Disallow calling rfactor() after fusing a pure var

Fixes #7854

src/Func.cpp

@@ -841,21 +841,6 @@ Func Stage::rfactor(vector<pair<RVar, Var>> preserved) {
                 << ", since it is already used in this Func's schedule elsewhere.\n"
                 << dump_argument_list();
         }
-        {
-            // Check that the RVar has not had a pure var fused into it
-            for (const Split &s : splits) {
-                if (s.is_fuse() && var_name_match(s.old_var, rv.name())) {
-                    const auto &iter = std::find_if(pure_vars_used.begin(), pure_vars_used.end(), [&s](const std::string &v) {
-                        return (var_name_match(v, s.outer) || var_name_match(v, s.inner));
-                    });
-                    user_assert(iter == pure_vars_used.end())
-                        << "In schedule for " << name()
-                        << ", can't perform rfactor() on " << rv.name()
-                        << " because the pure var " << (*iter)
-                        << " is fused into it.\n";
-                }
-            }
-        }
     }
 
     // If the operator is associative but non-commutative, rfactor() on inner
@@ -885,6 +870,10 @@ Func Stage::rfactor(vector<pair<RVar, Var>> preserved) {
         for (const Split &s : splits) {
             // If it's already applied, we should remove it from the split list.
             if (!apply_split_directive(s, rvars, predicates, args, values)) {
+                user_assert(!s.is_fuse())
+                    << "In schedule for " << name()
+                    << ", can't perform rfactor() after fusing " << s.outer
+                    << " and " << s.inner << "\n";
                 temp.push_back(s);
             }
         }

test/correctness/rfactor.cpp

@@ -992,23 +992,6 @@ int self_assignment_rfactor_test() {
     return 0;
 }
 
-int rfactor_with_partial_pure_fusion_test() {
-    Func f{"f"};
-    Var x{"x"}, y{"y"}, z{"z"};
-    RDom r({{0, 5}, {0, 5}, {0, 5}}, "r");
-    RVar rxy{"rxy"}, yrz{"yrz"};
-
-    f(x, y) = 0;
-    f(x, y) += r.x + r.y + r.z;
-
-    f.update()
-        .fuse(r.x, r.y, rxy)
-        .fuse(r.z, y, yrz)
-        .rfactor(rxy, z);
-
-    return 0;
-}
-
 }  // namespace
 
 int main(int argc, char **argv) {
@@ -1049,7 +1032,6 @@ int main(int argc, char **argv) {
         {"rfactor tile reorder test: checking output img correctness...", rfactor_tile_reorder_test},
         {"complex multiply rfactor test", complex_multiply_rfactor_test},
         {"argmin rfactor test", argmin_rfactor_test},
-        {"rfactor a fused rvar+rvar in the presence of a fused var+rvar", rfactor_with_partial_pure_fusion_test},
     };
 
     using Sharder = Halide::Internal::Test::Sharder;

test/error/CMakeLists.txt

@@ -94,6 +94,7 @@ tests(GROUPS error
       require_fail.cpp
       reuse_var_in_schedule.cpp
       reused_args.cpp
+      rfactor_after_var_and_rvar_fusion.cpp
       rfactor_fused_var_and_rvar.cpp
       rfactor_inner_dim_non_commutative.cpp
       round_up_and_blend_race.cpp

test/error/rfactor_after_var_and_rvar_fusion.cpp

@@ -0,0 +1,25 @@
+#include "Halide.h"
+
+using namespace Halide;
+
+int main(int argc, char **argv) {
+    Func f{"f"};
+    RDom r({{0, 5}, {0, 5}, {0, 5}}, "r");
+    Var x{"x"}, y{"y"};
+    f(x, y) = 0;
+    f(x, y) += r.x + r.y + r.z;
+
+    RVar rxy{"rxy"}, yrz{"yrz"};
+    Var z{"z"};
+
+    // Error: In schedule for f.update(0), can't perform rfactor() after fusing y and r$z
+    f.update()
+        .fuse(r.x, r.y, rxy)
+        .fuse(r.z, y, yrz)
+        .rfactor(rxy, z);
+
+    f.print_loop_nest();
+
+    printf("Success!\n");
+    return 0;
+}

test/error/rfactor_fused_var_and_rvar.cpp

@@ -12,7 +12,7 @@ int main(int argc, char **argv) {
     RVar rxy{"rxy"}, yrz{"yrz"}, yr{"yr"};
     Var z{"z"};
 
-    // Error: In schedule for f.update(0), can't perform rfactor() on yr because the pure var y is fused into it.
+    // Error: In schedule for f.update(0), can't perform rfactor() after fusing r$z and y
     f.update()
         .fuse(r.x, r.y, rxy)
         .fuse(y, r.z, yrz)