fix #16206, nim r / nim -r recompiles if cwd changes (#16349)

compiler/extccomp.nim

@@ -1026,6 +1026,8 @@ proc writeJsonBuildInstructions*(conf: ConfigRef) =
     lit $(%* conf.projectIsCmd)
     lit ",\L\"cmdInput\": "
     lit $(%* conf.cmdInput)
+    lit ",\L\"currentDir\": "
+    lit $(%* getCurrentDir())
 
     if optRun in conf.globalOptions or isDefined(conf, "nimBetterRun"):
       lit ",\L\"cmdline\": "
@@ -1046,14 +1048,23 @@ proc changeDetectedViaJsonBuildInstructions*(conf: ConfigRef; projectfile: Absol
   result = false
   try:
     let data = json.parseFile(jsonFile.string)
-    if not data.hasKey("depfiles") or not data.hasKey("cmdline"):
+    for key in "depfiles cmdline stdinInput currentDir".split:
+      if not data.hasKey(key): return true
+    if getCurrentDir() != data["currentDir"].getStr:
+      # fixes bug #16271
+      # Note that simply comparing `expandFilename(projectFile)` would
+      # not be sufficient in case other flags depend implicitly on `getCurrentDir`,
+      # and would require much more care. Simply re-compiling is safer for now.
+      # A better strategy for future work would be to cache (with an LRU cache)
+      # the N most recent unique build instructions, as done with `rdmd`,
+      # which is both robust and avoids recompilation when switching back and forth
+      # between projects, see https://github.com/timotheecour/Nim/issues/199
       return true
     let oldCmdLine = data["cmdline"].getStr
     if conf.commandLine != oldCmdLine:
       return true
     if hashNimExe() != data["nimexe"].getStr:
       return true
-    if not data.hasKey("stdinInput"): return true
     let stdinInput = data["stdinInput"].getBool
     let projectIsCmd = data["projectIsCmd"].getBool
     if conf.projectIsStdin or stdinInput: