feat: allow custom fiber API impls on unsupported arches

crates/fiber/Cargo.toml

@@ -39,3 +39,7 @@ backtrace = "0.3.68"
 # Assume presence of the standard library. Allows propagating
 # panic-unwinds across fiber invocations.
 std = []
+
+# Use embedder-provided stack-switching routines on unsupported architectures.
+# See `src/stackswitch/custom.rs` for the C ABI the embedder must implement.
+custom = []

crates/fiber/src/lib.rs

@@ -34,7 +34,9 @@ cfg_if::cfg_if! {
         use unix as imp;
         mod stackswitch;
     } else {
-        compile_error!("fibers are not supported on this platform");
+        mod nostd;
+        use nostd as imp;
+        mod stackswitch;
     }
 }
 

crates/fiber/src/stackswitch.rs

@@ -33,6 +33,10 @@ cfg_if::cfg_if! {
         mod riscv32imac;
         pub(crate) use supported::*;
         pub(crate) use riscv32imac::*;
+    } else if #[cfg(feature = "custom")] {
+        mod custom;
+        pub(crate) use supported::*;
+        pub(crate) use custom::*;
     } else {
         // No support for this platform. Don't fail compilation though and
         // instead defer the error to happen at runtime when a fiber is created.

crates/fiber/src/stackswitch/custom.rs

@@ -0,0 +1,18 @@
+//! Embedder-provided stack-switching routines.
+//!
+//! This module is used when the `custom` feature is enabled and the target
+//! architecture has no built-in inline-assembly implementation. It is the
+//! stack-switching analogue of Wasmtime's `custom-virtual-memory` feature:
+//! rather than relying on inline assembly for a known architecture, the
+//! routines below are declared as `extern "C"` imports which the embedder is
+//! expected to supply.
+
+unsafe extern "C" {
+    pub(crate) fn wasmtime_fiber_init(
+        top_of_stack: *mut u8,
+        entry: extern "C" fn(*mut u8, *mut u8) -> *mut u8,
+        entry_arg0: *mut u8,
+    );
+
+    pub(crate) fn wasmtime_fiber_switch(top_of_stack: *mut u8);
+}

crates/wasmtime/Cargo.toml

@@ -385,6 +385,9 @@ custom-native-signals = []
 # Same as `custom-virtual-memory` above, but for custom sync primitive APIs.
 custom-sync-primitives = []
 
+# Same as `custom-virtual-memory` above, but for custom fiber APIs.
+custom-fiber = ["wasmtime-fiber?/custom"]
+
 # Off-by-default support to profile the Pulley interpreter. This has a
 # performance hit, even when not profiling, so it's disabled by default at
 # compile time.