode 100644
index 00000000000..15d38d72160
--- /dev/null
+++ b/src/test/run-make-fulldeps/c-unwind-abi-catch-panic/main.rs
@@ -0,0 +1,44 @@
+//! A test for calling `C-unwind` functions across foreign function boundaries.
+//!
+//! This test triggers a panic when calling a foreign function that calls *back* into Rust.
+#![feature(c_unwind)]
+
+use std::panic::{catch_unwind, AssertUnwindSafe};
+
+fn main() {
+    // Call `add_small_numbers`, passing arguments that will NOT trigger a panic.
+    let (a, b) = (9, 1);
+    let c = unsafe { add_small_numbers(a, b) };
+    assert_eq!(c, 10);
+
+    // Call `add_small_numbers`, passing arguments that will trigger a panic, and catch it.
+    let caught_unwind = catch_unwind(AssertUnwindSafe(|| {
+        let (a, b) = (10, 1);
+        let _c = unsafe { add_small_numbers(a, b) };
+        unreachable!("should have unwound instead of returned");
+    }));
+
+    // Assert that we did indeed panic, then unwrap and downcast the panic into the sum.
+    assert!(caught_unwind.is_err());
+    let panic_obj = caught_unwind.unwrap_err();
+    let msg = panic_obj.downcast_ref::<String>().unwrap();
+    assert_eq!(msg, "11");
+}
+
+#[link(name = "add", kind = "static")]
+extern "C-unwind" {
+    /// An external function, defined in C.
+    ///
+    /// Returns the sum of two numbers, or panics if the sum is greater than 10.
+    fn add_small_numbers(a: u32, b: u32) -> u32;
+}
+
+/// This function will panic if `x` is greater than 10.
+///
+/// This function is called by `add_small_numbers`.
+#[no_mangle]
+pub extern "C-unwind" fn panic_if_greater_than_10(x: u32) {
+    if x > 10 {
+        panic!("{}", x); // That is too big!
+    }
+}
diff --git a/src/test/ui/allocator/no_std-alloc-error-handler-custom.rs b/src/test/ui/allocator/no_std-alloc-error-handler-custom.rs
index 4d40c7d0d22..c9b4abbfd3f 100644
--- a/src/test/ui/allocator/no_std-alloc-error-handler-custom.rs
+++ b/src/test/ui/allocator/no_std-alloc-error-handler-custom.rs
@@ -7,7 +7,7 @@
 // compile-flags:-C panic=abort
 // aux-build:helper.rs
 
-#![feature(start, rustc_private, new_uninit, panic_info_message)]
+#![feature(start, rustc_private, new_uninit, panic_info_message, lang_items)]
 #![feature(alloc_error_handler)]
 #![no_std]
 
@@ -84,6 +84,13 @@ fn panic(panic_info: &core::panic::PanicInfo) -> ! {
     }
 }
 
+// Because we are compiling this code with `-C panic=abort`, this wouldn't normally be needed.
+// However, `core` and `alloc` are both compiled with `-C panic=unwind`, which means that functions
+// in these libaries will refer to `rust_eh_personality` if LLVM can not *prove* the contents won't
+// unwind. So, for this test case we will define the symbol.
+#[lang = "eh_personality"]
+extern fn rust_eh_personality() {}
+
 #[derive(Debug)]
 struct Page([[u64; 32]; 16]);
 
diff --git a/src/test/ui/allocator/no_std-alloc-error-handler-default.rs b/src/test/ui/allocator/no_std-alloc-error-handler-default.rs
index 4f8c44f1763..d6cd4a6af85 100644
--- a/src/test/ui/allocator/no_std-alloc-error-handler-default.rs
+++ b/src/test/ui/allocator/no_std-alloc-error-handler-default.rs
@@ -8,7 +8,7 @@
 // aux-build:helper.rs
 // gate-test-default_alloc_error_handler
 
-#![feature(start, rustc_private, new_uninit, panic_info_message)]
+#![feature(start, rustc_private, new_uninit, panic_info_message, lang_items)]
 #![feature(default_alloc_error_handler)]
 #![no_std]
 
@@ -71,6 +71,13 @@ fn panic(panic_info: &core::panic::PanicInfo) -> ! {
     }
 }
 
+// Because we are compiling this code with `-C panic=abort`, this wouldn't normally be needed.
+// However, `core` and `alloc` are both compiled with `-C panic=unwind`, which means that functions
+// in these libaries will refer to `rust_eh_personality` if LLVM can not *prove* the contents won't
+// unwind. So, for this test case we will define the symbol.
+#[lang = "eh_personality"]
+extern fn rust_eh_personality() {}
+
 #[derive(Debug)]
 struct Page([[u64; 32]; 16]);
 
diff --git a/src/test/ui/codemap_tests/unicode.stderr b/src/test/ui/codemap_tests/unicode.stderr
index 61c3f4f1c98..b7ba4fa46d9 100644
--- a/src/test/ui/codemap_tests/unicode.stderr
+++ b/src/test/ui/codemap_tests/unicode.stderr
@@ -4,7 +4,7 @@ error[E0703]: invalid ABI: found `路濫狼á́́`
 LL | extern "路濫狼á́́" fn foo() {}
    |        ^^^^^^^^^ invalid ABI
    |
-   = help: valid ABIs: Rust, C, cdecl, stdcall, fastcall, vectorcall, thiscall, aapcs, win64, sysv64, ptx-kernel, msp430-interrupt, x86-interrupt, amdgpu-kernel, efiapi, avr-interrupt, avr-non-blocking-interrupt, C-cmse-nonsecure-call, system, rust-intrinsic, rust-call, platform-intrinsic, unadjusted
+   = help: valid ABIs: Rust, C, C-unwind, cdecl, stdcall, stdcall-unwind, fastcall, vectorcall, thiscall, thiscall-unwind, aapcs, win64, sysv64, ptx-kernel, msp430-interrupt, x86-interrupt, amdgpu-kernel, efiapi, avr-interrupt, avr-non-blocking-interrupt, C-cmse-nonsecure-call, system, system-unwind, rust-intrinsic, rust-call, platform-intrinsic, unadjusted
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/parser/issue-8537.stderr b/src/test/ui/parser/issue-8537.stderr
index 3f63c080210..85e2b77d867 100644
--- a/src/test/ui/parser/issue-8537.stderr
+++ b/src/test/ui/parser/issue-8537.stderr
@@ -4,7 +4,7 @@ error[E0703]: invalid ABI: found `invalid-ab_isize`
 LL |   "invalid-ab_isize"
    |   ^^^^^^^^^^^^^^^^^^ invalid ABI
    |
-   = help: valid ABIs: Rust, C, cdecl, stdcall, fastcall, vectorcall, thiscall, aapcs, win64, sysv64, ptx-kernel, msp430-interrupt, x86-interrupt, amdgpu-kernel, efiapi, avr-interrupt, avr-non-blocking-interrupt, C-cmse-nonsecure-call, system, rust-intrinsic, rust-call, platform-intrinsic, unadjusted
+   = help: valid ABIs: Rust, C, C-unwind, cdecl, stdcall, stdcall-unwind, fastcall, vectorcall, thiscall, thiscall-unwind, aapcs, win64, sysv64, ptx-kernel, msp430-interrupt, x86-interrupt, amdgpu-kernel, efiapi, avr-interrupt, avr-non-blocking-interrupt, C-cmse-nonsecure-call, system, system-unwind, rust-intrinsic, rust-call, platform-intrinsic, unadjusted
 
 error: aborting due to previous error
 
diff --git a/src/test/ui/unwind-abis/feature-gate-c-unwind-enabled.rs b/src/test/ui/unwind-abis/feature-gate-c-unwind-enabled.rs
new file mode 100644
index 00000000000..6ff5dbda2d5
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-c-unwind-enabled.rs
@@ -0,0 +1,12 @@
+// Test that the "C-unwind" ABI is feature-gated, and *can* be used when the
+// `c_unwind` feature gate is enabled.
+
+// check-pass
+
+#![feature(c_unwind)]
+
+extern "C-unwind" fn f() {}
+
+fn main() {
+    f();
+}
diff --git a/src/test/ui/unwind-abis/feature-gate-c-unwind.rs b/src/test/ui/unwind-abis/feature-gate-c-unwind.rs
new file mode 100644
index 00000000000..f02a368d4e0
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-c-unwind.rs
@@ -0,0 +1,9 @@
+// Test that the "C-unwind" ABI is feature-gated, and cannot be used when the
+// `c_unwind` feature gate is not used.
+
+extern "C-unwind" fn f() {}
+//~^ ERROR C-unwind ABI is experimental and subject to change [E0658]
+
+fn main() {
+    f();
+}
diff --git a/src/test/ui/unwind-abis/feature-gate-c-unwind.stderr b/src/test/ui/unwind-abis/feature-gate-c-unwind.stderr
new file mode 100644
index 00000000000..f4c785a235f
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-c-unwind.stderr
@@ -0,0 +1,12 @@
+error[E0658]: C-unwind ABI is experimental and subject to change
+  --> $DIR/feature-gate-c-unwind.rs:4:8
+   |
+LL | extern "C-unwind" fn f() {}
+   |        ^^^^^^^^^^
+   |
+   = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
+   = help: add `#![feature(c_unwind)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/unwind-abis/feature-gate-stdcall-unwind.rs b/src/test/ui/unwind-abis/feature-gate-stdcall-unwind.rs
new file mode 100644
index 00000000000..7d4dc8c9343
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-stdcall-unwind.rs
@@ -0,0 +1,13 @@
+// ignore-arm stdcall isn't supported
+// ignore-aarch64 stdcall isn't supported
+// ignore-riscv64 stdcall isn't supported
+
+// Test that the "stdcall-unwind" ABI is feature-gated, and cannot be used when
+// the `c_unwind` feature gate is not used.
+
+extern "stdcall-unwind" fn f() {}
+//~^ ERROR stdcall-unwind ABI is experimental and subject to change [E0658]
+
+fn main() {
+    f();
+}
diff --git a/src/test/ui/unwind-abis/feature-gate-stdcall-unwind.stderr b/src/test/ui/unwind-abis/feature-gate-stdcall-unwind.stderr
new file mode 100644
index 00000000000..e3d569f464f
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-stdcall-unwind.stderr
@@ -0,0 +1,12 @@
+error[E0658]: stdcall-unwind ABI is experimental and subject to change
+  --> $DIR/feature-gate-stdcall-unwind.rs:8:8
+   |
+LL | extern "stdcall-unwind" fn f() {}
+   |        ^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
+   = help: add `#![feature(c_unwind)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/unwind-abis/feature-gate-system-unwind.rs b/src/test/ui/unwind-abis/feature-gate-system-unwind.rs
new file mode 100644
index 00000000000..26c2de4e817
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-system-unwind.rs
@@ -0,0 +1,9 @@
+// Test that the "system-unwind" ABI is feature-gated, and cannot be used when
+// the `c_unwind` feature gate is not used.
+
+extern "system-unwind" fn f() {}
+//~^ ERROR system-unwind ABI is experimental and subject to change [E0658]
+
+fn main() {
+    f();
+}
diff --git a/src/test/ui/unwind-abis/feature-gate-system-unwind.stderr b/src/test/ui/unwind-abis/feature-gate-system-unwind.stderr
new file mode 100644
index 00000000000..87877336475
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-system-unwind.stderr
@@ -0,0 +1,12 @@
+error[E0658]: system-unwind ABI is experimental and subject to change
+  --> $DIR/feature-gate-system-unwind.rs:4:8
+   |
+LL | extern "system-unwind" fn f() {}
+   |        ^^^^^^^^^^^^^^^
+   |
+   = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
+   = help: add `#![feature(c_unwind)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/src/test/ui/unwind-abis/feature-gate-thiscall-unwind.rs b/src/test/ui/unwind-abis/feature-gate-thiscall-unwind.rs
new file mode 100644
index 00000000000..2f4cefccc19
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-thiscall-unwind.rs
@@ -0,0 +1,13 @@
+// ignore-arm thiscall isn't supported
+// ignore-aarch64 thiscall isn't supported
+// ignore-riscv64 thiscall isn't supported
+
+// Test that the "thiscall-unwind" ABI is feature-gated, and cannot be used when
+// the `c_unwind` feature gate is not used.
+
+extern "thiscall-unwind" fn f() {}
+//~^ ERROR thiscall-unwind ABI is experimental and subject to change [E0658]
+
+fn main() {
+    f();
+}
diff --git a/src/test/ui/unwind-abis/feature-gate-thiscall-unwind.stderr b/src/test/ui/unwind-abis/feature-gate-thiscall-unwind.stderr
new file mode 100644
index 00000000000..b103bb8d565
--- /dev/null
+++ b/src/test/ui/unwind-abis/feature-gate-thiscall-unwind.stderr
@@ -0,0 +1,12 @@
+error[E0658]: thiscall-unwind ABI is experimental and subject to change
+  --> $DIR/feature-gate-thiscall-unwind.rs:8:8
+   |
+LL | extern "thiscall-unwind" fn f() {}
+   |        ^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #74990 <https://github.com/rust-lang/rust/issues/74990> for more information
+   = help: add `#![feature(c_unwind)]` to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.