fixes ptr to cstring warnings[backport] (#20848)

* fix =#13790 ptr char (+friends) should not implicitly convert to cstring

* Apply suggestions from code review

* first round; compiles on windows

* nimPreviewSlimSystem

* conversion is unsafe, cast needed

* fixes more tests

* fixes asyncnet

* another try another error

* last one

* true

* one more

* why bugs didn't show at once

* add `nimPreviewCstringConversion` switch

* typo

* fixes ptr to cstring warnings[backport]

* add fixes

Co-authored-by: xflywind <43030857+xflywind@users.noreply.github.com>

lib/std/formatfloat.nim

@@ -49,7 +49,7 @@ proc writeFloatToBufferSprintf*(buf: var array[65, char]; value: BiggestFloat):
   ##
   ## returns the amount of bytes written to `buf` not counting the
   ## terminating '\0' character.
-  var n: int = c_sprintf(addr buf, "%.16g", value)
+  var n: int = c_sprintf(cast[cstring](addr buf), "%.16g", value)
   var hasDot = false
   for i in 0..n-1:
     if buf[i] == ',':
@@ -90,7 +90,7 @@ proc addFloatRoundtrip*(result: var string; x: float | float32) =
   else:
     var buffer {.noinit.}: array[65, char]
     let n = writeFloatToBufferRoundtrip(buffer, x)
-    result.addCstringN(cstring(buffer[0].addr), n)
+    result.addCstringN(cast[cstring](buffer[0].addr), n)
 
 proc addFloatSprintf*(result: var string; x: float) =
   when nimvm:
@@ -98,7 +98,7 @@ proc addFloatSprintf*(result: var string; x: float) =
   else:
     var buffer {.noinit.}: array[65, char]
     let n = writeFloatToBufferSprintf(buffer, x)
-    result.addCstringN(cstring(buffer[0].addr), n)
+    result.addCstringN(cast[cstring](buffer[0].addr), n)
 
 proc nimFloatToString(a: float): cstring =
   ## ensures the result doesn't print like an integer, i.e. return 2.0, not 2

lib/std/private/oscommon.nim

@@ -63,7 +63,7 @@ when defined(windows) and not weirdTarget:
     template findNextFile*(a, b: untyped): untyped = findNextFileA(a, b)
     template getCommandLine*(): untyped = getCommandLineA()
 
-    template getFilename*(f: untyped): untyped = $cstring(addr f.cFileName)
+    template getFilename*(f: untyped): untyped = $cast[cstring](addr f.cFileName)
 
   proc skipFindData*(f: WIN32_FIND_DATA): bool {.inline.} =
     # Note - takes advantage of null delimiter in the cstring

lib/std/private/osdirs.nim

@@ -231,7 +231,7 @@ iterator walkDir*(dir: string; relative = false, checkDir = false,
         while true:
           var x = readdir(d)
           if x == nil: break
-          var y = $cstring(addr x.d_name)
+          var y = $cast[cstring](addr x.d_name)
           if y != "." and y != "..":
             var s: Stat
             let path = dir / y

lib/std/private/strimpl.nim

@@ -106,7 +106,7 @@ func find*(s, sub: cstring, start: Natural = 0, last = 0): int =
   if sub.len > s.len - start: return -1
   if sub.len == 1: return find(s, sub[0], start, last)
   if last == 0 and s.len > start:
-    let found = c_strstr(s[start].unsafeAddr, sub)
+    let found = c_strstr(cast[cstring](s[start].unsafeAddr), sub)
     if not found.isNil:
       result = cast[ByteAddress](found) -% cast[ByteAddress](s)
     else:

lib/std/private/win_setenv.nim

@@ -94,9 +94,9 @@ else:
         var buf = newSeq[char](requiredSize + 1)
         let buf2 = buf[0].addr
         if wcstombs(buf2, wideName, csize_t(requiredSize + 1)) != high(csize_t):
-          var ptrToEnv = c_getenv(buf2)
+          var ptrToEnv = c_getenv(cast[cstring](buf2))
           ptrToEnv[0] = '\0'
-          ptrToEnv = c_getenv(buf2)
+          ptrToEnv = c_getenv(cast[cstring](buf2))
           ptrToEnv[1] = '='
 
     # And now, we have to update the outer environment to have a proper empty value.

lib/std/syncio.nim

@@ -467,7 +467,7 @@ proc readLine*(f: File, line: var string): bool {.tags: [ReadIOEffect],
     while true:
       # fixes #9634; this pattern may need to be abstracted as a template if reused;
       # likely other io procs need this for correctness.
-      fgetsSuccess = c_fgets(addr line[pos], sp.cint, f) != nil
+      fgetsSuccess = c_fgets(cast[cstring](addr line[pos]), sp.cint, f) != nil
       if fgetsSuccess: break
       when not defined(nimscript):
         if errno == EINTR: