[mypyc] Fix reference leak in mypyc bytes concatenation (#21469)

Fixes [Mypyc #1192](mypyc/mypyc#1192)

Fix a reference leak in mypyc-generated code for `bytes + bytes`.

The `bytes + bytes` primitive for `CPyBytes_Concat` was marked as
stealing the
left operand with `steals=[True, False]`. However, `CPyBytes_Concat`
does not
steal or decref either argument; it allocates and returns a new `bytes`
object.

Because of this mismatch, the refcount pass skipped emitting a `dec_ref`
for
owned left operands. For code such as:

```python
return (1).to_bytes(4, "big") + (2).to_bytes(4, "big")
````

the generated IR decrefed only the right operand after
`CPyBytes_Concat`, leaving
the owned left operand alive. Chained concatenations amplified the leak
because
intermediate concat results could also become leaked left operands.

This PR removes the incorrect `steals=[True, False]` annotation from the
`bytes + bytes` primitive, so both operands are treated as non-stolen
and owned
operands are decrefed normally after the call.

The tests verify that the generated IR decrefs both owned operands after
`CPyBytes_Concat`.

Author: colinxu2020

mypyc/primitives/bytes_ops.py

@@ -64,7 +64,6 @@
     return_type=bytes_rprimitive,
     c_function_name="CPyBytes_Concat",
     error_kind=ERR_MAGIC,
-    steals=[True, False],
 )
 
 # bytes * int

mypyc/test-data/refcount.test

@@ -2125,3 +2125,31 @@ L3:
     r8 = box(None, 1)
     inc_ref r8
     return r8
+
+[case testBytesConcatRefcount]
+def f(a: bytes, b: bytes) -> bytes:
+    return b"1" + a + b
+[out]
+def f(a, b):
+    a, b, r0, r1, r2 :: bytes
+L0:
+    r0 = b'1'
+    r1 = CPyBytes_Concat(r0, a)
+    r2 = CPyBytes_Concat(r1, b)
+    dec_ref r1
+    return r2
+
+[case testChainedBytesConcatRefcount]
+def f(a: bytes, b: bytes, c: bytes) -> bytes:
+    return b"1" + a + b + c
+[out]
+def f(a, b, c):
+    a, b, c, r0, r1, r2, r3 :: bytes
+L0:
+    r0 = b'1'
+    r1 = CPyBytes_Concat(r0, a)
+    r2 = CPyBytes_Concat(r1, b)
+    dec_ref r1
+    r3 = CPyBytes_Concat(r2, c)
+    dec_ref r2
+    return r3