Allow while with an empty body

hy/compiler.py

@@ -1828,7 +1828,7 @@ class HyASTCompiler(object):
         return ret
 
     @builds("while")
-    @checkargs(min=2)
+    @checkargs(min=1)
     def compile_while_expression(self, expr):
         expr.pop(0)  # "while"
         cond = expr.pop(0)
@@ -1867,7 +1867,8 @@ class HyASTCompiler(object):
 
         ret = cond_compiled + asty.While(
             expr, test=cond_compiled.force_expr,
-            body=body.stmts, orelse=orel.stmts)
+            body=body.stmts or [asty.Pass(expr)],
+            orelse=orel.stmts)
         ret.contains_yield = body.contains_yield
 
         return ret

tests/compilers/test_ast.py

@@ -97,7 +97,6 @@ def test_ast_invalid_unary_op():
 def test_ast_bad_while():
     "Make sure AST can't compile invalid while"
     cant_compile("(while)")
-    cant_compile("(while (True))")
 
 
 def test_ast_good_do():

tests/native_tests/language.hy

@@ -283,7 +283,21 @@
     (setv fact (* fact count))
     (setv count (- count 1)))
   (assert (= count 0))
-  (assert (= fact 120)))
+  (assert (= fact 120))
+
+  (setv l [])
+  (defn f []
+    (.append l 1)
+    (len l))
+  (while (!= (f) 4))
+  (assert (= l [1 1 1 1]))
+
+  (setv l [])
+  (defn f []
+    (.append l 1)
+    (len l))
+  (while (!= (f) 4) (do))
+  (assert (= l [1 1 1 1])))
 
 (defn test-while-loop-else []
   (setv count 5)