Always use 0 for offset of StaticArray's @buffer (#13319)

spec/compiler/codegen/offsetof_spec.cr

@@ -56,4 +56,12 @@ describe "Code gen: offsetof" do
       (pointerof(f).as(Void*) + offsetof(Foo, @y).to_i64).as(UInt32*).value == f.y
       CRYSTAL
   end
+
+  it "returns offset of `StaticArray#@buffer`" do
+    run(<<-CRYSTAL).to_b.should be_true
+      x = uninitialized Int32[4]
+      pointerof(x.@buffer).value = 12345
+      (pointerof(x).as(Void*) + offsetof(Int32[4], @buffer).to_i64).as(Int32*).value == x.@buffer
+      CRYSTAL
+  end
 end

spec/compiler/interpreter/pointers_spec.cr

@@ -121,6 +121,32 @@ describe Crystal::Repl::Interpreter do
       CRYSTAL
     end
 
+    it "pointerof read `StaticArray#@buffer` (1)" do
+      interpret(<<-CRYSTAL).should eq(2)
+        struct StaticArray(T, N)
+          def to_unsafe
+            pointerof(@buffer)
+          end
+
+          def x
+            @buffer
+          end
+        end
+
+        foo = uninitialized Int32[4]
+        foo.to_unsafe.value = 2
+        foo.x
+        CRYSTAL
+    end
+
+    it "pointerof read `StaticArray#@buffer` (2)" do
+      interpret(<<-CRYSTAL).should eq(2)
+        foo = uninitialized Int32[4]
+        pointerof(foo.@buffer).value = 2
+        foo.@buffer
+        CRYSTAL
+    end
+
     it "interprets pointer set and get (union type)" do
       interpret(<<-CRYSTAL).should eq(10)
         ptr = Pointer(Int32 | Bool).malloc(1_u64)

src/compiler/crystal/codegen/codegen.cr

@@ -105,13 +105,13 @@ module Crystal
     end
 
     def offset_of(type, element_index)
-      return 0_u64 if type.extern_union?
+      return 0_u64 if type.extern_union? || type.is_a?(StaticArrayInstanceType)
       llvm_typer.offset_of(llvm_typer.llvm_type(type), element_index)
     end
 
     def instance_offset_of(type, element_index)
-      # extern unions must be value types, which always use the above
-      # `offset_of` instead
+      # extern unions and static arrays must be value types, which always use
+      # the above `offset_of` instead
       llvm_typer.offset_of(llvm_typer.llvm_struct_type(type), element_index + 1)
     end
   end