Rework the tag macro to make it reusable

Factor out parts that are unique to instantiations such as the name, the
documentation, the underlying type, the public conversion methods.

Also adds the ability to document individual items and remodels the
syntax to more closely resemble ordinary enum definitions.

src/decoder/ifd.rs

@@ -10,27 +10,35 @@ use {TiffError, TiffFormatError, TiffResult, TiffUnsupportedError};
 use self::Value::{Ascii, List, Rational, Unsigned, Signed, SRational};
 
 macro_rules! tags {
-    {$(
-        $tag:ident
-        $val:expr;
-    )*} => {
-
-        /// TIFF tag
-        #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
-        pub enum Tag {
-            $($tag,)*
-            Unknown(u16)
+    {
+        // Permit arbitrary meta items, which include documentation.
+        $( #[$enum_attr:meta] )*
+        pub enum $name:ident($ty:ty) {
+            // Each of the `Name = Val,` permitting documentation.
+            $($(#[$ident_attr:meta])* $tag:ident = $val:expr,)*
         }
-        impl Tag {
-            pub fn from_u16(n: u16) -> Tag {
-                $(if n == $val { Tag::$tag } else)* {
-                    Tag::Unknown(n)
+    } => {
+        $( #[$enum_attr] )*
+        #[derive(Clone, Copy, PartialEq, Eq, Debug, Hash)]
+        pub enum $name {
+            $($(#[$ident_attr])* $tag,)*
+            Unknown($ty),
+            #[doc(hidden)]
+            __NonExhaustive,
+        }
+
+        impl $name {
+            fn __from_inner_type(n: u16) -> Self {
+                $(if n == $val { $name::$tag } else)* {
+                    $name::Unknown(n)
                 }
             }
-            pub fn to_u16(&self) -> u16 {
+
+            fn __to_inner_type(&self) -> u16 {
                 match *self {
-                    $( Tag::$tag => $val, )*
-                    Tag::Unknown(n) => n,
+                    $( $name::$tag => $val, )*
+                    $name::Unknown(n) => n,
+                    $name::__NonExhaustive => unreachable!(),
                 }
             }
         }
@@ -39,47 +47,60 @@ macro_rules! tags {
 
 // Note: These tags appear in the order they are mentioned in the TIFF reference
 tags! {
+/// TIFF tags
+pub enum Tag(u16) {
     // Baseline tags:
-    Artist 315;
+    Artist = 315,
     // grayscale images PhotometricInterpretation 1 or 3
-    BitsPerSample 258;
-    CellLength 265; // TODO add support
-    CellWidth 264; // TODO add support
+    BitsPerSample = 258,
+    CellLength = 265, // TODO add support
+    CellWidth = 264, // TODO add support
     // palette-color images (PhotometricInterpretation 3)
-    ColorMap 320; // TODO add support
-    Compression 259; // TODO add support for 2 and 32773
-    Copyright 33_432;
-    DateTime 306;
-    ExtraSamples 338; // TODO add support
-    FillOrder 266; // TODO add support
-    FreeByteCounts 289; // TODO add support
-    FreeOffsets 288; // TODO add support
-    GrayResponseCurve 291; // TODO add support
-    GrayResponseUnit 290; // TODO add support
-    HostComputer 316;
-    ImageDescription 270;
-    ImageLength 257;
-    ImageWidth 256;
-    Make 271;
-    MaxSampleValue 281; // TODO add support
-    MinSampleValue 280; // TODO add support
-    Model 272;
-    NewSubfileType 254; // TODO add support
-    Orientation 274; // TODO add support
-    PhotometricInterpretation 262;
-    PlanarConfiguration 284;
-    ResolutionUnit 296; // TODO add support
-    RowsPerStrip 278;
-    SamplesPerPixel 277;
-    Software 305;
-    StripByteCounts 279;
-    StripOffsets 273;
-    SubfileType 255; // TODO add support
-    Threshholding 263; // TODO add support
-    XResolution 282;
-    YResolution 283;
+    ColorMap = 320, // TODO add support
+    Compression = 259, // TODO add support for 2 and 32773
+    Copyright = 33_432,
+    DateTime = 306,
+    ExtraSamples = 338, // TODO add support
+    FillOrder = 266, // TODO add support
+    FreeByteCounts = 289, // TODO add support
+    FreeOffsets = 288, // TODO add support
+    GrayResponseCurve = 291, // TODO add support
+    GrayResponseUnit = 290, // TODO add support
+    HostComputer = 316,
+    ImageDescription = 270,
+    ImageLength = 257,
+    ImageWidth = 256,
+    Make = 271,
+    MaxSampleValue = 281, // TODO add support
+    MinSampleValue = 280, // TODO add support
+    Model = 272,
+    NewSubfileType = 254, // TODO add support
+    Orientation = 274, // TODO add support
+    PhotometricInterpretation = 262,
+    PlanarConfiguration = 284,
+    ResolutionUnit = 296, // TODO add support
+    RowsPerStrip = 278,
+    SamplesPerPixel = 277,
+    Software = 305,
+    StripByteCounts = 279,
+    StripOffsets = 273,
+    SubfileType = 255, // TODO add support
+    Threshholding = 263, // TODO add support
+    XResolution = 282,
+    YResolution = 283,
     // Advanced tags
-    Predictor 317;
+    Predictor = 317,
+}
+}
+
+impl Tag {
+    pub fn from_u16(val: u16) -> Self {
+        Self::__from_inner_type(val)
+    }
+
+    pub fn to_u16(&self) -> u16 {
+        Self::__to_inner_type(self)
+    }
 }
 
 #[derive(Clone, Copy, Debug, FromPrimitive)]
@@ -93,6 +114,8 @@ pub enum Type {
     SSHORT = 8,
     SLONG = 9,
     SRATIONAL = 10,
+    #[doc(hidden)] // Do not match against this.
+    __NonExhaustive,
 }
 
 #[allow(unused_qualifications)]
@@ -104,6 +127,8 @@ pub enum Value {
     Rational(u32, u32),
     SRational(i32, i32),
     Ascii(String),
+    #[doc(hidden)] // Do not match against this.
+    __NonExhaustive,
 }
 
 impl Value {