add compile-time evaluation of slice()

tests/functional/builtins/codegen/test_slice.py

@@ -432,3 +432,74 @@ def test_slice_bytes32_calldata_extended(get_contract, code, result):
         c.bar(3, "0x0001020304050607080910111213141516171819202122232425262728293031", 5).hex()
         == result
     )
+
+
+code_compruntime = [
+    (
+        "bytes32",
+        """
+@external
+@view
+def baz() -> Bytes[16]:
+    return slice(0x1234567891234567891234567891234567891234567891234567891234567891, 0, 16)
+        """,
+        """
+@external
+@view
+def baz(val: bytes32) -> Bytes[16]:
+    return slice(val, 0, 16)
+        """,
+        "0x1234567891234567891234567891234567891234567891234567891234567891",
+        "12345678912345678912345678912345",
+    ),
+    (
+        "string",
+        """
+@external
+@view
+def baz() -> String[5]:
+    return slice("why hello! how are you?", 4, 5)
+        """,
+        """
+@external
+@view
+def baz(val: String[100]) -> String[5]:
+    return slice(val, 4, 5)
+        """,
+        "why hello! how are you?",
+        "hello",
+    ),
+    (
+        "bytes",
+        """
+@external
+@view
+def baz() -> Bytes[6]:
+    return slice(b'gm sir, how are you ?', 0, 6)
+        """,
+        """
+@external
+@view
+def baz(val: Bytes[100]) -> Bytes[6]:
+    return slice(val, 0, 6)
+        """,
+        b"gm sir, how are you ?",
+        "gm sir".encode("utf-8").hex(),
+    ),
+]
+
+
+@pytest.mark.parametrize(
+    "name,compcode,runcode,arg,result", code_compruntime, ids=[el[0] for el in code_compruntime]
+)
+def test_comptime_runtime(get_contract, name, compcode, runcode, arg, result):
+    c1 = get_contract(compcode)
+    c2 = get_contract(runcode)
+    ret1 = c1.baz()
+    ret2 = c2.baz(arg)
+    if hasattr(ret1, "hex"):
+        assert ret1.hex() == result
+        assert ret2.hex() == result
+    else:
+        assert ret1 == result
+        assert ret2 == result

vyper/builtins/functions.py

@@ -294,6 +294,30 @@ class Slice(BuiltinFunction):
     ]
     _return_type = None
 
+    def evaluate(self, node):
+        (lit, st, le) = node.args[:3]
+        if (
+            isinstance(lit, (vy_ast.Bytes, vy_ast.Str, vy_ast.Hex))
+            and isinstance(st, vy_ast.Int)
+            and isinstance(le, vy_ast.Int)
+        ):
+            (st_val, le_val) = (st.value, le.value)
+            if isinstance(lit, vy_ast.Bytes):
+                sublit = lit.value[st_val : (st_val + le_val)]
+                return vy_ast.Bytes.from_node(node, value=sublit)
+            elif isinstance(lit, vy_ast.Str):
+                sublit = lit.value[st_val : (st_val + le_val)]
+                return vy_ast.Str.from_node(node, value=sublit)
+            else:
+                length = len(lit.value) // 2 - 1
+                if length != 32:
+                    # TODO unreachable?
+                    raise UnfoldableNode
+                sublit = lit.value[st_val : (2 + st_val + (le_val * 2))]
+                return vy_ast.Bytes.from_node(node, value=sublit)
+        else:
+            raise UnfoldableNode
+
     def fetch_call_return(self, node):
         arg_type, _, _ = self.infer_arg_types(node)