Detect hash collisions and produce an error.

stylance-cli/src/main.rs

@@ -1,5 +1,7 @@
+use anyhow::bail;
 use std::{
     borrow::Cow,
+    collections::HashMap,
     fs::{self, File},
     io::{BufWriter, Write},
     path::{Path, PathBuf},
@@ -115,6 +117,19 @@ fn run(run_params: &RunParams) -> anyhow::Result<()> {
         }
     }
 
+    {
+        let mut map = HashMap::new();
+        for file in modified_css_files.iter() {
+            if let Some(previous_file) = map.insert(&file.hash, file) {
+                bail!(
+                    "The following files had a hash collision:\n{}\n{}\nConsider increasing the hash_len setting.",
+                    file.path.to_string_lossy(),
+                    previous_file.path.to_string_lossy()
+                );
+            }
+        }
+    }
+
     if let Some(output_file) = &run_params.config.output_file {
         if let Some(parent) = output_file.parent() {
             fs::create_dir_all(parent)?;