fixes #23837; cursor now processes distinct types with a destructor (#23845)

fixes #23837

compiler/varpartitions.nim

@@ -106,6 +106,7 @@ type
     unanalysableMutation: bool
     inAsgnSource, inConstructor, inNoSideEffectSection: int
     inConditional, inLoop: int
+    inConvHasDestructor: int
     owner: PSym
     g: ModuleGraph
 
@@ -427,9 +428,17 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
     # primitive literals including the empty are harmless:
     discard
 
-  of nkExprEqExpr, nkExprColonExpr, nkHiddenStdConv, nkHiddenSubConv, nkCast, nkConv:
+  of nkExprEqExpr, nkExprColonExpr, nkHiddenStdConv, nkHiddenSubConv, nkCast:
     destMightOwn(c, dest, n[1])
 
+  of nkConv:
+    if hasDestructor(n.typ):
+      inc c.inConvHasDestructor
+      destMightOwn(c, dest, n[1])
+      dec c.inConvHasDestructor
+    else:
+      destMightOwn(c, dest, n[1])
+
   of nkIfStmt, nkIfExpr:
     for i in 0..<n.len:
       destMightOwn(c, dest, n[i].lastSon)
@@ -481,7 +490,7 @@ proc destMightOwn(c: var Partitions; dest: var VarIndex; n: PNode) =
 
   of nkCallKinds:
     if n.typ != nil:
-      if hasDestructor(n.typ):
+      if hasDestructor(n.typ) or c.inConvHasDestructor > 0:
         # calls do construct, what we construct must be destroyed,
         # so dest cannot be a cursor:
         dest.flags.incl ownsData

tests/destructor/t23837.nim

@@ -0,0 +1,51 @@
+discard """
+  output: '''
+Deallocating OwnedString
+HelloWorld
+'''
+  matrix: "--cursorinference:on; --cursorinference:off"
+  target: "c"
+"""
+
+# bug #23837
+{.
+  emit: [
+    """
+#include <stdlib.h>
+#include <string.h>
+char *allocCString() {
+    char *result = (char *) malloc(10 + 1);
+    strcpy(result, "HelloWorld");
+    return result;
+}
+
+"""
+  ]
+.}
+
+proc rawWrapper(): cstring {.importc: "allocCString", cdecl.}
+proc free(p: pointer) {.importc: "free", cdecl.}
+
+# -------------------------
+
+type OwnedString = distinct cstring
+
+proc `=destroy`(s: OwnedString) =
+  free(cstring s)
+  echo "Deallocating OwnedString"
+
+func `$`(s: OwnedString): string {.borrow.}
+
+proc leakyWrapper(): string =
+  let ostring = rawWrapper().OwnedString
+  $ostring
+
+# -------------------------
+
+proc main() =
+  # destructor not called - definitely lost: 11 bytes in 1 blocks
+  # doesn't leak with --cursorInference:off
+  let s = leakyWrapper()
+  echo s
+
+main()