defines the gcRefc symbol which allows writing specific code for refc (#20009)

* define gcRefc symbols

* add comments

* add a changelog item

* Update changelog.md

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>

* Update changelog.md

Co-authored-by: Yardanico <tiberiumk12@gmail.com>

Co-authored-by: konsumlamm <44230978+konsumlamm@users.noreply.github.com>
Co-authored-by: Yardanico <tiberiumk12@gmail.com>

changelog.md

@@ -110,6 +110,8 @@ becomes an alias for `addr`.
   In the command line, this is defined as `-d:a.b.c`. Older versions can
   use accents as in ``defined(`a.b.c`)`` to access such defines.
 
+- Defines the `gcRefc` symbol which allows writing specific code for the refc GC.
+
 ## Compiler changes
 
 - `nim` can now compile version 1.4.0 as follows: `nim c --lib:lib --stylecheck:off compiler/nim`,

compiler/commands.nim

@@ -542,6 +542,7 @@ proc processMemoryManagementOption(switch, arg: string, pass: TCmdLinePass,
       incl conf.globalOptions, optTlsEmulation # Boehm GC doesn't scan the real TLS
     of "refc":
       unregisterArcOrc(conf)
+      defineSymbol(conf.symbols, "gcrefc")
       conf.selectedGC = gcRefc
     of "markandsweep":
       unregisterArcOrc(conf)

tests/stdlib/tstrutils2.nim

@@ -1,10 +1,15 @@
+discard """
+  matrix: "--gc:refc; --gc:orc"
+"""
+
 import "$lib/.." / compiler/strutils2
 
 block: # setLen
   var a = "abc"
   a.setLen 0
   a.setLen 3, isInit = false
-  doAssert a[1] == 'b'
+  when defined(gcRefc): # bug #19763
+    doAssert a[1] == 'b'
   a.setLen 0
   a.setLen 3, isInit = true
   doAssert a[1] == '\0'