u8 tags for smaller enums

100% of the serialized enums during libcore compilation fit into the
smaller tag, and this eliminates hitting the leb128 code for
coding/decoding when we can statically guarantee that's not required.

30% of all leb128 integers serialized in libcore (12981183 total) come
from the usize's removed here.

compiler/rustc_macros/src/serialize.rs

@@ -76,8 +76,17 @@ fn decodable_body(
                 ty_name,
                 variants.len()
             );
+            let tag = if variants.len() < u8::MAX as usize {
                 quote! {
-                match ::rustc_serialize::Decoder::read_usize(__decoder) {
+                    ::rustc_serialize::Decoder::read_u8(__decoder) as usize
+                }
+            } else {
+                quote! {
+                    ::rustc_serialize::Decoder::read_usize(__decoder)
+                }
+            };
+            quote! {
+                match #tag {
                     #match_inner
                     n => panic!(#message, n),
                 }
@@ -206,12 +215,21 @@ fn encodable_body(
                     variant_idx += 1;
                     result
                 });
+                if variant_idx < u8::MAX as usize {
+                    quote! {
+                        let disc = match *self {
+                            #encode_inner
+                        };
+                        ::rustc_serialize::Encoder::emit_u8(__encoder, disc as u8);
+                    }
+                } else {
                     quote! {
                         let disc = match *self {
                             #encode_inner
                         };
                         ::rustc_serialize::Encoder::emit_usize(__encoder, disc);
                     }
+                }
             };
 
             let mut variant_idx = 0usize;