[lang] Raise an error when struct-for indices number mismatch

python/taichi/lang/impl.py

@@ -59,6 +59,16 @@ def expr_init_func(rhs):  # temporary solution to allow passing in tensors as
     return expr_init(rhs)
 
 
+def begin_frontend_struct_for(group, loop_range):
+    if group.size() != len(loop_range.shape):
+        raise IndexError(
+            'Number of struct-for indices does not match loop variable dimensionality '
+            f'({group.size()} != {len(loop_range.shape)}). Maybe you wanted to '
+            'use "for I in ti.grouped(x)" to group all indices into a single vector I?'
+        )
+    taichi_lang_core.begin_frontend_struct_for(group, loop_range.ptr)
+
+
 def wrap_scalar(x):
     if type(x) in [int, float]:
         return Expr(x)

python/taichi/lang/transformer.py

@@ -437,7 +437,7 @@ def visit_struct_for(self, node, is_grouped):
     ___loop_var = 0
     {} = ti.make_var_vector(size=len(___loop_var.loop_range().shape))
     ___expr_group = ti.make_expr_group({})
-    ti.core.begin_frontend_struct_for(___expr_group, ___loop_var.loop_range().ptr)
+    ti.begin_frontend_struct_for(___expr_group, ___loop_var.loop_range())
     ti.core.end_frontend_range_for()
             '''.format(vars, vars)
             t = ast.parse(template).body[0]
@@ -450,7 +450,7 @@ def visit_struct_for(self, node, is_grouped):
 {}
     ___loop_var = 0
     ___expr_group = ti.make_expr_group({})
-    ti.core.begin_frontend_struct_for(___expr_group, ___loop_var.loop_range().ptr)
+    ti.begin_frontend_struct_for(___expr_group, ___loop_var.loop_range())
     ti.core.end_frontend_range_for()
             '''.format(var_decl, vars)
             t = ast.parse(template).body[0]

taichi/transforms/simplify.cpp

@@ -1176,7 +1176,9 @@ class Simplify : public IRVisitor {
   }
 
   void visit(StructForStmt *for_stmt) override {
-    TI_ASSERT(current_struct_for == nullptr);
+    TI_ASSERT_INFO(current_struct_for == nullptr,
+                   "Nested struct-fors are not supported for now. "
+                   "Please try to use range-fors for inner loops.");
     current_struct_for = for_stmt;
     for_stmt->body->accept(this);
     current_struct_for = nullptr;

tests/python/test_for_group_mismatch.py

@@ -0,0 +1,96 @@
+import taichi as ti
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def test_struct_for_mismatch():
+    x = ti.var(ti.f32, (3, 4))
+
+    @ti.kernel
+    def func():
+        for i in x:
+            print(i)
+
+    func()
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def test_struct_for_mismatch2():
+    x = ti.var(ti.f32, (3, 4))
+
+    @ti.kernel
+    def func():
+        for i, j, k in x:
+            print(i, j, k)
+
+    func()
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def _test_grouped_struct_for_mismatch():
+    # doesn't work for now
+    # need grouped refactor
+    # for now, it just throw a unfriendly message:
+    # AssertionError: __getitem__ cannot be called in Python-scope
+    x = ti.var(ti.f32, (3, 4))
+
+    @ti.kernel
+    def func():
+        for i, j in ti.grouped(x):
+            print(i, j)
+
+    func()
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def _test_ndrange_for_mismatch():
+    # doesn't work for now
+    # need ndrange refactor
+    @ti.kernel
+    def func():
+        for i in ti.ndrange(3, 4):
+            print(i)
+
+    func()
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def _test_ndrange_for_mismatch2():
+    # doesn't work for now
+    # need ndrange and grouped refactor
+    @ti.kernel
+    def func():
+        for i, j, k in ti.ndrange(3, 4):
+            print(i, j, k)
+
+    func()
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def _test_grouped_ndrange_for_mismatch():
+    # doesn't work for now
+    # need ndrange and grouped refactor
+    @ti.kernel
+    def func():
+        for i in ti.grouped(ti.ndrange(3, 4)):
+            print(i)
+
+    func()
+
+
+@ti.must_throw(IndexError)
+@ti.host_arch_only
+def _test_static_ndrange_for_mismatch():
+    # doesn't work for now
+    # need ndrange and static refactor
+    @ti.kernel
+    def func():
+        for i in ti.static(ti.ndrange(3, 4)):
+            print(i)
+
+    func()