[Fix][TVMScript] Fix LetStmt printing logic (#13900)

This PR is the bug fix reported in #13892. Initially, we mix the logic of `LetStmt` docsifying method with and without concise scoping. For example, in
```python
x = T.var("int32")
with T.let(x, 0):
```
`x` in the `LetStmt` works as a right value, while in
```python
x: T.int32 = 0
```
`x` in the `LetStmt` works as a left value as result.
Our old logic mixed them together to generate the wrong code for the first case.
Meanwhile, during the fix, we found another bug in concise scoping check. For example, we have
```python
x = T.var("int32")
y = T.var("int32")
with T.let(x, y):
  with T.let(y, 0):
```
here we should not output
```python
x = T.var("int32")
y = T.var("int32")
with T.let(x, y):
  y: int32 = 0
```
becase this will define a new `y_1: int32 = 0` indeed, due the the variable shadowing logic of the parser, which is different from the `y` we define and refer to.
Our concise scoping `v: ... = ...` should launch if and only if the `v` is never defined before.
Otherwise, we use `with T.let(v, ...):` instead.

src/script/printer/tir/stmt.cc

@@ -57,13 +57,12 @@ TVM_STATIC_IR_FUNCTOR(IRDocsifier, vtable)
 TVM_STATIC_IR_FUNCTOR(IRDocsifier, vtable)
     .set_dispatch<tir::LetStmt>("", [](tir::LetStmt stmt, ObjectPath p, IRDocsifier d) -> Doc {
       bool concise = AllowConciseScoping(d);
-      ExprDoc rhs = d->AsDoc<ExprDoc>(stmt->value, p->Attr("value"));
-      With<TIRFrame> f(d, stmt);
-      ExprDoc lhs = d->IsVarDefined(stmt->var) ? d->GetVarDoc(stmt->var).value()
-                                               : DefineVar(stmt->var, *f, d);
-      AsDocBody(stmt->body, p->Attr("body"), f->get(), d);
-      Array<StmtDoc>* stmts = &(*f)->stmts;
-      if (concise) {
+      if (concise && !d->IsVarDefined(stmt->var)) {
+        ExprDoc rhs = d->AsDoc<ExprDoc>(stmt->value, p->Attr("value"));
+        With<TIRFrame> f(d, stmt);
+        ExprDoc lhs = DefineVar(stmt->var, *f, d);
+        AsDocBody(stmt->body, p->Attr("body"), f->get(), d);
+        Array<StmtDoc>* stmts = &(*f)->stmts;
         Type type = stmt->var->type_annotation;
         Optional<ExprDoc> type_doc =
             d->AsDoc<ExprDoc>(type, p->Attr("var")->Attr("type_annotation"));
@@ -75,6 +74,11 @@ TVM_STATIC_IR_FUNCTOR(IRDocsifier, vtable)
         stmts->insert(stmts->begin(), AssignDoc(lhs, rhs, type_doc));
         return StmtBlockDoc(*stmts);
       } else {
+        ExprDoc lhs = d->AsDoc<ExprDoc>(stmt->var, p->Attr("var"));
+        ExprDoc rhs = d->AsDoc<ExprDoc>(stmt->value, p->Attr("value"));
+        With<TIRFrame> f(d, stmt);
+        AsDocBody(stmt->body, p->Attr("body"), f->get(), d);
+        Array<StmtDoc>* stmts = &(*f)->stmts;
         rhs = TIR(d, "let")->Call({lhs, rhs});
         return ScopeDoc(NullOpt, rhs, *stmts);
       }

tests/python/unittest/test_tvmscript_printer_tir.py

@@ -254,6 +254,7 @@ def test_let_stmt():
     _assert_print(
         obj,
         """
+v = T.var("float32")
 with T.let(v, T.float32(10)):
     T.evaluate(0)
 """,

tests/python/unittest/test_tvmscript_roundtrip.py

@@ -3543,6 +3543,32 @@ def func():
     return func
 
 
+def let_stmt_var():
+    @T.prim_func
+    def func():
+        x = T.var("int32")
+        y = T.var("int32")
+        with T.let(x, 0):
+            with T.let(y, 0):
+                T.evaluate(0)
+        T.evaluate(0)
+
+    return func
+
+
+def let_stmt_value():
+    @T.prim_func
+    def func():
+        x = T.var("int32")
+        y = T.var("int32")
+        with T.let(x, y):
+            with T.let(y, 0):
+                T.evaluate(0)
+        T.evaluate(0)
+
+    return func
+
+
 ir_generator = tvm.testing.parameter(
     opt_gemm_normalize,
     opt_gemm_lower,
@@ -3601,6 +3627,8 @@ def func():
     *nested_boolean_expressions(),
     multi_env_threads,
     intrinsic_pow,
+    let_stmt_var,
+    let_stmt_value,
 )