Add a var! macro. (#548)(#550) r=emily

src/lib.rs

@@ -51,6 +51,16 @@ pub use mentat_db::{
     new_connection,
 };
 
+/// Produce the appropriate `Variable` for the provided valid ?-prefixed name.
+/// This lives here because we can't re-export macros:
+/// https://github.com/rust-lang/rust/issues/29638.
+#[macro_export]
+macro_rules! var {
+    ( ? $var:ident ) => {
+        $crate::Variable::from_valid_name(concat!("?", stringify!($var)))
+    };
+}
+
 /// Produce the appropriate `NamespacedKeyword` for the provided namespace and name.
 /// This lives here because we can't re-export macros:
 /// https://github.com/rust-lang/rust/issues/29638.
@@ -121,4 +131,13 @@ mod tests {
         assert_eq!(kw!(:foo/bar), NamespacedKeyword::new("foo", "bar"));
         assert_eq!(kw!(:org.mozilla.foo/bar_baz), NamespacedKeyword::new("org.mozilla.foo", "bar_baz"));
     }
+
+    #[test]
+    fn test_var() {
+        let foo_baz = var!(?foo_baz);
+        let vu = var!(?vü);
+        assert_eq!(foo_baz, Variable::from_valid_name("?foo_baz"));
+        assert_eq!(vu, Variable::from_valid_name("?vü"));
+        assert_eq!(foo_baz.as_str(), "?foo_baz");
+    }
 }