Support - as a CLI argument

Fixes #389

CHANGELOG.md

@@ -11,6 +11,9 @@ which improves diff results in some cases.
 
 ### Command Line Interface
 
+Difftastic will now read a file from stdin if given `-` as a path
+argument.
+
 `--list-languages` now respects the value of `--color`, whose default
 only uses styling when stdout is a TTY.
 

manual/src/usage.md

@@ -1,12 +1,14 @@
 # Usage
 
-## Diffing Files
+## File Arguments
+
+### Diffing Files
 
 ```
 $ difft sample_files/before.js sample_files/after.js
 ```
 
-## Diffing Directories
+### Diffing Directories
 
 ```
 $ difft sample_files/dir_before/ sample_files/dir_after/
@@ -18,6 +20,14 @@ with the same name.
 The `--skip-unchanged` option is useful when diffing directories that
 contain many unchanged files.
 
+### Reading stdin
+
+You can read a file from stdin by specifying `-` as the file path.
+
+```
+$ cat sample_files/before.js | difft - sample_files/after.js
+```
+
 ## Language Detection
 
 Difftastic guesses the language used based on the file extension, file

src/files.rs

@@ -1,5 +1,6 @@
 //! File reading utilities.
 
+use std::io::Read;
 use std::{
     fs,
     io::ErrorKind::*,
@@ -41,7 +42,7 @@ pub fn read_files_or_die(
     }
 }
 
-/// Read a path provided in a CLI argument, handling /dev/null
+/// Read a path provided in a CLI argument, handling /dev/null and -
 /// correctly.
 fn read_cli_path(path: &Path) -> std::io::Result<Vec<u8>> {
     // Treat /dev/null as an empty file, even on platforms like
@@ -51,6 +52,16 @@ fn read_cli_path(path: &Path) -> std::io::Result<Vec<u8>> {
         return Ok(vec![]);
     }
 
+    if path == Path::new("-") {
+        let stdin = std::io::stdin();
+        let mut handle = stdin.lock();
+
+        let mut bytes = vec![];
+        handle.read_to_end(&mut bytes)?;
+
+        return Ok(bytes);
+    }
+
     fs::read(path)
 }
 

src/main.rs

@@ -280,7 +280,7 @@ fn diff_file_content(
     }
 
     // Prefer the RHS path for language detection, unless it's /dev/null.
-    let (guess_src, guess_path) = if rhs_display_path == "/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 {