improve fn pointer signature check to allow some casts that should be permitted

Also properly check the "non-capturing Fn to fn" case

src/terminator/mod.rs

@@ -72,15 +72,8 @@ pub(super) fn eval_terminator(
                             ty::TyFnDef(_, _, real_sig) => {
                                 let sig = self.erase_lifetimes(&sig);
                                 let real_sig = self.erase_lifetimes(&real_sig);
-                                match instance.def {
-                                    // FIXME: this needs checks for weird transmutes
-                                    // we need to bail here, because noncapturing closures as fn ptrs fail the checks
-                                    ty::InstanceDef::ClosureOnceShim{..} => {}
-                                    _ => if sig.abi != real_sig.abi ||
-                                        sig.variadic != real_sig.variadic ||
-                                        sig.inputs_and_output != real_sig.inputs_and_output {
-                                        return Err(EvalError::FunctionPointerTyMismatch(real_sig, sig));
-                                    },
+                                if !self.check_sig_compat(sig, real_sig)? {
+                                    return Err(EvalError::FunctionPointerTyMismatch(real_sig, sig));
                                 }
                             },
                             ref other => bug!("instance def ty: {:?}", other),
@@ -138,6 +131,69 @@ pub(super) fn eval_terminator(
         Ok(())
     }
 
+    /// Decides whether it is okay to call the method with signature `real_sig` using signature `sig`
+    fn check_sig_compat(
+        &mut self,
+        sig: ty::FnSig<'tcx>,
+        real_sig: ty::FnSig<'tcx>,
+    ) -> EvalResult<'tcx, bool> {
+        fn check_ty_compat<'tcx>(
+            ty: ty::Ty<'tcx>,
+            real_ty: ty::Ty<'tcx>,
+        ) -> bool {
+            if ty == real_ty { return true; } // This is actually a fast pointer comparison
+            return match (&ty.sty, &real_ty.sty) {
+                // Permit changing the pointer type of raw pointers and references as well as
+                // mutability of raw pointers.
+                // TODO: Should not be allowed when fat pointers are involved.
+                (&TypeVariants::TyRawPtr(_), &TypeVariants::TyRawPtr(_)) => true,
+                (&TypeVariants::TyRef(_, _), &TypeVariants::TyRef(_, _)) =>
+                    ty.is_mutable_pointer() == real_ty.is_mutable_pointer(),
+                // rule out everything else
+                _ => false
+            }
+        }
+
+        if sig.abi == real_sig.abi &&
+            sig.variadic == real_sig.variadic &&
+            sig.inputs_and_output.len() == real_sig.inputs_and_output.len() &&
+            sig.inputs_and_output.iter().zip(real_sig.inputs_and_output).all(|(ty, real_ty)| check_ty_compat(ty, real_ty)) {
+            // Definitely good.
+            return Ok(true);
+        }
+
+        if sig.variadic || real_sig.variadic {
+            // We're not touching this
+            return Ok(false);
+        }
+
+        // We need to allow what comes up when a non-capturing closure is cast to a fn().
+        match (sig.abi, real_sig.abi) {
+            (Abi::Rust, Abi::RustCall) // check the ABIs.  This makes the test here non-symmetric.
+                if check_ty_compat(sig.output(), real_sig.output()) && real_sig.inputs_and_output.len() == 3 => {
+                // First argument of real_sig must be a ZST
+                let fst_ty = real_sig.inputs_and_output[0];
+                let layout = self.type_layout(fst_ty)?;
+                let size = layout.size(&self.tcx.data_layout).bytes();
+                if size == 0 {
+                    // Second argument must be a tuple matching the argument list of sig
+                    let snd_ty = real_sig.inputs_and_output[1];
+                    match snd_ty.sty {
+                        TypeVariants::TyTuple(tys, _) if sig.inputs().len() == tys.len() =>
+                            if sig.inputs().iter().zip(tys).all(|(ty, real_ty)| check_ty_compat(ty, real_ty)) {
+                                return Ok(true)
+                            },
+                        _ => {}
+                    }
+                }
+            }
+            _ => {}
+        };
+
+        // Nope, this doesn't work.
+        return Ok(false);
+    }
+
     fn eval_fn_call(
         &mut self,
         instance: ty::Instance<'tcx>,

tests/compile-fail/cast_fn_ptr2.rs

+fn main() {
+    fn f(_ : (i32,i32)) {}
+
+    let g = unsafe {
+        std::mem::transmute::<fn((i32,i32)), fn(i32)>(f)
+    };
+
+    g(42) //~ ERROR tried to call a function with sig fn((i32, i32)) through a function pointer of type fn(i32)
+}

tests/run-pass/cast_fn_ptr.rs

+fn main() {
+    fn f(_: *const u8) {}
+
+    let g = unsafe {
+        std::mem::transmute::<fn(*const u8), fn(*const i32)>(f)
+    };
+
+    g(&42 as *const _);
+}