Add a TargetIsa::display_enc() method.

The interpretation of an encoding depends on the target ISA, and so does
the proper printing of the encoding.

src/libcretonne/isa/encoding.rs

@@ -1,5 +1,7 @@
 //! The `Encoding` struct.
 
+use std::fmt;
+
 /// Bits needed to encode an instruction as binary machine code.
 ///
 /// The encoding consists of two parts, both specific to the target ISA: An encoding *recipe*, and
@@ -30,4 +32,47 @@ impl Encoding {
     pub fn bits(self) -> u16 {
         self.bits
     }
+
+    /// Is this a legal encoding, or the default placeholder?
+    pub fn is_legal(self) -> bool {
+        self != Self::default()
+    }
+}
+
+/// The default encoding is the illegal one.
+impl Default for Encoding {
+    fn default() -> Self {
+        Self::new(0xffff, 0xffff)
+    }
+}
+
+/// ISA-independent display of an encoding.
+impl fmt::Display for Encoding {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.is_legal() {
+            write!(f, "#{}/{:02x}", self.recipe, self.bits)
+        } else {
+            write!(f, "-")
+        }
+    }
+}
+
+/// Temporary object that holds enough context to properly display an encoding.
+/// This is meant to be created by `TargetIsa::display_enc()`.
+pub struct DisplayEncoding {
+    pub encoding: Encoding,
+    pub recipe_names: &'static [&'static str],
+}
+
+impl fmt::Display for DisplayEncoding {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.encoding.is_legal() {
+            write!(f,
+                   "{}/{:02x}",
+                   self.recipe_names[self.encoding.recipe()],
+                   self.encoding.bits)
+        } else {
+            write!(f, "-")
+        }
+    }
 }

src/libcretonne/isa/mod.rs

@@ -101,4 +101,12 @@ pub trait TargetIsa {
     ///
     /// This is just used for printing and parsing encodings in the textual IL format.
     fn recipe_names(&self) -> &'static [&'static str];
+
+    /// Create an object that can display an ISA-dependent encoding properly.
+    fn display_enc(&self, enc: Encoding) -> encoding::DisplayEncoding {
+        encoding::DisplayEncoding {
+            encoding: enc,
+            recipe_names: self.recipe_names(),
+        }
+    }
 }

src/libcretonne/isa/riscv/mod.rs

@@ -65,7 +65,7 @@ mod tests {
     use ir::{types, immediates};
 
     fn encstr(isa: &isa::TargetIsa, enc: isa::Encoding) -> String {
-        format!("{}/{:02x}", isa.recipe_names()[enc.recipe()], enc.bits())
+        isa.display_enc(enc).to_string()
     }
 
     #[test]