Implement std::str::FromStr for matching opcodes.

Replace the home-grown from_str function.

src/libcretonne/immediates.rs

@@ -7,6 +7,7 @@
 
 use std::fmt::{self, Display, Formatter};
 use std::mem;
+use std::str::FromStr;
 
 // Include code generated by `meta/gen_instr.py`. This file contains:
 //
@@ -32,9 +33,11 @@ fn simple_hash(s: &str) -> u32 {
     h
 }
 
-impl Opcode {
+impl FromStr for Opcode {
+    type Err = &'static str;
+
     /// Parse an Opcode name from a string.
-    pub fn from_str(s: &str) -> Option<Opcode> {
+    fn from_str(s: &str) -> Result<Opcode, &'static str> {
         let tlen = OPCODE_HASH_TABLE.len();
         assert!(tlen.is_power_of_two());
         let mut idx = simple_hash(s) as usize;
@@ -44,11 +47,11 @@ impl Opcode {
             let entry = OPCODE_HASH_TABLE[idx];
 
             if entry == Opcode::NotAnOpcode {
-                return None;
+                return Err("Unknown opcode");
             }
 
             if *opcode_name(entry) == *s {
-                return Some(entry);
+                return Ok(entry);
             }
 
             // Quadratic probing.
@@ -224,11 +227,11 @@ mod tests {
         assert_eq!(format!("{}", Opcode::IaddImm), "iadd_imm");
 
         // Check the matcher.
-        assert_eq!(Opcode::from_str("iadd"), Some(Opcode::Iadd));
+        assert_eq!("iadd".parse::<Opcode>(), Ok(Opcode::Iadd));
-        assert_eq!(Opcode::from_str("iadd_imm"), Some(Opcode::IaddImm));
+        assert_eq!("iadd_imm".parse::<Opcode>(), Ok(Opcode::IaddImm));
-        assert_eq!(Opcode::from_str("iadd\0"), None);
+        assert_eq!("iadd\0".parse::<Opcode>(), Err("Unknown opcode"));
-        assert_eq!(Opcode::from_str(""), None);
+        assert_eq!("".parse::<Opcode>(), Err("Unknown opcode"));
-        assert_eq!(Opcode::from_str("\0"), None);
+        assert_eq!("\0".parse::<Opcode>(), Err("Unknown opcode"));
     }
 
     #[test]