Apply suggestions from code review

Co-authored-by: rmsyn <117854522+rmsyn@users.noreply.github.com>

Author: romancardenas

riscv-rt/src/exceptions.rs

@@ -42,12 +42,8 @@ pub static __EXCEPTIONS: [Option<unsafe extern "C" fn(&TrapFrame)>; 16] = [
 #[export_name = "_dispatch_exception"]
 #[inline]
 unsafe extern "C" fn dispatch_exception(trap_frame: &TrapFrame, code: usize) {
-    if code < __EXCEPTIONS.len() {
-        match &__EXCEPTIONS[code] {
-            Some(handler) => handler(trap_frame),
-            None => ExceptionHandler(trap_frame),
-        }
-    } else {
-        ExceptionHandler(trap_frame);
+    match __EXCEPTIONS.get(code) {
+        Some(Some(handler)) => handler(trap_frame),
+        _ => ExceptionHandler(trap_frame),
     }
 }

riscv-rt/src/interrupts.rs

@@ -28,13 +28,9 @@ pub static __CORE_INTERRUPTS: [Option<unsafe extern "C" fn()>; 12] = [
 #[export_name = "_dispatch_core_interrupt"]
 #[inline]
 unsafe extern "C" fn dispatch_core_interrupt(code: usize) {
-    if code < __CORE_INTERRUPTS.len() {
-        match &__CORE_INTERRUPTS[code] {
-            Some(handler) => handler(),
-            None => DefaultHandler(),
-        }
-    } else {
-        DefaultHandler();
+    match __CORE_INTERRUPTS.get(code) {
+        Some(Some(handler)) => handler(),
+        _ => DefaultHandler(),
     }
 }
 

riscv/macros/src/lib.rs

@@ -121,20 +121,17 @@ impl PacEnumItem {
         };
         for v in variants.iter() {
             let ident = v.ident.clone();
-            let value = match &v.discriminant {
-                Some(d) => match &d.1 {
-                    syn::Expr::Lit(expr_lit) => match &expr_lit.lit {
-                        syn::Lit::Int(lit_int) => match lit_int.base10_parse::<usize>() {
-                            Ok(num) => num,
-                            Err(_) => {
-                                panic!("All variant discriminants must be unsigned integers")
-                            }
-                        },
-                        _ => panic!("All variant discriminants must be unsigned integers"),
-                    },
+           let value = match v.discriminant.as_ref() {
+                Some((_, syn::Expr::Lit(expr_lit))) => match &expr_lit.lit {
+                    syn::Lit::Int(lit_int) => {
+                        lit_int.base10_parse::<usize>().unwrap_or_else(|_| {
+                            panic!("All variant discriminants must be unsigned integers")
+                        })
+                    }
                     _ => panic!("All variant discriminants must be unsigned integers"),
                 },
-                _ => panic!("Variant must have a discriminant"),
+                None => panic!("Variant must have a discriminant"),
+                _ => panic!("All variant discriminants must be literal expressions"),
             };
 
             if numbers.insert(value, ident).is_some() {

riscv/src/interrupt/machine.rs

@@ -192,10 +192,12 @@ where
     }
 
     // Restore MSTATUS.PIE, MSTATUS.MPP, and SEPC
+    let mut after_mstatus = mstatus::read();
     if mstatus.mpie() {
-        mstatus::set_mpie();
+        after_mstatus.set_mpie(mstatus.mpie());
     }
-    mstatus::set_mpp(mstatus.mpp());
+    after_mstatus.set_mpp(mstatus.mpp());
+    mstatus::write(after_mstatus);
     mepc::write(mepc);
 
     r