Best-effort compatibility of GC.stat

Author: chrisseaton

doc/user/compatibility.md

@@ -157,6 +157,10 @@ Additionally, `FPE`, `ILL`, `KILL`, `SEGV`, `STOP`, and `VTALRM` cannot be trapp
 
 When TruffleRuby is run as part of a polyglot application, any signals that are handled by another language become unavailable for TruffleRuby to trap.
 
+### GC statistics
+
+TruffleRuby provides similar `GC.stat` statistics to MRI, but not all statistics are available, and some statistics may be approximations. Use `GC.stat.keys` to see which are provided with real or approximate values. Missing values will return `0`.
+
 ## Features with Very Low Performance
 
 ### `ObjectSpace`

spec/ruby/core/gc/stat_spec.rb

@@ -11,10 +11,6 @@
     GC.stat(:count).should be_kind_of(Integer)
   end
 
-  it "returns nil for a non-existent stat" do
-    GC.stat(:does_not_exist).should be_nil
-  end
-
   it "increases count after GC is run" do
     count = GC.stat(:count)
     GC.start

src/main/ruby/truffleruby/core/gc.rb

@@ -89,20 +89,12 @@ def self.stat(option = nil)
       heap_available_slots: committed,
       heap_live_slots: used,
       heap_free_slots: committed - used,
-      total_allocated_objects: 0,
       used: used,
       committed: committed,
       init: init,
       max: max,
-      peak_used: 0,
-      peak_committed: 0,
-      peak_init: 0,
-      peak_max: 0,
-      last_used: 0,
-      last_committed: 0,
-      last_init: 0,
-      last_max: 0,
     }
+    stat.default = 0
 
     memory_pool_names.each_with_index do |memory_pool_name, i|
       # Populate memory pool specific stats