fixes #15325 (#15340)

compiler/commands.nim

@@ -891,6 +891,9 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
     processOnOffSwitchG(conf, {optProfileVM}, arg, pass, info)
   of "sinkinference":
     processOnOffSwitch(conf, {optSinkInference}, arg, pass, info)
+  of "cursorinference":
+    # undocumented, for debugging purposes only:
+    processOnOffSwitch(conf, {optCursorInference}, arg, pass, info)
   of "panics":
     processOnOffSwitchG(conf, {optPanics}, arg, pass, info)
     if optPanics in conf.globalOptions:

compiler/injectdestructors.nim

@@ -1006,7 +1006,8 @@ proc injectDestructorCalls*(g: ModuleGraph; owner: PSym; n: PNode): PNode =
     echoCfg(c.g)
     echo n
 
-  computeCursors(owner, n, g.config)
+  if optCursorInference in g.config.options:
+    computeCursors(owner, n, g.config)
 
   var scope: Scope
   let body = p(n, c, scope, normal)

compiler/options.nim

@@ -41,6 +41,7 @@ type                          # please make sure we have under 32 options
     optMemTracker,
     optNilSeqs,
     optSinkInference          # 'sink T' inference
+    optCursorInference
 
 
   TOptions* = set[TOption]
@@ -372,7 +373,7 @@ const
   DefaultOptions* = {optObjCheck, optFieldCheck, optRangeCheck,
     optBoundsCheck, optOverflowCheck, optAssert, optWarns, optRefCheck,
     optHints, optStackTrace, optLineTrace, # consider adding `optStackTraceMsgs`
-    optTrMacros, optStyleCheck}
+    optTrMacros, optStyleCheck, optCursorInference}
   DefaultGlobalOptions* = {optThreadAnalysis,
     optExcessiveStackTrace, optListFullPaths}
 

compiler/varpartitions.nim

@@ -392,9 +392,18 @@ proc deps(c: var Partitions; dest, src: PNode) =
         analyseAsgn(c, c.s[vid], src)
         # do not borrow from a different local variable, this is easier
         # than tracking reassignments, consider 'var cursor = local; local = newNode()'
-        if src.kind == nkSym and (src.sym.kind in {skVar, skResult, skTemp} or
+        if src.kind == nkSym:
+          if (src.sym.kind in {skVar, skResult, skTemp} or
             (src.sym.kind in {skLet, skParam, skForVar} and hasDisabledAsgn(src.sym.typ))):
-          c.s[vid].flags.incl preventCursor
+            c.s[vid].flags.incl preventCursor
+          elif src.sym.kind in {skVar, skResult, skTemp, skLet, skForVar}:
+            # XXX: we need to compute variable alive ranges before doing anything else:
+            let srcid = variableId(c, src.sym)
+            if srcid >= 0 and preventCursor in c.s[srcid].flags:
+              # you cannot borrow from a local that lives shorter than 'vid':
+              if c.s[srcid].aliveStart > c.s[vid].aliveStart or
+                  c.s[srcid].aliveEnd < c.s[vid].aliveEnd:
+                c.s[vid].flags.incl preventCursor
 
     if src.kind == nkSym and hasDestructor(src.typ):
       rhsIsSink(c, src)