Auto merge of #121548 - RalfJung:ffi-unwind-intrinsics, r=davidtwco

ffi_unwind_calls: treat RustIntrinsic like regular Rust calls

Also add some comments to `abi_can_unwind` to explain what happens.

r? `@nbdd0121`  Cc `@BatmanAoD`

compiler/rustc_mir_transform/src/ffi_unwind_calls.rs

@@ -10,6 +10,10 @@ use rustc_target::spec::PanicStrategy;
 
 use crate::errors;
 
+/// Some of the functions declared as "may unwind" by `fn_can_unwind` can't actually unwind. In
+/// particular, `extern "C"` is still considered as can-unwind on stable, but we need to to consider
+/// it cannot-unwind here. So below we check `fn_can_unwind() && abi_can_unwind()` before concluding
+/// that a function call can unwind.
 fn abi_can_unwind(abi: Abi) -> bool {
     use Abi::*;
     match abi {
@@ -33,9 +37,8 @@ fn abi_can_unwind(abi: Abi) -> bool {
         | RiscvInterruptS
         | CCmseNonSecureCall
         | Wasm
-        | RustIntrinsic
         | Unadjusted => false,
-        Rust | RustCall | RustCold => true,
+        RustIntrinsic | Rust | RustCall | RustCold => unreachable!(), // these ABIs are already skipped earlier
     }
 }
 
@@ -81,14 +84,16 @@ fn has_ffi_unwind_calls(tcx: TyCtxt<'_>, local_def_id: LocalDefId) -> bool {
         let sig = ty.fn_sig(tcx);
 
         // Rust calls cannot themselves create foreign unwinds.
-        if let Abi::Rust | Abi::RustCall | Abi::RustCold = sig.abi() {
+        // We assume this is true for intrinsics as well.
+        if let Abi::RustIntrinsic | Abi::Rust | Abi::RustCall | Abi::RustCold = sig.abi() {
             continue;
         };
 
         let fn_def_id = match ty.kind() {
             ty::FnPtr(_) => None,
             &ty::FnDef(def_id, _) => {
-                // Rust calls cannot themselves create foreign unwinds.
+                // Rust calls cannot themselves create foreign unwinds (even if they use a non-Rust ABI).
+                // So the leak of the foreign unwind into Rust can only be elsewhere, not here.
                 if !tcx.is_foreign_item(def_id) {
                     continue;
                 }

library/core/src/intrinsics.rs

@@ -44,6 +44,15 @@
 //! * Sequentially consistent - sequentially consistent operations are
 //!   guaranteed to happen in order. This is the standard mode for working
 //!   with atomic types and is equivalent to Java's `volatile`.
+//!
+//! # Unwinding
+//!
+//! Rust intrinsics may, in general, unwind. If an intrinsic can never unwind, add the
+//! `#[rustc_nounwind]` attribute so that the compiler can make use of this fact.
+//!
+//! However, even for intrinsics that may unwind, rustc assumes that a Rust intrinsics will never
+//! initiate a foreign (non-Rust) unwind, and thus for panic=abort we can always assume that these
+//! intrinsics cannot unwind.
 
 #![unstable(
     feature = "core_intrinsics",
@@ -692,6 +701,7 @@ extern "rust-intrinsic" {
     /// The stabilized version of this intrinsic is available on the
     /// [`atomic`] signed integer types via the `fetch_min` method by passing
     /// [`Ordering::AcqRel`] as the `order`. For example, [`AtomicI32::fetch_min`].
+    #[rustc_nounwind]
     pub fn atomic_min_acqrel<T: Copy>(dst: *mut T, src: T) -> T;
     /// Minimum with the current value using a signed comparison.
     ///