feat(tools/cheddar): Check in new syntax highlighting tool

The first step with this tool will be to use it as a source-filter for
cgit. The second step is to use it as the Markdown renderer by
depending on one of the Markdown libraries, with integration for
rendering code snippets directly.

tools/cheddar/src/main.rs

@@ -0,0 +1,84 @@
+use std::env;
+use std::ffi::OsStr;
+use std::io::BufRead;
+use std::io;
+use std::path::Path;
+use syntect::easy::HighlightLines;
+use syntect::highlighting::ThemeSet;
+use syntect::parsing::{SyntaxSet, SyntaxReference};
+
+use syntect::html::{
+    append_highlighted_html_for_styled_line,
+    start_highlighted_html_snippet,
+    IncludeBackground,
+};
+
+fn syntax_from_args(syntaxes: &SyntaxSet) -> Option<&SyntaxReference> {
+    // The name of the file to be formatted is usually passed in as
+    // the first argument and can be used to determine a syntax set.
+    let args = env::args().collect::<Vec<String>>();
+    if args.len() != 2 {
+        return None
+    }
+
+    Path::new(&args[1])
+        .extension()
+        .and_then(OsStr::to_str)
+        .and_then(|ext| syntaxes.find_syntax_by_extension(ext))
+}
+
+fn should_continue(res: &io::Result<usize>) -> bool {
+    match *res {
+        Ok(n) => n > 0,
+        Err(_) => false,
+    }
+}
+
+fn main() {
+    let syntaxes = SyntaxSet::load_defaults_newlines();
+
+    let stdin = io::stdin();
+    let mut stdin = stdin.lock();
+    let mut linebuf = String::new();
+
+    // Get the first line, we might need it for syntax identification.
+    let mut read_result = stdin.read_line(&mut linebuf);
+
+    // Set up the highlighter
+    let ts = ThemeSet::load_defaults();
+    let theme = &ts.themes["InspiredGitHub"];
+
+    let syntax = syntax_from_args(&syntaxes)
+        .or_else(|| syntaxes.find_syntax_by_first_line(&linebuf))
+        .unwrap_or_else(|| syntaxes.find_syntax_plain_text());
+
+    let mut hl = HighlightLines::new(syntax, theme);
+    let (mut outbuf, bg) = start_highlighted_html_snippet(theme);
+
+    // Rather than using the `lines` iterator, read each line manually
+    // and maintain buffer state.
+    //
+    // This is done because the syntax highlighter requires trailing
+    // newlines to be efficient, and those are stripped in the lines
+    // iterator.
+    while should_continue(&read_result) {
+        let regions = hl.highlight(&linebuf, &syntaxes);
+
+        append_highlighted_html_for_styled_line(
+            &regions[..],
+            IncludeBackground::IfDifferent(bg),
+            &mut outbuf,
+        );
+
+        // immediately output the current state to avoid keeping
+        // things in memory
+        print!("{}", outbuf);
+
+        // merry go round again
+        linebuf.clear();
+        outbuf.clear();
+        read_result = stdin.read_line(&mut linebuf);
+    }
+
+    println!("</pre>");
+}