Add a --list-languages option

Fixes #292

CHANGELOG.md

@@ -14,6 +14,9 @@ Fixed an issue with HCL language detection on `.workflow` files.
 Fixed terminal width detection when only stderr is a TTY (e.g. when
 using difftastic with git). This was broken in 0.34.
 
+Added an option `--list-languages` which reports all the languages
+supported, along with the extensions associated with them.
+
 ## 0.34 (released 27th August 2022)
 
 ### Build

src/main.rs

@@ -32,6 +32,7 @@ extern crate log;
 
 use crate::diff::{dijkstra, unchanged};
 use crate::display::hunks::{matched_pos_to_hunks, merge_adjacent};
+use crate::parse::guess_language::LANG_EXTENSIONS;
 use crate::parse::syntax;
 use diff::changes::ChangeMap;
 use diff::dijkstra::ExceededGraphLimit;
@@ -126,6 +127,15 @@ fn main() {
                 }
             }
         }
+        Mode::ListLanguages => {
+            for (name, extensions) in LANG_EXTENSIONS {
+                print!("{:?}", name);
+                for extension in *extensions {
+                    print!(" .{}", extension);
+                }
+                println!();
+            }
+        }
         Mode::Diff {
             graph_limit,
             byte_limit,

src/options.rs

@@ -136,6 +136,10 @@ fn app() -> clap::Command<'static> {
                 .help("Override language detection. Inputs are assumed to have this file extension. When diffing directories, applies to all files.")
                 // TODO: support DFT_LANGUAGE for consistency
         )
+        .arg(
+            Arg::new("list-languages").long("list-languages")
+                .help("Print the all the languages supported by difftastic, along with their extensions.")
+        )
         .arg(
             Arg::new("byte-limit").long("byte-limit")
                 .takes_value(true)
@@ -191,6 +195,7 @@ pub enum Mode {
         /// The path that we should display for the RHS file.
         rhs_display_path: String,
     },
+    ListLanguages,
     DumpTreeSitter {
         path: String,
         language_override: Option<guess_language::Language>,
@@ -220,6 +225,10 @@ pub fn parse_args() -> Mode {
         None => None,
     };
 
+    if matches.is_present("list-languages") {
+        return Mode::ListLanguages;
+    }
+
     if let Some(path) = matches.value_of("dump-syntax") {
         return Mode::DumpSyntax {
             path: path.to_string(),

src/parse/guess_language.rs

@@ -64,7 +64,7 @@ pub enum Language {
     Zig,
 }
 
-const LANG_EXTENSIONS: &'static [(Language, &[&str])] = &[
+pub const LANG_EXTENSIONS: &'static [(Language, &[&str])] = &[
     (
         Bash,
         &[