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1f07fdd2dc
fixes #24664 ```nim proc main() = for i in 0..1: var s = "12345" s.add s echo s main() ``` In the given example, `add` contains two steps: `prepareAdd` and `appendString`. In the first step, a new buffer is created in order to store the final doubled string. But it doesn't copy the null terminator, neither zeromem the left unused spaces. It causes a problem because `appendString` will copy itself which doesn't end with `\0` properly so contaminated memory is copied instead. ``` var s = 12345\0 prepareAdd: var s = 12345xxxxx\0 appendString: var s = 1234512345x ```