Add "--list-languages" command line argument

src/main.rs

@@ -67,6 +67,7 @@ pub struct Options<'a> {
     pub language: Option<&'a str>,
     pub colored_output: bool,
     pub paging: bool,
+    pub term_width: usize,
 }
 
 enum OutputType {
@@ -453,6 +454,11 @@ fn run() -> Result<()> {
                 .default_value("auto")
                 .help("When to use the pager"),
         )
+        .arg(
+            Arg::with_name("list-languages")
+                .long("list-languages")
+                .help("Displays supported languages"),
+        )
         .subcommand(
             SubCommand::with_name("cache")
                 .about("Modify the syntax-definition and theme cache")
@@ -547,6 +553,7 @@ fn run() -> Result<()> {
                         interactive_terminal
                     },
                 },
+                term_width: console::Term::stdout().size().1 as usize,
             };
 
             let assets =
@@ -559,6 +566,50 @@ fn run() -> Result<()> {
                 )
             })?;
 
+            if app_matches.is_present("list-languages") {
+                let languages = assets.syntax_set.syntaxes();
+
+                let longest = languages
+                    .iter()
+                    .filter(|s| !s.hidden && s.file_extensions.len() > 0)
+                    .map(|s| s.name.len())
+                    .max()
+                    .unwrap_or(32); // Fallback width if they have no language definitions.
+
+                let separator = " ";
+                for lang in languages {
+                    if lang.hidden || lang.file_extensions.len() == 0 {
+                        continue;
+                    }
+                    print!("{:width$}{}", lang.name, separator, width = longest);
+
+                    // Line-wrapping for the possible file extension overflow.
+                    let desired_width = options.term_width - longest - separator.len();
+                    // Number of characters on this line so far, wrap before `desired_width`
+                    let mut num_chars = 0;
+
+                    let comma_separator = ", ";
+                    let mut extension = lang.file_extensions.iter().peekable();
+                    while let Some(word) = extension.next() {
+                        // If we can't fit this word in, then create a line break and align it in.
+                        let new_chars = word.len() + comma_separator.len();
+                        if num_chars + new_chars >= desired_width {
+                            num_chars = 0;
+                            print!("\n{:width$}{}", "", separator, width = longest);
+                        }
+
+                        num_chars += new_chars;
+                        print!("{}", Green.paint(word as &str));
+                        if extension.peek().is_some() {
+                            print!("{}", comma_separator);
+                        }
+                    }
+                    println!();
+                }
+
+                return Ok(());
+            }
+
             let mut output_type = get_output_type(options.paging);
             let handle = output_type.handle()?;
             let mut printer = Printer::new(handle, &options);

src/printer.rs

@@ -1,5 +1,4 @@
 use ansi_term::Style;
-use console::Term;
 use errors::*;
 use std::io::Write;
 use syntect::highlighting;
@@ -11,16 +10,12 @@ const PANEL_WIDTH: usize = 7;
 pub struct Printer<'a> {
     handle: &'a mut Write,
     colors: Colors,
-    term_width: usize,
     options: &'a Options<'a>,
     pub line_changes: Option<LineChanges>,
 }
 
 impl<'a> Printer<'a> {
     pub fn new(handle: &'a mut Write, options: &'a Options) -> Self {
-        let (_, term_width) = Term::stdout().size();
-        let term_width = term_width as usize;
-
         let colors = if options.colored_output {
             Colors::colored()
         } else {
@@ -30,7 +25,6 @@ impl<'a> Printer<'a> {
         Printer {
             handle,
             colors,
-            term_width,
             options,
             line_changes: None,
         }
@@ -141,7 +135,7 @@ impl<'a> Printer<'a> {
     }
 
     fn print_horizontal_line(&mut self, grid_char: char) -> Result<()> {
-        let hline = "─".repeat(self.term_width - (PANEL_WIDTH + 1));
+        let hline = "─".repeat(self.options.term_width - (PANEL_WIDTH + 1));
         let hline = format!("{}{}{}", "─".repeat(PANEL_WIDTH), grid_char, hline);
 
         writeln!(self.handle, "{}", self.colors.grid.paint(hline))?;