make implicit cstring conversions explicit (#19488)

The Nim manual says that an implicit conversion to cstring will
eventually not be allowed [1]:

    A Nim `string` is implicitly convertible to `cstring` for convenience.

    [...]

    Even though the conversion is implicit, it is not *safe*: The garbage collector
    does not consider a `cstring` to be a root and may collect the underlying
    memory. For this reason, the implicit conversion will be removed in future
    releases of the Nim compiler. Certain idioms like conversion of a `const` string
    to `cstring` are safe and will remain to be allowed.

And from Nim 1.6.0, such a conversion triggers a warning [2]:

    A dangerous implicit conversion to `cstring` now triggers a `[CStringConv]` warning.
    This warning will become an error in future versions! Use an explicit conversion
    like `cstring(x)` in order to silence the warning.

However, some files in this repo produced such a warning. For example,
before this commit, compiling `parsejson.nim` would produce:

    /foo/Nim/lib/pure/parsejson.nim(221, 37) Warning: implicit conversion to 'cstring' from a non-const location: my.buf; this will become a compile time error in the future [CStringConv]
    /foo/Nim/lib/pure/parsejson.nim(231, 39) Warning: implicit conversion to 'cstring' from a non-const location: my.buf; this will become a compile time error in the future [CStringConv]

This commit resolves the most visible `CStringConv` warnings, making the
cstring conversions explicit.

[1] https://github.com/nim-lang/Nim/blob/d2318d9ccfe6/doc/manual.md#cstring-type
[2] https://github.com/nim-lang/Nim/blob/d2318d9ccfe6/changelogs/changelog_1_6_0.md#type-system

1 files changed, 1 insertions, 1 deletions

diff --git a/lib/pure/logging.nim b/lib/pure/logging.nim
index 6751a372a..cbe8a827a 100644
--- a/lib/pure/logging.nim
+++ b/lib/pure/logging.nim
@@ -367,7 +367,7 @@ method log*(logger: ConsoleLogger, level: Level, args: varargs[string, `$`]) =
   if level >= logging.level and level >= logger.levelThreshold:
     let ln = substituteLog(logger.fmtStr, level, args)
     when defined(js):
-      let cln: cstring = ln
+      let cln = ln.cstring
       case level
       of lvlDebug: {.emit: "console.debug(`cln`);".}
       of lvlInfo:  {.emit: "console.info(`cln`);".}