fixes #26010; Double destroy with {.cursor.} (#26031)

fixes #26010

Cursors do not own their values and therefore cannot transfer ownership
through move.
Reject move(cursor) during semantic analysis and share the
cursor-location check
between semantic analysis and destructor injection.

(cherry picked from commit 99a696e0c4)

compiler/injectdestructors.nim

@@ -24,7 +24,7 @@ import std/[strtabs, tables, strutils, intsets]
 when defined(nimPreviewSlimSystem):
   import std/assertions
 
-from trees import exprStructuralEquivalent, getRoot, whichPragma
+from trees import exprStructuralEquivalent, getRoot, isCursor, whichPragma
 
 type
   Con = object
@@ -180,17 +180,6 @@ proc isFirstWrite(n: PNode; c: var Con): bool =
   let m = skipConvDfa(n)
   result = nfFirstWrite in m.flags
 
-proc isCursor(n: PNode): bool =
-  case n.kind
-  of nkSym:
-    sfCursor in n.sym.flags
-  of nkDotExpr:
-    isCursor(n[1])
-  of nkCheckedFieldExpr:
-    isCursor(n[0])
-  else:
-    false
-
 template isFullyUnpackedTuple(n: PNode): bool =
   ## we move out all elements of unpacked tuples,
   ## hence unpacked tuples themselves don't need to be destroyed

compiler/semmagic.nim

@@ -691,5 +691,10 @@ proc magicsAfterOverloadResolution(c: PContext, n: PNode,
     if n[1].kind in {nkStmtListExpr, nkBlockExpr,
               nkIfExpr, nkCaseStmt, nkTryStmt}:
       localError(c.config, n.info, "Nested expressions cannot be moved: '" & $n[1] & "'")
+  of mMove:
+    result = n
+    if isCursor(n[1]):
+      localError(c.config, n.info, errFailedMove,
+        "cannot move cursor '" & $n[1] & "'; a cursor does not own its value")
   else:
     result = n

compiler/trees.nim

@@ -225,6 +225,17 @@ proc getRoot*(n: PNode): PSym =
     else: result = nil
   else: result = nil
 
+proc isCursor*(n: PNode): bool =
+  case n.kind
+  of nkSym:
+    sfCursor in n.sym.flags
+  of nkDotExpr:
+    isCursor(n[1])
+  of nkCheckedFieldExpr:
+    isCursor(n[0])
+  else:
+    false
+
 proc stupidStmtListExpr*(n: PNode): bool =
   for i in 0..<n.len-1:
     if n[i].kind notin {nkEmpty, nkCommentStmt}: return false

tests/arc/t26010.nim

@@ -0,0 +1,23 @@
+discard """
+  action: reject
+  matrix: "--mm:orc; --mm:refc"
+  errormsg: "cannot move cursor 'a'; a cursor does not own its value"
+"""
+
+# bug #26010: a cursor is a non-owning alias and cannot transfer ownership.
+
+type Xxx = object
+
+proc `=destroy`(v: var Xxx) =
+  debugEcho "dest"
+
+proc test(v: ref Xxx) =
+  var a {.cursor.} = v
+  var b = move(a)
+  discard
+
+proc main() =
+  var x = new Xxx
+  test(x)
+
+main()