MIR Gen - Make mutating state (other than the if condition) within a match guard an error

3 files changed, 33 insertions, 1 deletions

diff --git a/src/mir/from_hir.hpp b/src/mir/from_hir.hpp
index 11a18a6d..b11a380a 100644
--- a/src/mir/from_hir.hpp
+++ b/src/mir/from_hir.hpp
@@ -103,6 +103,11 @@ TAGGED_UNION(ScopeType, Owning,
         ::std::map<unsigned int,VarState>    changed_args;
         bool exit_state_valid;
         SplitEnd    exit_state;
+        }),
+    // State which should end up with no mutation of variable states
+    (Freeze, struct {
+        //::std::map<unsigned int,VarState>    changed_slots;
+        //::std::map<unsigned int,VarState>    changed_args;
         })
     );
 
@@ -257,6 +262,7 @@ public:
     ScopeHandle new_scope_temp(const Span& sp);
     ScopeHandle new_scope_split(const Span& sp);
     ScopeHandle new_scope_loop(const Span& sp);
+    ScopeHandle new_scope_freeze(const Span& sp);
 
     /// Raises every variable defined in the source scope into the target scope
     void raise_all(const Span& sp, ScopeHandle src, const ScopeHandle& target);
diff --git a/src/mir/from_hir_match.cpp b/src/mir/from_hir_match.cpp
index cbb39b34..c10f170b 100644
--- a/src/mir/from_hir_match.cpp
+++ b/src/mir/from_hir_match.cpp
@@ -372,17 +372,18 @@ void MIR_LowerHIR_Match( MirBuilder& builder, MirConverter& conv, ::HIR::ExprNod
             ac.cond_start = builder.new_bb_unlinked();
             builder.set_cur_block( ac.cond_start );
 
+            auto freeze_scope = builder.new_scope_freeze(arm.m_cond->span());
             auto tmp_scope = builder.new_scope_temp(arm.m_cond->span());
             conv.visit_node_ptr( arm.m_cond );
             auto cond_lval = builder.get_result_in_if_cond(arm.m_cond->span());
             builder.terminate_scope( arm.m_code->span(), mv$(tmp_scope) );
             ac.cond_false = builder.new_bb_unlinked();
             builder.end_block(::MIR::Terminator::make_If({ mv$(cond_lval), ac.code, ac.cond_false }));
-            // TODO: Emit the `if` (to new blocks) and insert an early termination of the this split arm
 
             builder.set_cur_block(ac.cond_false);
             builder.end_split_arm(arm.m_cond->span(), match_scope, true, true);
             builder.pause_cur_block();
+            builder.terminate_scope( arm.m_code->span(), mv$(freeze_scope) );
 
             // NOTE: Paused so that later code (which knows what the false branch will be) can end it correctly
 
diff --git a/src/mir/mir_builder.cpp b/src/mir/mir_builder.cpp
index 54269144..5dcb2ed2 100644
--- a/src/mir/mir_builder.cpp
+++ b/src/mir/mir_builder.cpp
@@ -745,6 +745,14 @@ ScopeHandle MirBuilder::new_scope_loop(const Span& sp)
     DEBUG("START (loop) scope " << idx);
     return ScopeHandle { *this, idx };
 }
+ScopeHandle MirBuilder::new_scope_freeze(const Span& sp)
+{
+    unsigned int idx = m_scopes.size();
+    m_scopes.push_back( ScopeDef {sp, ScopeType::make_Freeze({})} );
+    m_scope_stack.push_back( idx );
+    DEBUG("START (freeze) scope " << idx);
+    return ScopeHandle { *this, idx };
+}
 void MirBuilder::terminate_scope(const Span& sp, ScopeHandle scope, bool emit_cleanup/*=true*/)
 {
     TRACE_FUNCTION_F("DONE scope " << scope.idx << " - " << (emit_cleanup ? "CLEANUP" : "NO CLEANUP"));
@@ -1510,6 +1518,9 @@ void MirBuilder::complete_scope(ScopeDef& sd)
         DEBUG("Loop");
         ),
     (Split,
+        ),
+    (Freeze,
+        //DEBUG("Freeze");
         )
     )
 
@@ -1886,6 +1897,18 @@ VarState& MirBuilder::get_slot_state_mut(const Span& sp, unsigned int idx, SlotT
                 }
             }
         }
+        else if( scope_def.data.is_Freeze() )
+        {
+            if( type == SlotType::Local && idx == m_if_cond_lval.as_Local() )
+            {
+            }
+            else
+            {
+                // NOTE: This is only used in match conditions
+                DEBUG("Mutating state of ?" << idx);
+                ERROR(sp, E0000, "Attempting to move/initialise a value where not allowed");
+            }
+        }
         else
         {
         }
@@ -2159,6 +2182,8 @@ void MirBuilder::drop_scope_values(const ScopeDef& sd)
         ),
     (Loop,
         // No values
+        ),
+    (Freeze,
         )
     )
 }