Don't trigger recursion errors on medium-size set literals (#1785)

Currently set literals get compiled into a chain of UNIONs, which
causes a big blowup in the nesting depth of the AST. As a result, we
trip a recursion error on set literals containing 60 one-element tuples,
which is a solidly medium-size input.

Fix this by compiling set literals into a tree of UNIONs instead of a
chain.

Fixes #475 to my satisfaction though others may disagree

edb/edgeql/compiler/expr.py

@@ -144,17 +144,16 @@ def compile_Set(
             # a set literal is just sugar for a UNION
             op = 'UNION'
 
+            # Turn it into a tree of UNIONs so we only blow up the nesting
+            # depth logarithmically.
+            # TODO: Introduce an N-ary operation that handles the whole thing?
+            mid = len(elements) // 2
+            ls, rs = elements[:mid], elements[mid:]
             bigunion = qlast.BinOp(
-                left=elements[0],
-                right=elements[1],
+                left=qlast.Set(elements=ls) if len(ls) > 1 else ls[0],
+                right=qlast.Set(elements=rs) if len(rs) > 1 else rs[0],
                 op=op
             )
-            for el in elements[2:]:
-                bigunion = qlast.BinOp(
-                    left=bigunion,
-                    right=el,
-                    op=op
-                )
             return dispatch.compile(bigunion, ctx=ctx)
     else:
         return setgen.new_empty_set(

tests/test_edgeql_select.py

@@ -6005,3 +6005,21 @@ async def test_edgeql_select_duplicate_definition_03(self):
                     todo
                 }
             """)
+
+    async def test_edgeql_select_big_set_literal(self):
+        res = await self.con.query("""
+            SELECT {
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+                 (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,), (1,),
+            };
+        """)
+
+        assert len(res) == 100