fixes #25488; Strings can be compared against nil (#25489)

fixes #25488
ref https://github.com/nim-lang/Nim/pull/20222

compiler/nifgen.nim

@@ -983,7 +983,7 @@ proc genericParamToNif(n: PNode; parent: PNode; c: var TranslationContext) =
     toNif n, parent, c
 
 proc addExternName(sym: PSym; c: var TranslationContext) =
-  if sym.loc.snippet != nil:
+  if sym.loc.snippet != "":
     c.b.addStrLit sym.loc.snippet
   else:
     c.b.addStrLit sym.name.s

lib/system/comparisons.nim

@@ -38,6 +38,21 @@ proc `==`*[T](x, y: ptr T): bool {.magic: "EqRef", noSideEffect.}
 proc `==`*[T: proc | iterator](x, y: T): bool {.magic: "EqProc", noSideEffect.}
   ## Checks that two `proc` variables refer to the same procedure.
 
+when true:
+  # guard against string converted to cstring implicitly; see also #bug #25488
+  proc isNil*(x: string): bool {.noSideEffect, error: "'isNil' is invalid for 'string'".}
+
+
+  # bug #9149; ensure that 'typeof(nil)' does not match *too* well by using 'typeof(nil) | typeof(nil)',
+  # especially for converters, see tests/overload/tconverter_to_string.nim
+  # Eventually we will be able to remove this hack completely.
+
+  proc `==`*(x: string; y: typeof(nil) | typeof(nil)): bool {.error: "'nil' is invalid for 'string'".} =
+    discard
+
+  proc `==`*(x: typeof(nil) | typeof(nil); y: string): bool {.error: "'nil' is invalid for 'string'".} =
+    discard
+
 proc `<=`*[Enum: enum](x, y: Enum): bool {.magic: "LeEnum", noSideEffect.}
 proc `<=`*(x, y: string): bool {.magic: "LeStr", noSideEffect.} =
   ## Compares two strings and returns true if `x` is lexicographically