1
00:00:06,600 --> 00:00:09,570
- Let's imagine we
define a simple enum type

2
00:00:09,570 --> 00:00:10,440
like we have here.

3
00:00:10,440 --> 00:00:12,360
I've defined an enum type called color.

4
00:00:12,360 --> 00:00:13,750
It has a couple of variants,

5
00:00:13,750 --> 00:00:16,050
well, three, red, green, and blue.

6
00:00:16,050 --> 00:00:17,940
And let's say in my code

7
00:00:17,940 --> 00:00:20,040
I use some of those variants,
but not all of them.

8
00:00:20,040 --> 00:00:21,390
I use the red variant

9
00:00:21,390 --> 00:00:24,030
but let's say I don't
use the green variant

10
00:00:24,030 --> 00:00:25,920
and I don't use the blue variant.

11
00:00:25,920 --> 00:00:28,560
Well, we saw this earlier.

12
00:00:28,560 --> 00:00:31,140
If there are variants that
you haven't used anywhere

13
00:00:31,140 --> 00:00:34,200
in your code, the Rust compiler
gives you a warning message

14
00:00:34,200 --> 00:00:35,730
about dead code.

15
00:00:35,730 --> 00:00:38,910
It'll say the green variant isn't used.

16
00:00:38,910 --> 00:00:40,380
Why have we defined it?

17
00:00:40,380 --> 00:00:41,760
The blue variant isn't used.

18
00:00:41,760 --> 00:00:43,500
Why have we defined that?

19
00:00:43,500 --> 00:00:45,360
So we have seen this example earlier.

20
00:00:45,360 --> 00:00:48,303
If you go back to the
project lesson four enums,

21
00:00:49,140 --> 00:00:53,400
in mytypes.rs, I defined
a color enum type.

22
00:00:53,400 --> 00:00:58,290
In main.rs, I had a function
called demo_simple_enums.

23
00:00:58,290 --> 00:01:00,450
It used the red variant

24
00:01:00,450 --> 00:01:03,840
but it didn't use the
green or the blue variant.

25
00:01:03,840 --> 00:01:06,900
When I run the application,
I get warnings.

26
00:01:06,900 --> 00:01:07,980
So let me just remind you

27
00:01:07,980 --> 00:01:09,210
what those warnings look like

28
00:01:09,210 --> 00:01:11,730
and then we're gonna see how to fix them.

29
00:01:11,730 --> 00:01:14,010
So here's the code for lesson four.

30
00:01:14,010 --> 00:01:17,070
And remember mytypes.rs,

31
00:01:17,070 --> 00:01:21,240
it defined a couple of enums,
a color enum, house location.

32
00:01:21,240 --> 00:01:23,970
We have a look at that
later on in the lesson.

33
00:01:23,970 --> 00:01:28,290
Let's focus on the color enum for now.

34
00:01:28,290 --> 00:01:33,290
Three variants, in my main
code, I import the module,

35
00:01:35,130 --> 00:01:37,290
I introduce the color enum

36
00:01:37,290 --> 00:01:40,140
into scope plus house location.

37
00:01:40,140 --> 00:01:44,593
In my main code when I call
demo_simple_enums, here it is,

38
00:01:46,860 --> 00:01:48,810
in this function I just used the red enum.

39
00:01:48,810 --> 00:01:52,110
I declared a variable
and used the red variant

40
00:01:52,110 --> 00:01:53,700
but not the green or the blue.

41
00:01:53,700 --> 00:01:57,450
I'm testing for them, but
I haven't actually used,

42
00:01:57,450 --> 00:02:00,120
I haven't actually assigned
the value to a variable.

43
00:02:00,120 --> 00:02:01,650
And that's the critical thing.

44
00:02:01,650 --> 00:02:05,190
So let me open the terminal
window and I'm gonna remind you

45
00:02:05,190 --> 00:02:07,350
about the warnings that we're gonna get.

46
00:02:07,350 --> 00:02:08,580
We're gonna get lots of warnings here

47
00:02:08,580 --> 00:02:11,283
about unused code, cargo run.

48
00:02:14,220 --> 00:02:17,210
So we had from the top,

49
00:02:17,210 --> 00:02:18,690
and I talked about this a bit earlier,

50
00:02:18,690 --> 00:02:20,580
we had lots of functions

51
00:02:20,580 --> 00:02:23,043
that exist that haven't been called,

52
00:02:24,120 --> 00:02:28,110
but also we have the color enum.

53
00:02:28,110 --> 00:02:30,000
The green variant isn't used

54
00:02:30,000 --> 00:02:32,610
and the blue variant
isn't used either, okay?

55
00:02:32,610 --> 00:02:37,023
So why do they exist if
I never, ever use them?

56
00:02:38,400 --> 00:02:41,040
So I mean it's a
legitimate question really,

57
00:02:41,040 --> 00:02:44,370
but maybe it's because you
haven't got round to it yet

58
00:02:44,370 --> 00:02:47,970
and you would just like the
compiler to stop complaining.

59
00:02:47,970 --> 00:02:52,970
So what you can do is you can
use an attribute, this syntax,

60
00:02:54,270 --> 00:02:59,270
a hash symbol or whatever
you call it, a hash symbol

61
00:02:59,430 --> 00:03:01,590
and then square brackets.

62
00:03:01,590 --> 00:03:05,430
This is called an attribute in Rust.

63
00:03:05,430 --> 00:03:08,280
And it's kind of like
a compiler directive.

64
00:03:08,280 --> 00:03:11,850
And if you use the allow
directive, you can specify

65
00:03:11,850 --> 00:03:14,850
or the allow attribute,
you can specify what kind

66
00:03:14,850 --> 00:03:17,400
of language features you want to allow

67
00:03:17,400 --> 00:03:19,170
without giving warnings.

68
00:03:19,170 --> 00:03:22,320
By default, it would give
you warnings for dead code

69
00:03:22,320 --> 00:03:23,970
but you can use this attribute

70
00:03:23,970 --> 00:03:27,300
to allow dead code in
your application, okay?

71
00:03:27,300 --> 00:03:30,930
So you're not gonna get warnings
from the compiler anymore.

72
00:03:30,930 --> 00:03:35,220
So what you can do is you
can qualify each variant

73
00:03:35,220 --> 00:03:38,310
like this and you would have
to do it for each variant.

74
00:03:38,310 --> 00:03:42,240
You can say, in my color
enum, the red variant,

75
00:03:42,240 --> 00:03:45,990
I'm going to allow that
red variant not to be used,

76
00:03:45,990 --> 00:03:47,760
and the green and the blue,

77
00:03:47,760 --> 00:03:52,760
you label each variant to
allow it not to be used

78
00:03:53,130 --> 00:03:54,713
in your application, okay?

79
00:03:56,040 --> 00:03:58,980
So we're gonna have a
look at how to do that

80
00:03:58,980 --> 00:04:01,380
in the application and
see how it helps matters.

81
00:04:01,380 --> 00:04:05,580
We'll define an enum that
does allow variants not

82
00:04:05,580 --> 00:04:09,420
to be used, and we'll see
what happens when we run it.

83
00:04:09,420 --> 00:04:13,470
The warnings disappear, so let's do that.

84
00:04:13,470 --> 00:04:17,973
It's in my other file,
mytypes_for_demo_purposes.

85
00:04:18,990 --> 00:04:22,110
I spent age years thinking
of the name for this module.

86
00:04:22,110 --> 00:04:25,620
So the module name is
mytypes_for_demo_purposes.

87
00:04:25,620 --> 00:04:26,940
I've defined a different color.

88
00:04:26,940 --> 00:04:30,090
This color enum type is in this module,

89
00:04:30,090 --> 00:04:31,650
namespace, if you like.

90
00:04:31,650 --> 00:04:36,090
It is different from the
color type defined in here.

91
00:04:36,090 --> 00:04:39,150
The module name is like a namespace.

92
00:04:39,150 --> 00:04:41,730
So technically this enum

93
00:04:41,730 --> 00:04:46,350
is of type mytypes colon colon color.

94
00:04:46,350 --> 00:04:51,350
And this enum type is of type
mytypes_for_demo_purposes,

95
00:04:55,230 --> 00:04:56,910
colon colon color,

96
00:04:56,910 --> 00:05:00,000
it is a different type
name in a different scope.

97
00:05:00,000 --> 00:05:05,000
And in this enum, I'm allowing
the variants not to be used.

98
00:05:05,310 --> 00:05:08,100
I'm allowing these to be dead code.

99
00:05:08,100 --> 00:05:10,590
And I've done it for
house location as well.

100
00:05:10,590 --> 00:05:14,310
So in my main code, I'm
going to use this module,

101
00:05:14,310 --> 00:05:16,860
I'm gonna import this module instead

102
00:05:16,860 --> 00:05:19,950
of importing the previous one.

103
00:05:19,950 --> 00:05:23,100
So in my main code right at the top

104
00:05:23,100 --> 00:05:26,370
I will comment these statements out.

105
00:05:26,370 --> 00:05:29,433
I'm no longer gonna use mytypes module.

106
00:05:30,300 --> 00:05:32,400
Instead I'm going to use

107
00:05:32,400 --> 00:05:36,810
the mytypes_for_demo_purposes
module, input that file,

108
00:05:36,810 --> 00:05:39,870
and also from that file
or from that module,

109
00:05:39,870 --> 00:05:41,880
introduce those enums into scope.

110
00:05:41,880 --> 00:05:46,880
So these enums now will not
complain about unused variants.

111
00:05:48,270 --> 00:05:50,370
So let's give that a try.

112
00:05:50,370 --> 00:05:52,233
Let's cargo run again.

113
00:05:53,760 --> 00:05:56,250
I'm hoping there will
not be any warnings now

114
00:05:56,250 --> 00:05:57,903
about unused variants.

115
00:05:59,880 --> 00:06:04,170
I'm still getting warnings
about unused functions.

116
00:06:04,170 --> 00:06:07,080
Okay, so that's a, you
know, rather disappointing.

117
00:06:07,080 --> 00:06:09,663
I've gotta say, not unexpected.

118
00:06:10,560 --> 00:06:15,360
I'm not getting any warnings
about variants being unused.

119
00:06:15,360 --> 00:06:16,770
So that was good.

120
00:06:16,770 --> 00:06:19,560
Now what I could do is I
could use the same technique

121
00:06:19,560 --> 00:06:22,680
for the functions, I could
say I've got this function,

122
00:06:22,680 --> 00:06:24,480
I will eventually be calling it

123
00:06:24,480 --> 00:06:28,380
when we get there in the
lesson later on, but not yet.

124
00:06:28,380 --> 00:06:33,180
So what I could do is I
could pinch this attribute

125
00:06:33,180 --> 00:06:35,310
and you can use it to qualify anything.

126
00:06:35,310 --> 00:06:39,990
You can qualify a variant to
say allow this not to be used.

127
00:06:39,990 --> 00:06:42,960
You can also use it to qualify functions.

128
00:06:42,960 --> 00:06:45,390
So you could put this attribute

129
00:06:45,390 --> 00:06:50,390
at the beginning of each
function, like so, okay?

130
00:06:50,730 --> 00:06:54,540
Other languages have
similar kind of features

131
00:06:54,540 --> 00:06:57,270
and this is how you can do it in Rust.

132
00:06:57,270 --> 00:07:00,240
Obviously if you are using this too much

133
00:07:00,240 --> 00:07:01,530
you're gonna start wondering

134
00:07:01,530 --> 00:07:05,490
why does this function even
exist if I'm not calling it.

135
00:07:05,490 --> 00:07:08,280
So you are kind of suppressing a warning

136
00:07:08,280 --> 00:07:10,470
that was actually something
you should have taken notice of

137
00:07:10,470 --> 00:07:12,060
but you can do it.

138
00:07:12,060 --> 00:07:14,943
So if I compile now,
let's see what happens.

139
00:07:15,960 --> 00:07:19,590
Beautiful, free of any warnings,

140
00:07:19,590 --> 00:07:21,780
because I'm allowing dead code.

141
00:07:21,780 --> 00:07:24,750
This is dead code, it's never called,

142
00:07:24,750 --> 00:07:26,433
I'm gonna allow dead code.

143
00:07:29,520 --> 00:07:31,590
The only caveat is that
it's a bit tiresome

144
00:07:31,590 --> 00:07:36,590
having to qualify every single
thing to allow dead code.

145
00:07:38,220 --> 00:07:40,470
Every single thing in my application

146
00:07:40,470 --> 00:07:43,200
I could qualify individually like that.

147
00:07:43,200 --> 00:07:45,093
There is another way to do it.

148
00:07:45,990 --> 00:07:50,070
You can define an attribute at
the top of your application.

149
00:07:50,070 --> 00:07:52,953
It's a slightly different
syntax, it looks like this.

150
00:07:53,820 --> 00:07:55,200
At the top of your application,

151
00:07:55,200 --> 00:07:58,650
you say you have an
exclamation mark here, okay?

152
00:07:58,650 --> 00:08:02,340
Technically your entire application

153
00:08:02,340 --> 00:08:04,860
in Rust is called a crate, okay?

154
00:08:04,860 --> 00:08:09,860
So this is a crate-scope attribute.

155
00:08:10,290 --> 00:08:14,190
It applies across the entire crate.

156
00:08:14,190 --> 00:08:16,560
The crate, all the terms in Rust

157
00:08:16,560 --> 00:08:18,030
are something to do with crabs,

158
00:08:18,030 --> 00:08:20,820
like crustaceans, Rustaceans.

159
00:08:20,820 --> 00:08:24,420
A crate is something
that would contain crabs.

160
00:08:24,420 --> 00:08:29,420
So in Rust terminology, your
entire application is a crate

161
00:08:29,760 --> 00:08:32,880
and this is a crate scope attribute

162
00:08:32,880 --> 00:08:36,180
or rather I should call it
a crate level attribute.

163
00:08:36,180 --> 00:08:39,480
It applies across the entire application.

164
00:08:39,480 --> 00:08:42,300
Allow dead code anywhere
in my application.

165
00:08:42,300 --> 00:08:45,690
So technically now I could revert back.

166
00:08:45,690 --> 00:08:50,580
If I comment those out and
uncomment my original files,

167
00:08:50,580 --> 00:08:54,330
like so, then it will allow dead code

168
00:08:54,330 --> 00:08:56,760
without complaining, very nice.

169
00:08:56,760 --> 00:08:57,813
When I input mytypes,

170
00:08:58,920 --> 00:09:01,830
even though there are some
variants here which aren't used,

171
00:09:01,830 --> 00:09:04,530
it'll say, aha, but I'm
going to allow that.

172
00:09:04,530 --> 00:09:07,560
So this is actually the easiest way

173
00:09:07,560 --> 00:09:10,863
to avoid warnings everywhere
about unused code.

174
00:09:12,030 --> 00:09:15,060
So fingers crossed, if I
run the application again,

175
00:09:15,060 --> 00:09:17,523
it will give me no warnings.

176
00:09:20,469 --> 00:09:21,302
I love it.