Fix indexing primitive refs with integers (broken in #1543), add tests

Apparently lua_tostring has the side effect of *converting* numbers to strings

library/LuaTypes.cpp

@@ -706,6 +706,8 @@ static type_identity *find_primitive_field(lua_State *state, int field, const ch
  *   Fields point to identity, or NULL for metafields.
  */
 static int meta_primitive_index(lua_State *state)
+{
+    if (lua_type(state, -1) == LUA_TSTRING)
     {
         const char *attr = lua_tostring(state, -1);
         if (strcmp(attr, "ref_target") == 0) {
@@ -717,6 +719,7 @@ static int meta_primitive_index(lua_State *state)
             }
             return 1;
         }
+    }
 
     uint8_t *ptr = get_object_addr(state, 1, 2, "read");
     auto type = find_primitive_field(state, 2, "read", &ptr);

test/structures/primitive_refs.lua

@@ -0,0 +1,92 @@
+utils = require('utils')
+
+function with_temp_ref(...)
+    local args = {...}
+    local dtype = 'int32_t'
+    if type(args[1]) == 'string' then
+        dtype = table.remove(args, 1)
+    end
+    local count = 1
+    if type(args[1]) == 'number' then
+        count = table.remove(args, 1)
+    end
+    local func = table.remove(args, 1)
+    assert(type(func) == 'function')
+    dfhack.with_temp_object(df.new(dtype, count), func, table.unpack(args))
+end
+
+function test.zero_initialize()
+    with_temp_ref(function(x)
+        expect.eq(x.value, 0)
+    end)
+end
+
+function test.zero_initialize_array()
+    with_temp_ref(2, function(x)
+        expect.eq(x:_displace(1).value, 0)
+    end)
+end
+
+function test.displace_zero()
+    with_temp_ref(function(x)
+        expect.eq(x, x:_displace(0))
+    end)
+end
+
+function test.displace_nonzero()
+    with_temp_ref(2, function(x)
+        expect.ne(x, x:_displace(1))
+        expect.eq(utils.addressof(x) + x:sizeof(), utils.addressof(x:_displace(1)))
+    end)
+end
+
+function test.displace_negative()
+    with_temp_ref(2, function(x)
+        expect.true_(x:_displace(1):_displace(-1).value)
+    end)
+end
+
+function test.index_read()
+    with_temp_ref(function(x)
+        expect.eq(x.value, x[0])
+    end)
+end
+
+function test.index_write()
+    with_temp_ref(function(x)
+        x[0] = 1
+        expect.eq(x.value, 1)
+        expect.eq(x[0], 1)
+        x.value = 2
+        expect.eq(x.value, 2)
+        expect.eq(x[0], 2)
+    end)
+end
+
+function test.index_write_multi()
+    local len = 3
+    with_temp_ref(len, function(x)
+        for i = 0, len - 1 do
+            x[i] = i * i
+        end
+        for i = 0, len - 1 do
+            expect.eq(x[i], i * i)
+        end
+    end)
+end
+
+function test.index_read_negative()
+    with_temp_ref(function(x)
+        expect.error_match(function()
+            expect.true_(x:_displace(1)[-1])
+        end, 'negative index')
+    end)
+end
+
+function test.index_write_negative()
+    with_temp_ref(function(x)
+        expect.error_match(function()
+            x:_displace(1)[-1] = 7
+        end, 'negative index')
+    end)
+end