MIR Gen - Support zero-arm matches - Since they can never be reached, emit a Diverge

3 files changed, 24 insertions, 1 deletions

diff --git a/src/mir/from_hir.cpp b/src/mir/from_hir.cpp
index a281cbaa..c6d9bcd3 100644
--- a/src/mir/from_hir.cpp
+++ b/src/mir/from_hir.cpp
@@ -378,7 +378,10 @@ namespace {
             
             if( node.m_arms.size() == 0 ) {
                 // Nothing
-                TODO(node.span(), "Handle zero-arm match - can only match over ! or similar");
+                //const auto& ty = node.m_value->m_res_type;
+                // TODO: Ensure that the type is a zero-variant enum or !
+                m_builder.end_split_arm_early(node.span());
+                m_builder.end_block( ::MIR::Terminator::make_Diverge({}) );
             }
             else if( node.m_arms.size() == 1 && node.m_arms[0].m_patterns.size() == 1 && ! node.m_arms[0].m_cond ) {
                 // - Shortcut: Single-arm match
diff --git a/src/mir/from_hir.hpp b/src/mir/from_hir.hpp
index c915cfcc..39890bbf 100644
--- a/src/mir/from_hir.hpp
+++ b/src/mir/from_hir.hpp
@@ -167,6 +167,7 @@ public:
     void terminate_scope(const Span& sp, ScopeHandle );
     void terminate_scope_early(const Span& sp, const ScopeHandle& );
     void end_split_arm(const Span& sp, const ScopeHandle& , bool reachable);
+    void end_split_arm_early(const Span& sp);
     
     const ScopeHandle& fcn_scope() const {
         return m_fcn_scope;
diff --git a/src/mir/mir_builder.cpp b/src/mir/mir_builder.cpp
index f4f9db7e..c5564045 100644
--- a/src/mir/mir_builder.cpp
+++ b/src/mir/mir_builder.cpp
@@ -480,6 +480,25 @@ void MirBuilder::end_split_arm(const Span& sp, const ScopeHandle& handle, bool r
     // TODO: Undo moves from within the other scope
     sd_split.arms.push_back( {} );
 }
+void MirBuilder::end_split_arm_early(const Span& sp)
+{
+    // Terminate all scopes until a split is found.
+    while( ! m_scope_stack.empty() && ! m_scopes.at( m_scope_stack.back() ).data.is_Split() )
+    {
+        auto& scope_def = m_scopes[m_scope_stack.back()];
+        // Fully drop the scope
+        drop_scope_values(scope_def);
+        m_scope_stack.pop_back();
+        complete_scope(scope_def);
+    }
+    
+    if( !m_scope_stack.empty() )
+    {
+        auto& sd = m_scopes[ m_scope_stack.back() ];
+        auto& sd_split = sd.data.as_Split();
+        sd_split.arms.back().has_early_terminated = true;
+    }
+}
 void MirBuilder::complete_scope(ScopeDef& sd)
 {
     sd.complete = true;