diff --git a/tools/cheddar/default.nix b/tools/cheddar/default.nix
index 5c86fec48..c65c782e6 100644
--- a/tools/cheddar/default.nix
+++ b/tools/cheddar/default.nix
@@ -1,20 +1,17 @@
-{ depot, ... }:
+{ pkgs, ... }:
 
-with depot.third_party;
-
-naersk.buildPackage {
+pkgs.naersk.buildPackage {
   src = ./.;
   doDoc = false;
   doCheck = false;
 
   override = x: {
-    # bat contains syntax highlighting packages for a lot more
-    # languages than what ships with syntect, and we can make use of
-    # them!
-    BAT_SYNTAXES = "${bat.src}/assets/syntaxes.bin";
+    # Use our custom bat syntax set, which is everything from upstream,
+    # plus additional languages we care about.
+    BAT_SYNTAXES = "${pkgs.bat_syntaxes}";
 
     # LLVM packages (why are they even required?) are not found
     # automatically if added to buildInputs, hence this ...
-    LIBCLANG_PATH = "${llvmPackages.libclang}/lib/libclang.so.10";
+    LIBCLANG_PATH = "${pkgs.llvmPackages.libclang}/lib/libclang.so.10";
   };
 }
diff --git a/tools/cheddar/src/main.rs b/tools/cheddar/src/main.rs
index 52e518cd8..01e6c868f 100644
--- a/tools/cheddar/src/main.rs
+++ b/tools/cheddar/src/main.rs
@@ -3,6 +3,7 @@ use comrak::nodes::{Ast, AstNode, NodeValue, NodeCodeBlock, NodeHtmlBlock};
 use comrak::{Arena, parse_document, format_html, ComrakOptions};
 use lazy_static::lazy_static;
 use std::cell::RefCell;
+use std::collections::HashMap;
 use std::env;
 use std::ffi::OsStr;
 use std::io::BufRead;
@@ -42,12 +43,23 @@ lazy_static! {
         unsafe_: true, // required for tagfilter
         ..ComrakOptions::default()
     };
+
+    // Configures a map of specific filenames to languages, for cases
+    // where the detection by extension or other heuristics fails.
+    static ref FILENAME_OVERRIDES: HashMap<&'static str, &'static str> = {
+        let mut map = HashMap::new();
+        // rules.pl is the canonical name of the submit rule file in
+        // Gerrit, which is written in Prolog.
+        map.insert("rules.pl", "Prolog");
+        map
+    };
 }
 
 // HTML fragment used when rendering inline blocks in Markdown documents.
 // Emulates the GitHub style (subtle background hue and padding).
 const BLOCK_PRE: &str = "<pre style=\"background-color:#f6f8fa;padding:16px;\">\n";
 
+#[derive(Debug, Default)]
 struct Args {
     /// Should Cheddar run as an about filter? (i.e. give special
     /// rendering treatment to Markdown documents)
@@ -55,16 +67,16 @@ struct Args {
 
     /// What file extension has been supplied (if any)?
     extension: Option<String>,
+
+    /// Which language to override the detection to (if any)?
+    lang_override: Option<&'static str>,
 }
 
 /// Parse the command-line flags passed to cheddar to determine
 /// whether it is running in about-filter mode (`--about-filter`) and
 /// what file extension has been supplied.
 fn parse_args() -> Args {
-    let mut args = Args {
-        about_filter: false,
-        extension: None,
-    };
+    let mut args = Args::default();
 
     for (i, arg) in env::args().enumerate() {
         if i == 0 {
@@ -76,6 +88,10 @@ fn parse_args() -> Args {
             continue;
         }
 
+        if let Some(lang) = (*FILENAME_OVERRIDES).get(arg.as_str()) {
+            args.lang_override = Some(lang);
+        }
+
         args.extension = Path::new(&arg)
             .extension()
             .and_then(OsStr::to_str)
@@ -244,7 +260,7 @@ fn format_markdown() {
         .expect("Markdown rendering failed");
 }
 
-fn format_code(extension: Option<&str>) {
+fn format_code(args: &Args) {
     let stdin = io::stdin();
     let mut stdin = stdin.lock();
     let mut linebuf = String::new();
@@ -255,8 +271,12 @@ fn format_code(extension: Option<&str>) {
     // Set up the highlighter
     let theme = &THEMES.themes["InspiredGitHub"];
 
-    let syntax = extension
-        .and_then(|e| SYNTAXES.find_syntax_by_extension(e))
+    let syntax = args.lang_override
+        .and_then(|l| SYNTAXES.find_syntax_by_name(l))
+        .or_else(|| match args.extension {
+            Some(ref ext) => SYNTAXES.find_syntax_by_extension(ext),
+            None => None,
+        })
         .or_else(|| SYNTAXES.find_syntax_by_first_line(&linebuf))
         .unwrap_or_else(|| SYNTAXES.find_syntax_plain_text());
 
@@ -296,6 +316,6 @@ fn main() {
 
     match args.extension.as_ref().map(String::as_str) {
         Some("md") if args.about_filter => format_markdown(),
-        extension  => format_code(extension),
+        _  => format_code(&args),
     }
 }