Fixes #504: Implement Display/Fail for SetError and LookupError;

lib/codegen/src/isa/mod.rs

@@ -120,12 +120,14 @@ pub fn lookup(triple: Triple) -> Result<Builder, LookupError> {
 }
 
 /// Describes reason for target lookup failure
-#[derive(PartialEq, Eq, Copy, Clone, Debug)]
+#[derive(Fail, PartialEq, Eq, Copy, Clone, Debug)]
 pub enum LookupError {
     /// Support for this target was disabled in the current build.
+    #[fail(display = "Support for this target is disabled")]
     SupportDisabled,
 
     /// Support for this target has not yet been implemented.
+    #[fail(display = "Support for this target has not been implemented yet")]
     Unsupported,
 }
 

lib/codegen/src/settings.rs

@@ -147,15 +147,18 @@ impl Configurable for Builder {
 }
 
 /// An error produced when changing a setting.
-#[derive(Debug, PartialEq, Eq)]
+#[derive(Fail, Debug, PartialEq, Eq)]
 pub enum SetError {
     /// No setting by this name exists.
+    #[fail(display = "No existing setting with this name")]
     BadName,
 
     /// Type mismatch for setting (e.g., setting an enum setting as a bool).
+    #[fail(display = "Trying to set a setting with the wrong type")]
     BadType,
 
     /// This is not a valid value for this setting.
+    #[fail(display = "Unexpected value for a setting")]
     BadValue,
 }