Display stdin CLI arguments as "(stdin)"

This improves display for #389, and makes language detection use
pattern matching on FileArgument rather than comparing literal
strings.

src/main.rs

@@ -234,6 +234,8 @@ fn diff_file(
     diff_file_content(
         lhs_display_path,
         rhs_display_path,
+        lhs_path,
+        rhs_path,
         &lhs_bytes,
         &rhs_bytes,
         display_options.tab_width,
@@ -246,6 +248,8 @@ fn diff_file(
 fn diff_file_content(
     lhs_display_path: &str,
     rhs_display_path: &str,
+    _lhs_path: &FileArgument,
+    rhs_path: &FileArgument,
     lhs_bytes: &[u8],
     rhs_bytes: &[u8],
     tab_width: usize,
@@ -283,13 +287,12 @@ fn diff_file_content(
         rhs_src.pop();
     }
 
-    // Prefer the RHS path for language detection, unless it's /dev/null.
-    let (guess_src, guess_path) = if rhs_display_path == "/dev/null" || rhs_display_path == "-" {
-        // TODO: take a Path directly instead.
-        (&lhs_src, Path::new(&lhs_display_path))
-    } else {
-        (&rhs_src, Path::new(&rhs_display_path))
+    let (guess_src, guess_path) = match rhs_path {
+        FileArgument::NamedPath(_) => (&rhs_src, Path::new(&rhs_display_path)),
+        FileArgument::Stdin => (&rhs_src, Path::new(&lhs_display_path)),
+        FileArgument::DevNull => (&lhs_src, Path::new(&lhs_display_path)),
     };
+
     let language = language_override.or_else(|| guess(guess_path, guess_src));
     let lang_config = language.map(tsp::from_language);
 
@@ -549,6 +552,8 @@ fn print_diff_result(display_options: &DisplayOptions, summary: &DiffResult) {
 
 #[cfg(test)]
 mod tests {
+    use std::ffi::OsStr;
+
     use super::*;
     use crate::options::{DEFAULT_BYTE_LIMIT, DEFAULT_GRAPH_LIMIT, DEFAULT_TAB_WIDTH};
 
@@ -558,6 +563,8 @@ mod tests {
         let res = diff_file_content(
             "foo.el",
             "foo.el",
+            &FileArgument::from_path_argument(OsStr::new("foo.el")),
+            &FileArgument::from_path_argument(OsStr::new("foo.el")),
             s.as_bytes(),
             s.as_bytes(),
             DEFAULT_TAB_WIDTH,

src/options.rs

@@ -314,18 +314,22 @@ pub fn parse_args() -> Mode {
 
     // TODO: document these different ways of calling difftastic.
     let (lhs_display_path, rhs_display_path, lhs_path, rhs_path, in_vcs) = match &args[..] {
-        [lhs_path, rhs_path] => (
-            lhs_path.to_owned(),
-            rhs_path.to_owned(),
-            FileArgument::from_cli_argument(lhs_path),
-            FileArgument::from_cli_argument(rhs_path),
-            false,
-        ),
+        [lhs_path, rhs_path] => {
+            let lhs_arg = FileArgument::from_cli_argument(lhs_path);
+            let rhs_arg = FileArgument::from_cli_argument(rhs_path);
+            (
+                lhs_arg.display(),
+                rhs_arg.display(),
+                lhs_arg,
+                rhs_arg,
+                false,
+            )
+        }
         [display_path, lhs_tmp_file, _lhs_hash, _lhs_mode, rhs_tmp_file, _rhs_hash, _rhs_mode] => {
             // https://git-scm.com/docs/git#Documentation/git.txt-codeGITEXTERNALDIFFcode
             (
-                display_path.to_owned(),
-                display_path.to_owned(),
+                display_path.to_string_lossy().to_string(),
+                display_path.to_string_lossy().to_string(),
                 FileArgument::from_path_argument(lhs_tmp_file),
                 FileArgument::from_path_argument(rhs_tmp_file),
                 true,
@@ -336,8 +340,8 @@ pub fn parse_args() -> Mode {
             // Rename file.
             // TODO: where does git document these 9 arguments?
             (
-                old_name.to_owned(),
-                new_name.to_owned(),
+                old_name.to_string_lossy().to_string(),
+                new_name.to_string_lossy().to_string(),
                 FileArgument::from_path_argument(lhs_tmp_file),
                 FileArgument::from_path_argument(rhs_tmp_file),
                 true,
@@ -439,8 +443,8 @@ pub fn parse_args() -> Mode {
         language_override,
         lhs_path,
         rhs_path,
-        lhs_display_path: lhs_display_path.to_string_lossy().to_string(),
-        rhs_display_path: rhs_display_path.to_string_lossy().to_string(),
+        lhs_display_path,
+        rhs_display_path,
     }
 }