Revert "implement unwinding abi's (RFC 2945)"

This reverts commit 0f33e9f.

Author: pnkfelix

compiler/rustc_middle/src/ty/layout.rs

@@ -2562,7 +2562,6 @@ 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
@@ -2587,16 +2586,17 @@ fn fn_can_unwind(
             //
             //  2. A Rust item using a non-Rust ABI (like `extern "C" fn foo() { ... }`).
             //
-            // 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
-                }
-                _ => false,
-            }
+            // 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
         }
     }
 }
@@ -2823,12 +2823,7 @@ 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,
-                sig.abi,
-            ),
+            can_unwind: fn_can_unwind(cx.tcx().sess.panic_strategy(), codegen_fn_attr_flags, conv),
         };
         fn_abi.adjust_for_abi(cx, sig.abi);
         debug!("FnAbi::new_internal = {:?}", fn_abi);

compiler/rustc_mir_build/src/build/mod.rs

@@ -563,7 +563,7 @@ 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);
@@ -573,26 +573,12 @@ fn should_abort_on_panic(tcx: TyCtxt<'_>, fn_def_id: LocalDefId, abi: Abi) -> bo
         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 {
-        // If an `#[unwind]` attribute was found, we should adhere to it.
+        None => false, // FIXME(#58794); should be `!(abi == Abi::Rust || abi == Abi::RustCall)`
         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.
-                _ => true,
-            }
-        }
     }
 }