Introduce target_spec_enum macro to avoid duplication

With this macro we only need to enumerate every variant once. This saves
a lot of duplication already between the definition, the `FromStr` impl
and the `ToJson` impl.

It also enables us to do further things with it like JSON schema
generation.

compiler/rustc_target/src/lib.rs

@@ -72,3 +72,63 @@ fn find_relative_libdir(sysroot: &Path) -> std::borrow::Cow<'static, str> {
         Some(libdir) => libdir.into(),
     }
 }
+
+macro_rules! target_spec_enum {
+    (
+        $( #[$attr:meta] )*
+        pub enum $name:ident {
+            $(
+                $( #[$variant_attr:meta] )*
+                $variant:ident = $string:literal,
+            )*
+        }
+        parse_error_type = $parse_error_type:literal;
+    ) => {
+        $( #[$attr] )*
+        #[derive(Clone, Copy, PartialEq, Eq, Hash, Debug, PartialOrd, Ord)]
+        pub enum $name {
+            $(
+                $( #[$variant_attr] )*
+                $variant,
+            )*
+        }
+
+        impl FromStr for $name {
+            type Err = String;
+
+            fn from_str(s: &str) -> Result<Self, Self::Err> {
+                Ok(match s {
+                    $( $string => Self::$variant, )*
+                    _ => {
+                        let all = [$( concat!("'", $string, "'") ),*].join(", ");
+                        return Err(format!("invalid {}: '{s}'. allowed values: {all}", $parse_error_type));
+                    }
+                })
+            }
+        }
+
+        impl $name {
+            pub fn desc(&self) -> &'static str {
+                match self {
+                    $( Self::$variant => $string, )*
+                }
+            }
+        }
+
+        impl crate::json::ToJson for $name {
+            fn to_json(&self) -> crate::json::Json {
+                self.desc().to_json()
+            }
+        }
+
+        crate::json::serde_deserialize_from_str!($name);
+
+
+        impl std::fmt::Display for $name {
+            fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+                f.write_str(self.desc())
+            }
+        }
+    };
+}
+use target_spec_enum;