Support diffing directories where files are only in one side

This was broken in 1e9f43768. Add test.

Fixes #500

src/files.rs

@@ -3,6 +3,7 @@
 use std::io::Read;
 use std::{
     fs,
+    io::ErrorKind::*,
     path::{Path, PathBuf},
 };
 
@@ -12,7 +13,11 @@ use walkdir::WalkDir;
 use crate::exit_codes::EXIT_BAD_ARGUMENTS;
 use crate::options::FileArgument;
 
-pub fn read_files_or_die(lhs_path: &FileArgument, rhs_path: &FileArgument) -> (Vec<u8>, Vec<u8>) {
+pub fn read_files_or_die(
+    lhs_path: &FileArgument,
+    rhs_path: &FileArgument,
+    missing_as_empty: bool,
+) -> (Vec<u8>, Vec<u8>) {
     let lhs_res = read_file_arg(lhs_path);
     let rhs_res = read_file_arg(rhs_path);
 
@@ -20,6 +25,12 @@ pub fn read_files_or_die(lhs_path: &FileArgument, rhs_path: &FileArgument) -> (V
         // Both files exist, the happy case.
         (Ok(lhs_src), Ok(rhs_src)) => (lhs_src, rhs_src),
 
+        // Proceed if we've been given two paths and only one
+        // exists. This is important for mercurial diffs when a file
+        // has been removed.
+        (Ok(lhs_src), Err(e)) if missing_as_empty && e.kind() == NotFound => (lhs_src, vec![]),
+        (Err(e), Ok(rhs_src)) if missing_as_empty && e.kind() == NotFound => (vec![], rhs_src),
+
         (lhs_res, rhs_res) => {
             // Something else went wrong. Print both errors
             // encountered.

src/main.rs

@@ -237,6 +237,7 @@ fn main() {
                         &rhs_path,
                         &display_options,
                         &diff_options,
+                        false,
                         language_override,
                     );
                     print_diff_result(&display_options, &diff_result);
@@ -280,9 +281,10 @@ fn diff_file(
     rhs_path: &FileArgument,
     display_options: &DisplayOptions,
     diff_options: &DiffOptions,
+    missing_as_empty: bool,
     language_override: Option<parse::guess_language::Language>,
 ) -> DiffResult {
-    let (lhs_bytes, rhs_bytes) = read_files_or_die(lhs_path, rhs_path);
+    let (lhs_bytes, rhs_bytes) = read_files_or_die(lhs_path, rhs_path, missing_as_empty);
     diff_file_content(
         lhs_display_path,
         rhs_display_path,
@@ -618,6 +620,7 @@ fn diff_directories<'a>(
             &FileArgument::NamedPath(rhs_path),
             &display_options,
             &diff_options,
+            true,
             language_override,
         )
     })

tests/cli.rs

@@ -131,3 +131,15 @@ fn slightly_invalid_utf8() {
     let predicate_fn = predicate::str::contains("using");
     cmd.assert().stdout(predicate_fn);
 }
+
+#[test]
+fn directory_arguments() {
+    let mut cmd = Command::cargo_bin("difft").unwrap();
+
+    cmd.arg("sample_files/dir_before")
+        .arg("sample_files/dir_after");
+
+    let predicate_fn = predicate::str::contains("only_in_before.c")
+        .and(predicate::str::contains("only_in_after_dir.rs"));
+    cmd.assert().stdout(predicate_fn);
+}