add unreachable instruction to the mono IR

compiler/gen/src/llvm/build.rs

@@ -1366,6 +1366,13 @@ pub fn build_exp_stmt<'a, 'ctx, 'env>(
             value
         }
 
+        Unreachable => {
+            // used in exception handling
+            env.builder.build_unreachable();
+
+            env.context.i64_type().const_zero().into()
+        }
+
         Switch {
             branches,
             default_branch,

compiler/gen_dev/src/lib.rs

@@ -335,6 +335,7 @@ where
             Stmt::Ret(sym) => {
                 self.set_last_seen(*sym, stmt);
             }
+            Stmt::Unreachable => {}
             Stmt::Inc(sym, following) => {
                 self.set_last_seen(*sym, stmt);
                 self.scan_ast(following);

compiler/mono/src/borrow.rs

@@ -166,7 +166,7 @@ impl<'a> ParamMap<'a> {
                 }
                 Inc(_, _) | Dec(_, _) => unreachable!("these have not been introduced yet"),
 
-                Ret(_) | Jump(_, _) | RuntimeError(_) => {
+                Ret(_) | Unreachable | Jump(_, _) | RuntimeError(_) => {
                     // these are terminal, do nothing
                 }
             }
@@ -470,7 +470,7 @@ impl<'a> BorrowInfState<'a> {
             }
             Inc(_, _) | Dec(_, _) => unreachable!("these have not been introduced yet"),
 
-            Ret(_) | RuntimeError(_) => {
+            Ret(_) | RuntimeError(_) | Unreachable => {
                 // these are terminal, do nothing
             }
         }

compiler/mono/src/inc_dec.rs

@@ -35,6 +35,8 @@ pub fn occuring_variables(stmt: &Stmt<'_>) -> (MutSet<Symbol>, MutSet<Symbol>) {
                 result.insert(*symbol);
             }
 
+            Unreachable => {}
+
             Inc(symbol, cont) | Dec(symbol, cont) => {
                 result.insert(*symbol);
                 stack.push(cont);
@@ -673,6 +675,8 @@ impl<'a> Context<'a> {
                 }
             }
 
+            Unreachable => (stmt, MutSet::default()),
+
             Jump(j, xs) => {
                 let empty = MutSet::default();
                 let j_live_vars = match self.jp_live_vars.get(j) {
@@ -813,6 +817,8 @@ pub fn collect_stmt(
             vars
         }
 
+        Unreachable => vars,
+
         RuntimeError(_) => vars,
     }
 }

compiler/mono/src/ir.rs

@@ -754,6 +754,7 @@ pub enum Stmt<'a> {
         ret_layout: Layout<'a>,
     },
     Ret(Symbol),
+    Unreachable,
     Inc(Symbol, &'a Stmt<'a>),
     Dec(Symbol, &'a Stmt<'a>),
     Join {
@@ -1103,6 +1104,8 @@ impl<'a> Stmt<'a> {
                 .append(symbol_to_doc(alloc, *symbol))
                 .append(";"),
 
+            Unreachable => alloc.text("unreachable;"),
+
             Switch {
                 cond_symbol,
                 branches,
@@ -4436,6 +4439,8 @@ fn substitute_in_stmt_help<'a>(
             }
         }
 
+        Unreachable => None,
+
         RuntimeError(_) => None,
     }
 }

compiler/mono/src/tail_recursion.rs

@@ -187,6 +187,7 @@ fn insert_jumps<'a>(
             None => None,
         },
 
+        Unreachable => None,
         Ret(_) => None,
         Jump(_, _) => None,
         RuntimeError(_) => None,