feat: allow using the throwaway ('_') var

examples/sizes.txt

@@ -73,6 +73,7 @@
   stubs/String                                     777       141    636       141      0 
   stubs/Uint64                                     371         8    363         8      0 
   template_variables/TemplateVariables             168       155     13       155      0 
+  throwaway/Throwaway                               45        31     14        31      0 
   tictactoe/TicTacToe                              894       692    202       680     12 
   too_many_permutations                            108       106      2       106      0 
   transaction/Transaction                          893       849     44       849      0 

src/puya/awst/nodes.py

@@ -1021,7 +1021,7 @@ class AssignmentStatement(Statement):
     value: Expression = attrs.field(validator=[lvalue_expr_validator])
 
     def __attrs_post_init__(self) -> None:
-        if self.value.wtype != self.target.wtype:
+        if not wtypes.is_valid_assignment(self.target.wtype, self.value.wtype):
             raise CodeError(
                 f"Assignment target type {self.target.wtype}"
                 f" differs from expression value type {self.value.wtype}",
@@ -1052,7 +1052,7 @@ def __init__(self, value: Expression, target: Lvalue, source_location: SourceLoc
                 "Tuple unpacking in assignment expressions is not supported",
                 target.source_location,
             )
-        if value.wtype != target.wtype:
+        if not wtypes.is_valid_assignment(target.wtype, value.wtype):
             raise CodeError(
                 f"Assignment target type {target.wtype}"
                 f" differs from expression value type {value.wtype}",

src/puya/awst/wtypes.py

@@ -75,6 +75,11 @@ def is_valid_utf8_literal(value: object) -> typing.TypeGuard[str]:
     lvalue=False,
 )
 
+throwaway_type: typing.Final = WType(
+    name="throwaway",
+    stub_name="typing.Any",
+)
+
 bool_wtype: typing.Final = WType(
     name="bool",
     stub_name="bool",
@@ -565,3 +570,18 @@ def arc4_to_avm_equivalent_wtype(arc4_wtype: WType) -> WType:
         return bool_wtype
 
     raise InternalError(f"Invalid arc4_wtype: {arc4_wtype}")
+
+
+def is_valid_assignment(target: WType, source: WType) -> bool:
+    if target == throwaway_type:
+        return True
+    if (
+        isinstance(target, WTuple)
+        and isinstance(source, WTuple)
+        and len(target.types) == len(source.types)
+    ):
+        return all(
+            is_valid_assignment(target_item, source_item)
+            for target_item, source_item in zip(target.types, source.types, strict=True)
+        )
+    return target == source

src/puya/awst_build/eb/throwaway.py

@@ -0,0 +1,73 @@
+import typing
+from collections.abc import Sequence
+
+import mypy.nodes
+
+from puya.awst import wtypes
+from puya.awst.nodes import AssignmentExpression, Expression, Literal, Lvalue, VarExpression
+from puya.awst_build.eb.base import ExpressionBuilder
+from puya.errors import CodeError, InternalError
+from puya.parse import SourceLocation
+
+
+class ThrowawayExpressionBuilder(ExpressionBuilder):
+    wtype = wtypes.throwaway_type
+
+    def __init__(self, expr: Expression):
+        super().__init__(expr.source_location)
+        match expr:
+            case VarExpression() as target:
+                pass
+            case AssignmentExpression(target=target):
+                # this is an odd use case...
+                pass
+            case _:
+                raise InternalError("Expected a VarExpression", expr.source_location)
+        self._expr = target
+
+    def lvalue(self) -> Lvalue:
+        return self._expr
+
+    def rvalue(self) -> typing.Never:
+        self._raise_error(self.source_location)
+
+    def delete(self, location: SourceLocation) -> typing.Never:
+        self._raise_error(location)
+
+    def index(self, index: ExpressionBuilder | Literal, location: SourceLocation) -> typing.Never:
+        self._raise_error(location)
+
+    def call(
+        self,
+        args: Sequence[ExpressionBuilder | Literal],
+        arg_kinds: list[mypy.nodes.ArgKind],
+        arg_names: list[str | None],
+        location: SourceLocation,
+    ) -> typing.Never:
+        self._raise_error(location)
+
+    def member_access(self, name: str, location: SourceLocation) -> typing.Never:
+        self._raise_error(location)
+
+    def iterate(self) -> typing.Never:
+        self._raise_error(self.source_location)
+
+    def bool_eval(self, location: SourceLocation, *, negate: bool = False) -> typing.Never:
+        self._raise_error(location)
+
+    def unary_plus(self, location: SourceLocation) -> typing.Never:
+        self._raise_error(location)
+
+    def unary_minus(self, location: SourceLocation) -> typing.Never:
+        self._raise_error(location)
+
+    def bitwise_invert(self, location: SourceLocation) -> typing.Never:
+        self._raise_error(location)
+
+    def contains(
+        self, item: ExpressionBuilder | Literal, location: SourceLocation
+    ) -> typing.Never:
+        self._raise_error(location)
+
+    def _raise_error(self, location: SourceLocation) -> typing.Never:
+        raise CodeError("'_' variables can only be assigned to", location)

src/puya/awst_build/eb/type_registry.py

@@ -20,6 +20,7 @@
     string,
     struct,
     template_variables,
+    throwaway,
     transaction,
     tuple as tuple_,
     uint64,
@@ -141,6 +142,7 @@
     wtypes.WGroupTransaction: transaction.GroupTransactionExpressionBuilder,
     wtypes.WInnerTransaction: transaction.InnerTransactionExpressionBuilder,
     wtypes.WInnerTransactionFields: transaction.InnerTxnParamsExpressionBuilder,
+    wtypes.throwaway_type: throwaway.ThrowawayExpressionBuilder,
 }
 
 
@@ -157,5 +159,8 @@ def var_expression(expr: Expression) -> ExpressionBuilder:
     try:
         builder = WTYPE_TO_BUILDER[expr.wtype]
     except KeyError:
-        builder = WTYPE_TO_BUILDER[type(expr.wtype)]
+        try:
+            builder = WTYPE_TO_BUILDER[type(expr.wtype)]
+        except KeyError as ex:
+            raise InternalError(f"Unhandled wtype: {expr.wtype}", expr.source_location) from ex
     return builder(expr)

src/puya/awst_build/subroutine.py

@@ -45,7 +45,6 @@
     VarExpression,
     WhileLoop,
 )
-from puya.awst.wtypes import WType
 from puya.awst_build import constants, intrinsic_data
 from puya.awst_build.base_mypy_visitor import BaseMyPyVisitor
 from puya.awst_build.context import ASTConversionModuleContext
@@ -123,6 +122,7 @@ def __init__(
         func_loc = self._location(func_def)
         self.contract_method_info = contract_method_info
         self._is_bool_context = False
+        self._is_lvalue_context = False
         self.func_def = func_def
         self._precondition(
             func_def.abstract_status == mypy.nodes.NOT_ABSTRACT,
@@ -164,7 +164,11 @@ def __init__(
                 func_loc,
             )
         # TODO: this should be more than just type?
-        self._symtable = dict[str, WType]()
+        # mypy has special behaviour to treat '_' as Any, so predefine the '_' symbol as
+        # a type that can only be assigned to
+        self._symtable = {
+            "_": wtypes.throwaway_type,
+        }
         args = list[SubroutineArgument]()
         for arg, arg_type in zip(mypy_args, mypy_arg_types, strict=True):
             if arg.kind.is_star():
@@ -267,6 +271,18 @@ def _set_bool_context(self, *, is_bool_context: bool) -> Iterator[None]:
     _enter_bool_context = partialmethod(_set_bool_context, is_bool_context=True)
     _leave_bool_context = partialmethod(_set_bool_context, is_bool_context=False)
 
+    @contextlib.contextmanager
+    def _set_lvalue_context(self, *, is_lvalue_context: bool) -> Iterator[None]:
+        was_lvalue_context = self._is_lvalue_context
+        self._is_lvalue_context = is_lvalue_context
+        try:
+            yield
+        finally:
+            self._is_lvalue_context = was_lvalue_context
+
+    _enter_lvalue_context = partialmethod(_set_lvalue_context, is_lvalue_context=True)
+    _enter_rvalue_context = partialmethod(_set_lvalue_context, is_lvalue_context=False)
+
     def visit_expression_stmt(self, stmt: mypy.nodes.ExpressionStmt) -> ExpressionStatement | None:
         stmt_loc = self._location(stmt)
         if isinstance(stmt.expr, mypy.nodes.StrExpr):
@@ -391,9 +407,10 @@ def _handle_state_proxy_assignment(
             ]
 
     def resolve_lvalue(self, lvalue: mypy.nodes.Expression) -> Lvalue:
-        builder_or_literal = lvalue.accept(self)
-        builder = require_expression_builder(builder_or_literal)
-        return builder.lvalue()
+        with self._enter_lvalue_context():
+            builder_or_literal = lvalue.accept(self)
+            builder = require_expression_builder(builder_or_literal)
+            return builder.lvalue()
 
     def empty_statement(self, _tmt: mypy.nodes.Statement) -> None:
         return None
@@ -708,9 +725,6 @@ def _visit_ref_expr_maybe_aliased(
                     " as a singular lvalue in an assignment statement",
                     expr_loc,
                 )
-                if var_name == "_":
-                    # TODO: ignore "_"
-                    raise CodeError("_ is not currently supported as a variable name", expr_loc)
                 local_type = lazy_setdefault(
                     self._symtable,
                     key=var_name,
@@ -845,7 +859,7 @@ def visit_call_expr(self, call: mypy.nodes.CallExpr) -> ExpressionBuilder | Lite
             args_context: typing.Any = self._enter_bool_context
         else:
             args_context = contextlib.nullcontext
-        with args_context():
+        with args_context(), self._enter_rvalue_context():
             args = [arg.accept(self) for arg in call.args]
         return callee_builder.call(
             args=args,
@@ -1147,11 +1161,14 @@ def visit_bytes_expr(self, expr: mypy.nodes.BytesExpr) -> Literal:
 
     def visit_tuple_expr(self, mypy_expr: mypy.nodes.TupleExpr) -> ExpressionBuilder:
         items = [
-            require_expression_builder(
-                mypy_item.accept(self),
-                msg="Python literals (other than True/False) are not valid as tuple elements",
-            ).rvalue()
-            for mypy_item in mypy_expr.items
+            eb.lvalue() if self._is_lvalue_context else eb.rvalue()
+            for eb in (
+                require_expression_builder(
+                    mypy_item.accept(self),
+                    msg="Python literals (other than True/False) are not valid as tuple elements",
+                )
+                for mypy_item in mypy_expr.items
+            )
         ]
         if not items:
             raise CodeError(

test_cases/throwaway/contract.py

@@ -0,0 +1,20 @@
+from puyapy import Account, Bytes, Contract, Global, Txn, UInt64, subroutine
+
+
+class Throwaway(Contract):
+    def approval_program(self) -> bool:
+        tup = get_tuple()
+        args, sender, _ = tup
+        _, _, approval = tup
+        assert sender == Global.creator_address
+        assert args == 0
+        assert approval
+        return True
+
+    def clear_state_program(self) -> bool:
+        return True
+
+
+@subroutine
+def get_tuple() -> tuple[UInt64, Account, Bytes]:
+    return Txn.num_app_args, Txn.sender, Txn.approval_program

test_cases/throwaway/out/Throwaway.approval.mir

@@ -0,0 +1,48 @@
+// Op                     //                                           Op Description                      Stack (out)                                  X stack                                 Source code                                                    Source line
+
+#pragma version 10
+
+// test_cases.throwaway.contract.Throwaway.approval_program() -> uint64:
+main_block@0:
+    callsub get_tuple     //                                                                               {get_tuple}.0,{get_tuple}.1,{get_tuple}.2                                            get_tuple()                                                    throwaway/contract.py:6
+    cover 2               //                                           store tup.2#0 to l-stack (no copy)  tup.2#0,{get_tuple}.0,{get_tuple}.1                                                  tup = get_tuple()                                              throwaway/contract.py:6
+    //                    virtual: store tup.1#0 to l-stack (no copy)  tup.2#0,tup.1#0,{get_tuple}.0                                                    tup = get_tuple()                       throwaway/contract.py:6
+    //                    virtual: store tup.0#0 to l-stack (no copy)  tup.2#0,tup.0#0,tup.1#0                                                          tup = get_tuple()                       throwaway/contract.py:6
+    global CreatorAddress //                                                                               tup.2#0,tup.0#0,tup.1#0,{global}                                                     Global.creator_address                                         throwaway/contract.py:9
+    //                    virtual: store tmp%0#0 to l-stack (no copy)  tup.2#0,tup.0#0,tup.1#0,tmp%0#0                                                  Global.creator_address                  throwaway/contract.py:9
+    //                    virtual: load tup.1#0 from l-stack (no copy) tup.2#0,tup.0#0,tmp%0#0,tup.1#0                                                  sender == Global.creator_address        throwaway/contract.py:9
+    //                    virtual: load tmp%0#0 from l-stack (no copy) tup.2#0,tup.0#0,tup.1#0,tmp%0#0                                                  sender == Global.creator_address        throwaway/contract.py:9
+    ==                    //                                                                               tup.2#0,tup.0#0,{==}                                                                 sender == Global.creator_address                               throwaway/contract.py:9
+    //                    virtual: store tmp%1#0 to l-stack (no copy)  tup.2#0,tup.0#0,tmp%1#0                                                          sender == Global.creator_address        throwaway/contract.py:9
+    //                    virtual: load tmp%1#0 from l-stack (no copy) tup.2#0,tup.0#0,tmp%1#0                                                          assert sender == Global.creator_address throwaway/contract.py:9
+    assert                //                                                                               tup.2#0,tup.0#0                                                                      assert sender == Global.creator_address                        throwaway/contract.py:9
+    //                    virtual: load tup.0#0 from l-stack (no copy) tup.2#0,tup.0#0                                                                  args == 0                               throwaway/contract.py:10
+    !                     //                                                                               tup.2#0,{!}                                                                          args == 0                                                      throwaway/contract.py:10
+    //                    virtual: store tmp%2#0 to l-stack (no copy)  tup.2#0,tmp%2#0                                                                  args == 0                               throwaway/contract.py:10
+    //                    virtual: load tmp%2#0 from l-stack (no copy) tup.2#0,tmp%2#0                                                                  assert args == 0                        throwaway/contract.py:10
+    assert                //                                                                               tup.2#0                                                                              assert args == 0                                               throwaway/contract.py:10
+    //                    virtual: load tup.2#0 from l-stack (no copy) tup.2#0                                                                          approval                                throwaway/contract.py:11
+    len                   //                                                                               {len}                                                                                approval                                                       throwaway/contract.py:11
+    //                    virtual: store tmp%3#0 to l-stack (no copy)  tmp%3#0                                                                          approval                                throwaway/contract.py:11
+    //                    virtual: load tmp%3#0 from l-stack (no copy) tmp%3#0                                                                          assert approval                         throwaway/contract.py:11
+    assert                //                                                                                                                                                                    assert approval                                                throwaway/contract.py:11
+    int 1                 //                                                                               1                                                                                    True                                                           throwaway/contract.py:12
+    return                //                                                                                                                                                                    return True                                                    throwaway/contract.py:12
+
+
+// test_cases.throwaway.contract.get_tuple() -> uint64, bytes, bytes:
+get_tuple:
+    proto 0 3             //                                                                                                                                                                    @subroutine\ndef get_tuple() -> tuple[UInt64, Account, Bytes]: throwaway/contract.py:18-19
+
+get_tuple_block@0:
+    txn NumAppArgs        //                                                                               {txn}                                                                                Txn.num_app_args                                               throwaway/contract.py:20
+    //                    virtual: store tmp%0#0 to l-stack (no copy)  tmp%0#0                                                                          Txn.num_app_args                        throwaway/contract.py:20
+    txn Sender            //                                                                               tmp%0#0,{txn}                                                                        Txn.sender                                                     throwaway/contract.py:20
+    //                    virtual: store tmp%1#0 to l-stack (no copy)  tmp%0#0,tmp%1#0                                                                  Txn.sender                              throwaway/contract.py:20
+    txn ApprovalProgram   //                                                                               tmp%0#0,tmp%1#0,{txn}                                                                Txn.approval_program                                           throwaway/contract.py:20
+    //                    virtual: store tmp%2#0 to l-stack (no copy)  tmp%0#0,tmp%1#0,tmp%2#0                                                          Txn.approval_program                    throwaway/contract.py:20
+    uncover 2             //                                           load tmp%0#0 from l-stack (no copy) tmp%1#0,tmp%2#0,tmp%0#0                                                              return Txn.num_app_args, Txn.sender, Txn.approval_program      throwaway/contract.py:20
+    uncover 2             //                                           load tmp%1#0 from l-stack (no copy) tmp%2#0,tmp%0#0,tmp%1#0                                                              return Txn.num_app_args, Txn.sender, Txn.approval_program      throwaway/contract.py:20
+    uncover 2             //                                           load tmp%2#0 from l-stack (no copy) tmp%0#0,tmp%1#0,tmp%2#0                                                              return Txn.num_app_args, Txn.sender, Txn.approval_program      throwaway/contract.py:20
+    retsub                //                                                                               tmp%0#0,tmp%1#0,tmp%2#0                                                              return Txn.num_app_args, Txn.sender, Txn.approval_program      throwaway/contract.py:20
+