adds nimPreviewCStringComparisons for cstring comparisons (#24946)

todo: We can also give a deprecation message for `ltPtr`/`lePtr`
matching for cstring in `magicsAfterOverloadResolution`

follow up https://github.com/nim-lang/Nim/pull/24942

changelog.md

@@ -21,6 +21,8 @@ errors.
 
 - The bare `except:` now panics on `Defect`. Use `except Exception:` or `except Defect:` to catch `Defect`. `--legacy:noPanicOnExcept` is provided for a transition period.
 
+- With `-d:nimPreviewCStringComparisons`, comparsions (`<`, `>`, `<=`, `>=`) between cstrings switch from reference semantics to value semantics like `==` and `!=`.
+
 ## Standard library additions and changes
 
 [//]: # "Additions:"

compiler/nim.cfg

@@ -11,6 +11,7 @@ define:nimPreviewRangeDefault
 define:nimPreviewNonVarDestructor
 define:nimPreviewCheckedClose
 define:nimPreviewAsmSemSymbol
+define:nimPreviewCStringComparisons
 threads:off
 
 #import:"$projectpath/testability"

lib/system.nim

@@ -2738,36 +2738,37 @@ func ltCStringVm(x, y: cstring): bool {.inline.} =
 func leCStringVm(x, y: cstring): bool {.inline.} =
   discard "implemented in the vm ops"
 
-func `<`*(x, y: cstring): bool {.inline.} =
-  if x == y:
-    result = false
-  elif x == nil:
-    result = true
-  elif y == nil:
-    result = false
-  else:
-    when nimvm:
-      result = ltCStringVm(x, y)
+when defined(nimPreviewCStringComparisons):
+  func `<`*(x, y: cstring): bool {.inline.} =
+    if x == y:
+      result = false
+    elif x == nil:
+      result = true
+    elif y == nil:
+      result = false
     else:
-      when defined(js):
-        result = pointer(x) < pointer(y)
+      when nimvm:
+        result = ltCStringVm(x, y)
       else:
-        result = strcmp(x, y) < 0
+        when defined(js):
+          result = pointer(x) < pointer(y)
+        else:
+          result = strcmp(x, y) < 0
 
-func `<=`*(x, y: cstring): bool {.inline.} =
-  if x == y: result = true
-  elif x == nil:
-    result = true
-  elif y == nil:
-    result = false
-  else:
-    when nimvm:
-      result = leCStringVm(x, y)
+  func `<=`*(x, y: cstring): bool {.inline.} =
+    if x == y: result = true
+    elif x == nil:
+      result = true
+    elif y == nil:
+      result = false
     else:
-      when defined(js):
-        result = pointer(x) <= pointer(y)
+      when nimvm:
+        result = leCStringVm(x, y)
       else:
-        result = strcmp(x, y) <= 0
+        when defined(js):
+          result = pointer(x) <= pointer(y)
+        else:
+          result = strcmp(x, y) <= 0
 
 template closureScope*(body: untyped): untyped =
   ## Useful when creating a closure in a loop to capture local loop variables by

tests/config.nims

@@ -46,3 +46,5 @@ when not defined(testsConciseTypeMismatch):
 switch("experimental", "vtables")
 switch("experimental", "openSym")
 switch("experimental", "typeBoundOps")
+
+switch("define", "nimPreviewCStringComparisons")