Speed up make_simplified_union for Literal[string]s (issue #9169) (#9192)

make_simplified_union no longer runs for 0.5 seconds every time it's called with a 
Union containing 270 literals of strings.

Like I explained in #9169, this only fixes half of the problem and I'm not capable of 
fixing the other half. This function is still called 1098 times for the "reduced" example 
code, and IMO it should be called only once.

Author: Akuli

mypy/typeops.py

@@ -344,21 +344,35 @@ def make_simplified_union(items: Sequence[Type],
     from mypy.subtypes import is_proper_subtype
 
     removed = set()  # type: Set[int]
-    for i, ti in enumerate(items):
-        if i in removed: continue
-        # Keep track of the truishness info for deleted subtypes which can be relevant
-        cbt = cbf = False
-        for j, tj in enumerate(items):
-            if i != j and is_proper_subtype(tj, ti, keep_erased_types=keep_erased):
-                # We found a redundant item in the union.
-                removed.add(j)
-                cbt = cbt or tj.can_be_true
-                cbf = cbf or tj.can_be_false
-        # if deleted subtypes had more general truthiness, use that
-        if not ti.can_be_true and cbt:
-            items[i] = true_or_false(ti)
-        elif not ti.can_be_false and cbf:
-            items[i] = true_or_false(ti)
+
+    # Avoid slow nested for loop for Union of Literal of strings (issue #9169)
+    if all((isinstance(item, LiteralType) and
+            item.fallback.type.fullname == 'builtins.str')
+           for item in items):
+        seen = set()    # type: Set[str]
+        for index, item in enumerate(items):
+            assert isinstance(item, LiteralType)
+            assert isinstance(item.value, str)
+            if item.value in seen:
+                removed.add(index)
+            seen.add(item.value)
+
+    else:
+        for i, ti in enumerate(items):
+            if i in removed: continue
+            # Keep track of the truishness info for deleted subtypes which can be relevant
+            cbt = cbf = False
+            for j, tj in enumerate(items):
+                if i != j and is_proper_subtype(tj, ti, keep_erased_types=keep_erased):
+                    # We found a redundant item in the union.
+                    removed.add(j)
+                    cbt = cbt or tj.can_be_true
+                    cbf = cbf or tj.can_be_false
+            # if deleted subtypes had more general truthiness, use that
+            if not ti.can_be_true and cbt:
+                items[i] = true_or_false(ti)
+            elif not ti.can_be_false and cbf:
+                items[i] = true_or_false(ti)
 
     simplified_set = [items[i] for i in range(len(items)) if i not in removed]
     return UnionType.make_union(simplified_set, line, column)