feat: require type annotations for loop variables (vyperlang#3596)

this commit changes the vyper language to require type annotations for
loop variables. that is, before, the following was allowed:
```vyper
for i in [1, 2, 3]:
    pass
```

now, `i` is required to have a type annotation:
```vyper
for i: uint256 in [1, 2, 3]:
    pass
```

this makes the annotation of loop variables consistent with the rest of
vyper (it was previously a special case, that loop variables did not
need to be annotated).

the approach taken in this commit is to add a pre-parsing step which
lifts out the type annotation into a separate data structure, and then
splices it back in during the post-processing steps in
`vyper/ast/parse.py`.

this commit also simplifies a lot of analysis regarding for loops.
notably, the possible types for the loop variable no longer needs to be
iterated over, we can just propagate the type provided by the user. for
this reason we also no longer need to use the typechecker speculation
machinery for inferring the type of the loop variable. however, the
NodeMetadata code is not removed because it might come in handy at a
later date.

---------

Co-authored-by: Charles Cooper <cooper.charles.m@gmail.com>

examples/auctions/blind_auction.vy

@@ -107,7 +107,7 @@ def reveal(_numBids: int128, _values: uint256[128], _fakes: bool[128], _secrets:
 
     # Calculate refund for sender
     refund: uint256 = 0
-    for i in range(MAX_BIDS):
+    for i: int128 in range(MAX_BIDS):
         # Note that loop may break sooner than 128 iterations if i >= _numBids
         if (i >= _numBids):
             break

examples/tokens/ERC1155ownable.vy

@@ -205,7 +205,7 @@ def balanceOfBatch(accounts: DynArray[address, BATCH_SIZE], ids: DynArray[uint25
     assert len(accounts) == len(ids), "ERC1155: accounts and ids length mismatch"
     batchBalances: DynArray[uint256, BATCH_SIZE] = []
     j: uint256 = 0
-    for i in ids:
+    for i: uint256 in ids:
         batchBalances.append(self.balanceOf[accounts[j]][i])
         j += 1
     return batchBalances
@@ -243,7 +243,7 @@ def mintBatch(receiver: address, ids: DynArray[uint256, BATCH_SIZE], amounts: Dy
     assert len(ids) == len(amounts), "ERC1155: ids and amounts length mismatch"
     operator: address = msg.sender
 
-    for i in range(BATCH_SIZE):
+    for i: uint256 in range(BATCH_SIZE):
         if i >= len(ids):
             break
         self.balanceOf[receiver][ids[i]] += amounts[i]
@@ -277,7 +277,7 @@ def burnBatch(ids: DynArray[uint256, BATCH_SIZE], amounts: DynArray[uint256, BAT
     assert len(ids) == len(amounts), "ERC1155: ids and amounts length mismatch"
     operator: address = msg.sender 
 
-    for i in range(BATCH_SIZE):
+    for i: uint256 in range(BATCH_SIZE):
         if i >= len(ids):
             break
         self.balanceOf[msg.sender][ids[i]] -= amounts[i]
@@ -333,7 +333,7 @@ def safeBatchTransferFrom(sender: address, receiver: address, ids: DynArray[uint
     assert sender == msg.sender or self.isApprovedForAll[sender][msg.sender], "Caller is neither owner nor approved operator for this ID"
     assert len(ids) == len(amounts), "ERC1155: ids and amounts length mismatch"
     operator: address = msg.sender
-    for i in range(BATCH_SIZE):
+    for i: uint256 in range(BATCH_SIZE):
         if i >= len(ids):
             break
         id: uint256 = ids[i]

examples/voting/ballot.vy

@@ -54,7 +54,7 @@ def directlyVoted(addr: address) -> bool:
 def __init__(_proposalNames: bytes32[2]):
     self.chairperson = msg.sender
     self.voterCount = 0
-    for i in range(2):
+    for i: int128 in range(2):
         self.proposals[i] = Proposal({
             name: _proposalNames[i],
             voteCount: 0
@@ -82,7 +82,7 @@ def _forwardWeight(delegate_with_weight_to_forward: address):
     assert self.voters[delegate_with_weight_to_forward].weight > 0
 
     target: address = self.voters[delegate_with_weight_to_forward].delegate
-    for i in range(4):
+    for i: int128 in range(4):
         if self._delegated(target):
             target = self.voters[target].delegate
             # The following effectively detects cycles of length <= 5,
@@ -157,7 +157,7 @@ def vote(proposal: int128):
 def _winningProposal() -> int128:
     winning_vote_count: int128 = 0
     winning_proposal: int128 = 0
-    for i in range(2):
+    for i: int128 in range(2):
         if self.proposals[i].voteCount > winning_vote_count:
             winning_vote_count = self.proposals[i].voteCount
             winning_proposal = i

examples/wallet/wallet.vy

@@ -14,7 +14,7 @@ seq: public(int128)
 
 @external
 def __init__(_owners: address[5], _threshold: int128):
-    for i in range(5):
+    for i: uint256 in range(5):
         if _owners[i] != empty(address):
             self.owners[i] = _owners[i]
     self.threshold = _threshold
@@ -47,7 +47,7 @@ def approve(_seq: int128, to: address, _value: uint256, data: Bytes[4096], sigda
     assert self.seq == _seq
     # # Iterates through all the owners and verifies that there signatures,
     # # given as the sigdata argument are correct
-    for i in range(5):
+    for i: uint256 in range(5):
         if sigdata[i][0] != 0:
             # If an invalid signature is given for an owner then the contract throws
             assert ecrecover(h2, sigdata[i][0], sigdata[i][1], sigdata[i][2]) == self.owners[i]

tests/functional/builtins/codegen/test_empty.py

@@ -423,7 +423,7 @@ def test_empty(xs: int128[111], ys: Bytes[1024], zs: Bytes[31]) -> bool: view
 @internal
 def write_junk_to_memory():
     xs: int128[1024] = empty(int128[1024])
-    for i in range(1024):
+    for i: uint256 in range(1024):
         xs[i] = -(i + 1)
 @internal
 def priv(xs: int128[111], ys: Bytes[1024], zs: Bytes[31]) -> bool:
@@ -469,7 +469,7 @@ def test_return_empty(get_contract_with_gas_estimation):
 @internal
 def write_junk_to_memory():
     xs: int128[1024] = empty(int128[1024])
-    for i in range(1024):
+    for i: uint256 in range(1024):
         xs[i] = -(i + 1)
 
 @external

tests/functional/builtins/codegen/test_mulmod.py

@@ -20,7 +20,7 @@ def test_uint256_mulmod_complex(get_contract_with_gas_estimation):
 @external
 def exponential(base: uint256, exponent: uint256, modulus: uint256) -> uint256:
     o: uint256 = 1
-    for i in range(256):
+    for i: uint256 in range(256):
         o = uint256_mulmod(o, o, modulus)
         if exponent & shift(1, 255 - i) != 0:
             o = uint256_mulmod(o, base, modulus)

tests/functional/builtins/codegen/test_slice.py

@@ -17,7 +17,7 @@ def test_basic_slice(get_contract_with_gas_estimation):
 @external
 def slice_tower_test(inp1: Bytes[50]) -> Bytes[50]:
     inp: Bytes[50] = inp1
-    for i in range(1, 11):
+    for i: uint256 in range(1, 11):
         inp = slice(inp, 1, 30 - i * 2)
     return inp
     """

tests/functional/codegen/features/iteration/test_break.py

@@ -11,7 +11,7 @@ def test_break_test(get_contract_with_gas_estimation):
 def foo(n: decimal) -> int128:
     c: decimal = n * 1.0
     output: int128 = 0
-    for i in range(400):
+    for i: int128 in range(400):
         c = c / 1.2589
         if c < 1.0:
             output = i
@@ -35,12 +35,12 @@ def test_break_test_2(get_contract_with_gas_estimation):
 def foo(n: decimal) -> int128:
     c: decimal = n * 1.0
     output: int128 = 0
-    for i in range(40):
+    for i: int128 in range(40):
         if c < 10.0:
             output = i * 10
             break
         c = c / 10.0
-    for i in range(10):
+    for i: int128 in range(10):
         c = c / 1.2589
         if c < 1.0:
             output = output + i
@@ -63,12 +63,12 @@ def test_break_test_3(get_contract_with_gas_estimation):
 def foo(n: int128) -> int128:
     c: decimal = convert(n, decimal)
     output: int128 = 0
-    for i in range(40):
+    for i: int128 in range(40):
         if c < 10.0:
             output = i * 10
             break
         c /= 10.0
-    for i in range(10):
+    for i: int128 in range(10):
         c /= 1.2589
         if c < 1.0:
             output = output + i
@@ -108,7 +108,7 @@ def foo():
         """
 @external
 def foo():
-    for i in [1, 2, 3]:
+    for i: uint256 in [1, 2, 3]:
         b: uint256 = i
     if True:
         break

tests/functional/codegen/features/iteration/test_continue.py

@@ -7,7 +7,7 @@ def test_continue1(get_contract_with_gas_estimation):
     code = """
 @external
 def foo() -> bool:
-    for i in range(2):
+    for i: uint256 in range(2):
         continue
         return False
     return True
@@ -21,7 +21,7 @@ def test_continue2(get_contract_with_gas_estimation):
 @external
 def foo() -> int128:
     x: int128 = 0
-    for i in range(3):
+    for i: int128 in range(3):
         x += 1
         continue
         x -= 1
@@ -36,7 +36,7 @@ def test_continue3(get_contract_with_gas_estimation):
 @external
 def foo() -> int128:
     x: int128 = 0
-    for i in range(3):
+    for i: int128 in range(3):
         x += i
         continue
     return x
@@ -50,7 +50,7 @@ def test_continue4(get_contract_with_gas_estimation):
 @external
 def foo() -> int128:
     x: int128 = 0
-    for i in range(6):
+    for i: int128 in range(6):
         if i % 2 == 0:
             continue
         x += 1
@@ -83,7 +83,7 @@ def foo():
         """
 @external
 def foo():
-    for i in [1, 2, 3]:
+    for i: uint256 in [1, 2, 3]:
         b: uint256 = i
     if True:
         continue