fix & add test for basic hot code reloading case (#21915)

fixes #21885

compiler/cgen.nim

@@ -2180,7 +2180,10 @@ proc finalCodegenActions*(graph: ModuleGraph; m: BModule; n: PNode): PNode =
 
     if m.hcrOn:
       # make sure this is pulled in (meaning hcrGetGlobal() is called for it during init)
-      cgsym(m, "programResult")
+      let sym = magicsys.getCompilerProc(m.g.graph, "programResult")
+      # ignore when not available, could be a module imported early in `system`
+      if sym != nil:
+        cgsymImpl m, sym
       if m.inHcrInitGuard:
         endBlock(m.initProc)
 

testament/categories.nim

@@ -80,6 +80,7 @@ proc runBasicDLLTest(c, r: var TResults, cat: Category, options: string, isOrc =
   testSpec r, makeTest("tests/dll/client.nim", options & " --threads:on" & rpath, cat)
   testSpec r, makeTest("tests/dll/nimhcr_unit.nim", options & " --threads:off" & rpath, cat)
   testSpec r, makeTest("tests/dll/visibility.nim", options & " --threads:off" & rpath, cat)
+  testSpec r, makeTest("tests/dll/nimhcr_basic.nim", options & " --threads:off" & rpath, cat)
 
   if "boehm" notin options:
     # force build required - see the comments in the .nim file for more details

tests/dll/nimhcr_basic.nim

@@ -0,0 +1,7 @@
+discard """
+  output: '''
+Hello world
+'''
+"""
+
+echo "Hello world"