Fix unimplemented backend error

Cargo.toml

@@ -36,7 +36,7 @@ harness = false
 
 [lints.rust.unexpected_cfgs]
 level = "warn"
-check-cfg = ["cfg(nightly)", "cfg(backend, values(\"itanium\", \"seh\", \"panic\"))"]
+check-cfg = ["cfg(nightly)", "cfg(backend, values(\"itanium\", \"seh\", \"panic\", \"unimplemented\"))"]
 
 [profile.dev]
 panic = "unwind"

build.rs

@@ -31,6 +31,9 @@ fn main() {
     {
         println!("cargo::rustc-cfg=backend=\"seh\"");
     } else {
+        #[cfg(feature = "std")]
         println!("cargo::rustc-cfg=backend=\"panic\"");
+        #[cfg(not(feature = "std"))]
+        println!("cargo::rustc-cfg=backend=\"unimplemented\"");
     }
 }

src/backend/mod.rs

@@ -169,11 +169,11 @@ mod imp;
 #[path = "seh.rs"]
 mod imp;
 
-#[cfg(all(backend = "panic", feature = "std"))]
+#[cfg(backend = "panic")]
 #[path = "panic.rs"]
 mod imp;
 
-#[cfg(all(backend = "panic", not(feature = "std")))]
+#[cfg(backend = "unimplemented")]
 #[path = "unimplemented.rs"]
 mod imp;
 

src/backend/unimplemented.rs

@@ -1,19 +1,27 @@
-use super::Backend;
+use super::{RethrowHandle, ThrowByValue};
 
 pub(crate) struct ActiveBackend;
 
 compile_error!("Lithium does not support no_std in this configuration");
 
-unsafe impl Backend for ActiveBackend {
-    type ExceptionHeader = ();
+unsafe impl ThrowByValue for ActiveBackend {
+    type RethrowHandle<E> = UnimplementedRethrowHandle;
 
-    fn new_header() {}
+    unsafe fn throw<E>(_cause: E) -> ! {
+        unimplemented!()
+    }
 
-    unsafe fn throw(_ex: *mut ()) -> ! {
+    unsafe fn intercept<Func: FnOnce() -> R, R, E>(
+        _func: Func,
+    ) -> Result<R, (E, Self::RethrowHandle<E>)> {
         unimplemented!()
     }
+}
+
+pub(crate) struct UnimplementedRethrowHandle;
 
-    fn intercept<Func: FnOnce() -> R, R>(_func: Func) -> Result<R, *mut ()> {
+impl RethrowHandle for UnimplementedRethrowHandle {
+    unsafe fn rethrow<F>(self, _new_cause: F) -> ! {
         unimplemented!()
     }
 }