Make the new native gc internal only (#696)

## Summary
Made the new `native` gc not accessible via command line arguments or
via `compileOption`.

## Details
The latest `native` gc was created to better represent the gc the JS and
VM backends use. But it has no meaning for the C backend and instead
redirected to orc. As there is no reason to explicitly choose the
`native` gc via command-line arguments I removed the option. I also
removed the option for the `compileOption` proc because one should
query the used backend instead.

compiler/front/main.nim

@@ -441,7 +441,7 @@ proc customizeForBackend*(graph: ModuleGraph, conf: ConfigRef,
     of excNone, excNative: conf.exc = excGoto
     of excGoto:            discard
 
-    if conf.selectedGC in {gcUnselected, gcNative}:
+    if conf.selectedGC == gcUnselected:
       # the default gc for the C backend is ORC. We can't just set it to
       # ``gcOrc`` directly, however, as additional defines, etc. are required
       # XXX: the dependency on ``optionsprocessor`` hints that a different

compiler/front/optionsprocessor.nim

@@ -589,7 +589,7 @@ proc processCompile(conf: ConfigRef; filename: string) =
 # multiple names for the same garbage collector.
 const
   gcNames = @[
-    "native", "boehm", "refc", "markandsweep", "destructors", "arc", "orc",
+    "boehm", "refc", "markandsweep", "destructors", "arc", "orc",
     "hooks", "go", "none", "stack", "regions",]
 
   cmdNames = @[
@@ -644,7 +644,6 @@ func testCompileOptionArg*(conf: ConfigRef; switch, arg: string): CompileOptArgC
   case switch.normalize
   of "gc":
     case arg.normalize
-    of "native": asResult conf.selectedGC == gcNative
     of "boehm": asResult conf.selectedGC == gcBoehm
     of "refc": asResult conf.selectedGC == gcRefc
     of "markandsweep": asResult conf.selectedGC == gcMarkAndSweep
@@ -1154,8 +1153,6 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass,
     expectArg(switch, arg)
     if pass in {passCmd2, passPP}:
       case arg.normalize
-      of "native":
-        conf.selectedGC = gcNative
       of "boehm":
         conf.selectedGC = gcBoehm
         defineSymbol(conf, "boehmgc")

doc/advopt.txt

@@ -117,9 +117,9 @@ Advanced options:
   --skipUserCfg:on|off      do not read the user's configuration file
   --skipParentCfg:on|off    do not read the parent dirs' configuration files
   --skipProjCfg:on|off      do not read the project's configuration file
-  --gc:native|refc|arc|orc|markAndSweep|boehm|go|none|regions
+  --gc:refc|arc|orc|markAndSweep|boehm|go|none|regions
                             select the GC to use; default depends on the selected
-                            backend ('orc' for C, 'native' for Javascript and the VM)
+                            backend ('orc' for C, the native GC for Javascript and the VM)
   --exceptions:native|goto
                             select the exception handling implementation
   --index:on|off            turn index file generation on|off

doc/gc.rst

@@ -34,8 +34,6 @@ Multi-paradigm Memory Management Strategies
 
 To choose the memory management strategy use the `--gc:` switch.
 
---gc:native   The native GC for the selected backend. Redirects to `--gc:orc`
-  for the C backend.
 --gc:refc    This is the default GC. It's a
   deferred reference counting based garbage collector
   with a simple Mark&Sweep backup GC in order to collect cycles. Heaps are thread-local.
@@ -67,7 +65,6 @@ ORC                Shared   Cycle Collector   No             `--gc:orc`
 Boehm              Shared   Cycle Collector   Yes            `--gc:boehm`
 Go                 Shared   Cycle Collector   Yes            `--gc:go`
 None               Manual   Manual            Manual         `--gc:none`
-Native             Depends  Depends           Depends        `--gc:native`
 ================== ======== ================= ============== ===================
 
 .. default-role:: code