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add runnableExamples for prepareStrMutation (#17227)

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* Update lib/system.nim

Co-authored-by: Timothee Cour <timothee.cour2@gmail.com>
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flywind 2021-03-02 19:52:07 +08:00 • committed by GitHub
commit dfa0a6b4a6
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2 changed files with 9 additions and 2 deletions
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lib/system.nim
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@ -3124,5 +3124,12 @@ when not defined(createNimHcr) and not defined(nimscript):
when notJSnotNims and not defined(nimSeqsV2):
proc prepareStrMutation*(s: var string) {.inline.} =
## String literals (e.g. "abc", etc) in the ARC/ORC mode are "copy on write",
## therefore you should call `prepareStrMutation` before modifying the strings.
## therefore you should call `prepareStrMutation` before modifying the strings
## via `addr`.
runnableExamples("--gc:arc"):
var x = "abc"
var y = "defgh"
prepareStrMutation(y) # without this, you may get a `SIGBUS` or `SIGSEGV`
moveMem(addr y[0], addr x[0], x.len)
assert y == "abcgh"
discard