uucore: fsext: Provide From<&str> helper for MetadataTimeField

We assume that the string has been validated by clap ahead of
time (just like the current code does).

Author: drinkcat

src/uu/du/src/du.rs

@@ -648,12 +648,9 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
     };
 
     let time = matches.contains_id(options::TIME).then(|| {
-        match matches.get_one::<String>(options::TIME).map(AsRef::as_ref) {
-            None | Some("ctime" | "status") => MetadataTimeField::Modification,
-            Some("access" | "atime" | "use") => MetadataTimeField::Access,
-            Some("birth" | "creation") => MetadataTimeField::Birth,
-            _ => unreachable!("should be caught by clap"),
-        }
+        matches
+            .get_one::<String>(options::TIME)
+            .map_or(MetadataTimeField::Modification, |s| s.as_str().into())
     });
 
     let size_format = if matches.get_flag(options::HUMAN_READABLE) {

src/uu/ls/src/ls.rs

@@ -464,14 +464,7 @@ fn extract_sort(options: &clap::ArgMatches) -> Sort {
 /// A `MetadataTimeField` variant representing the time to use.
 fn extract_time(options: &clap::ArgMatches) -> MetadataTimeField {
     if let Some(field) = options.get_one::<String>(options::TIME) {
-        match field.as_str() {
-            "ctime" | "status" => MetadataTimeField::Change,
-            "access" | "atime" | "use" => MetadataTimeField::Access,
-            "mtime" | "modification" => MetadataTimeField::Modification,
-            "birth" | "creation" => MetadataTimeField::Birth,
-            // below should never happen as clap already restricts the values.
-            _ => unreachable!("Invalid field for --time"),
-        }
+        field.as_str().into()
     } else if options.get_flag(options::time::ACCESS) {
         MetadataTimeField::Access
     } else if options.get_flag(options::time::CHANGE) {

src/uucore/src/lib/features/fsext.rs

@@ -135,6 +135,22 @@ pub enum MetadataTimeField {
     Birth,
 }
 
+impl From<&str> for MetadataTimeField {
+    /// Get a `MetadataTimeField` from a string, we expect the value
+    /// to come from clap, and be constrained there (e.g. if Modification is
+    /// not supported), and the default branch should not be reached.
+    fn from(value: &str) -> Self {
+        match value {
+            "ctime" | "status" => MetadataTimeField::Change,
+            "access" | "atime" | "use" => MetadataTimeField::Access,
+            "mtime" | "modification" => MetadataTimeField::Modification,
+            "birth" | "creation" => MetadataTimeField::Birth,
+            // below should never happen as clap already restricts the values.
+            _ => unreachable!("Invalid metadata time field."),
+        }
+    }
+}
+
 // The implementations for get_system_time are separated because some options, such
 // as ctime will not be available
 #[cfg(unix)]