Fix: don't return the trailing zero

spec/compiler/macro/macro_methods_spec.cr

@@ -592,8 +592,8 @@ module Crystal
       end
 
       it "executes to_utf16" do
-        assert_macro %({{"hello".to_utf16}}), "::Slice(::UInt16).literal(104_u16, 101_u16, 108_u16, 108_u16, 111_u16, 0_u16)"
-        assert_macro %({{"TEST 😐🐙 ±∀ の".to_utf16}}), "::Slice(::UInt16).literal(84_u16, 69_u16, 83_u16, 84_u16, 32_u16, 55357_u16, 56848_u16, 55357_u16, 56345_u16, 32_u16, 177_u16, 8704_u16, 32_u16, 12398_u16, 0_u16)"
+        assert_macro %({{"hello".to_utf16}}), "(::Slice(::UInt16).literal(104_u16, 101_u16, 108_u16, 108_u16, 111_u16, 0_u16))[0, 5]"
+        assert_macro %({{"TEST 😐🐙 ±∀ の".to_utf16}}), "(::Slice(::UInt16).literal(84_u16, 69_u16, 83_u16, 84_u16, 32_u16, 55357_u16, 56848_u16, 55357_u16, 56345_u16, 32_u16, 177_u16, 8704_u16, 32_u16, 12398_u16, 0_u16))[0, 14]"
       end
 
       it "executes to_i" do

src/compiler/crystal/macros/methods.cr

@@ -823,12 +823,17 @@ module Crystal
       when "to_utf16"
         interpret_check_args do
           slice = @value.to_utf16
+
           # include the trailing zero that isn't counted in the slice but was
           # generated by String#to_utf16 so the literal can be passed to C
           # functions that expect a null terminated UInt16*
-          slice = Slice(UInt16).new(slice.to_unsafe, slice.size + 1)
-          args = slice.to_a { |codepoint| NumberLiteral.new(codepoint).as(ASTNode) }
-          Call.new(Generic.new(Path.global("Slice"), [Path.global("UInt16")] of ASTNode), "literal", args)
+          args = Slice(UInt16).new(slice.to_unsafe, slice.size + 1).to_a do |codepoint|
+            NumberLiteral.new(codepoint).as(ASTNode)
+          end
+          literal_node = Call.new(Generic.new(Path.global("Slice"), [Path.global("UInt16")] of ASTNode), "literal", args)
+
+          # but keep the trailing zero hidden in the exposed slice
+          Call.new(literal_node, "[]", [NumberLiteral.new("0", :i32), NumberLiteral.new(slice.size)] of ASTNode)
         end
       when "tr"
         interpret_check_args do |first, second|