opcode.rs: Use Parsable trait for opcode parsing

src/parser/instruction.rs

@@ -34,8 +34,7 @@ impl Parsable for Instruction {
         T: crate::TokenStream<'text>,
     {
         // parse an opcode
-        let insn = stream.expect(Type::Insn)?;
+        let opcode: Opcode = Opcode::parse(p, stream)?;
-        let opcode: Opcode = insn.parse()?;
         // resolve the opcode to a final opcode and an encoding
         let (opcode, encoding) = opcode.resolve();
         // parse the encoding

src/parser/instruction/opcode.rs

@@ -124,11 +124,18 @@ impl Opcode {
     }
 }
 
+impl Parsable for Opcode {
+    fn parse<'text, T>(_: &Parser, stream: &mut T) -> Result<Self, Error>
+    where T: TokenStream<'text> {
+        stream.expect(Type::Insn)?.parse().map_err(|e: Error| e.context(stream.context()))
+    }
+}
+
 impl FromStr for Opcode {
     type Err = Error;
     fn from_str(s: &str) -> Result<Self, Self::Err> {
         use Opcode::*;
-        //TODO: Reduce allocations here
+        //TODO: Reduce allocations here?
         let s = s.to_ascii_lowercase();
         Ok(match s.as_str() {
             "rrc" => Rrc,