Mark MappingTarget as #[non_exhaustive] for increased semver flexibility

This will allow us to add new enum variants in the future without breaking
semver compatibility. See
https://doc.rust-lang.org/reference/attributes/type_system.html#the-non_exhaustive-attribute

Since we already added an enum variant since v0.18.3, now is a good time to mark
it as `#[non_exhaustive]`.

src/bin/bat/main.rs

@@ -74,13 +74,9 @@ fn get_syntax_mapping_to_paths<'a>(
 ) -> HashMap<&'a str, Vec<String>> {
     let mut map = HashMap::new();
     for mapping in mappings {
-        match mapping {
-            (_, MappingTarget::MapToUnknown) => {}
-            (_, MappingTarget::MapExtensionToUnknown) => {}
-            (matcher, MappingTarget::MapTo(s)) => {
-                let globs = map.entry(*s).or_insert_with(Vec::new);
-                globs.push(matcher.glob().glob().into());
-            }
+        if let (matcher, MappingTarget::MapTo(s)) = mapping {
+            let globs = map.entry(*s).or_insert_with(Vec::new);
+            globs.push(matcher.glob().glob().into());
         }
     }
     map

src/syntax_mapping.rs

@@ -8,6 +8,7 @@ use globset::{Candidate, GlobBuilder, GlobMatcher};
 pub mod ignored_suffixes;
 
 #[derive(Debug, Clone, Copy, PartialEq)]
+#[non_exhaustive]
 pub enum MappingTarget<'a> {
     /// For mapping a path to a specific syntax.
     MapTo(&'a str),