fix: limit loading of grammars

src/main.rs

@@ -64,6 +64,11 @@ struct AppConfig {
     #[arg(short = 'g', long, value_name = "DIR", default_value = None)]
     syntax_root: Option<PathBuf>,
 
+    /// Grammars to load, separated by commas.
+    /// Useful to avoid the memory required to load all languages.
+    #[arg(short = 'L', long = "lang", value_name = "NAME", value_delimiter = ',')]
+    languages: Vec<String>,
+
     /// The log level to use
     #[arg(short = 'l', long, value_name = "LEVEL", default_value_t = Level::INFO)]
     log_level: Level,

src/syntax.rs

@@ -1,4 +1,11 @@
-use std::{borrow::Cow, collections::HashMap, error::Error, fs, path::Path, sync::OnceLock};
+use std::{
+    borrow::Cow,
+    collections::{HashMap, HashSet},
+    error::Error,
+    fs,
+    path::Path,
+    sync::OnceLock,
+};
 
 use tracing::{info, warn};
 use tree_house::{
@@ -67,12 +74,20 @@ struct LanguageRegistry {
 }
 
 impl LanguageRegistry {
-    fn new(syntax_root: &Path) -> Self {
+    /// Loads grammars from `syntax_root`. When `languages` is non-empty, only
+    /// grammars with those names are loaded; otherwise every grammar found is.
+    fn new(syntax_root: &Path, languages: &[String]) -> Self {
+        let filter: Option<HashSet<&str>> =
+            (!languages.is_empty()).then(|| languages.iter().map(String::as_str).collect());
+
         let mut by_name = HashMap::new();
         let mut configs = Vec::new();
 
         for entry in fs::read_dir(syntax_root).into_iter().flatten().flatten() {
             let name = entry.file_name().to_string_lossy().into_owned();
+            if filter.as_ref().is_some_and(|f| !f.contains(name.as_str())) {
+                continue;
+            }
             let so_path = entry.path().join(format!("{name}.so"));
             if !so_path.exists() {
                 continue;
@@ -87,6 +102,13 @@ impl LanguageRegistry {
             }
         }
 
+        // Warn about explicitly requested grammars that were not found.
+        for lang in languages {
+            if !by_name.contains_key(lang) {
+                warn!("Requested grammar {lang} was not found in the syntax root");
+            }
+        }
+
         info!("Loaded {} syntax highlighting grammars", configs.len());
         Self { by_name, configs }
     }
@@ -138,7 +160,7 @@ pub fn init(config: &AppConfig) {
     match &config.syntax_root {
         Some(syntax_root) => {
             info!("Loading syntax files from {}", syntax_root.display());
-            REGISTRY.get_or_init(|| LanguageRegistry::new(syntax_root));
+            REGISTRY.get_or_init(|| LanguageRegistry::new(syntax_root, &config.languages));
         }
         None => {
             warn!(