Enable a panic-free Jefe for the first time

The supervisor task in Hubris is not permitted to panic, since it's
responsible for handling panics.

Jefe has historically contained a bunch of (static) panics, many of
which aren't actually possible at runtime. I've been gradually grinding
away at these in my other PRs.

As of #1937, it's now possible to build a _minimal_ Jefe (like we use on
the G0) that contains no panics. So I've enabled that on donglet, and
turned on the userlib/no-panic feature that will statically ensure it
remains true.

Turning on dump support in Jefe causes a bunch of panics to reappear,
because humpty is panic-heavy. That's a task for another day.

app/donglet/app-g031.toml

@@ -17,6 +17,7 @@ max-sizes = {flash = 4096, ram = 512}
 start = true
 stacksize = 368
 notifications = ["fault", "timer"]
+features = ["no-panic"]
 
 [tasks.sys]
 name = "drv-stm32xx-sys"

task/jefe/Cargo.toml

@@ -31,6 +31,7 @@ build-util = { path = "../../build/util" }
 [features]
 dump = []
 nano = [ "ringbuf/disabled" ]
+no-panic = [ "userlib/no-panic" ]
 
 # This section is here to discourage RLS/rust-analyzer from doing test builds,
 # since test builds don't work for cross compilation.

task/jefe/src/main.rs

@@ -310,9 +310,20 @@ impl idol_runtime::NotificationHandler for ServerImpl<'_> {
             let mut next_task = 1;
             while let Some(fault_index) = kipc::find_faulted_task(next_task) {
                 let fault_index = usize::from(fault_index);
-                next_task = fault_index + 1;
-
-                let status = &mut self.task_states[fault_index];
+                // This addition cannot overflow in practice, because the number
+                // of tasks in the system is very much smaller than 2**32. So we
+                // use wrapping add, because currently the compiler doesn't
+                // understand this property.
+                next_task = fault_index.wrapping_add(1);
+
+                // Safety: `fault_index` is from the kernel, and the kernel will
+                // not give us an out-of-range task index.
+                //
+                // TODO: it might be nice to fold this into a utility function
+                // in kipc or something
+                let status = unsafe {
+                    self.task_states.get_unchecked_mut(fault_index)
+                };
 
                 // If we're aware that this task is in a fault state, don't
                 // bother making a syscall to enquire.