Move CSS hashing back to bun_css

src/base64/Cargo.toml

@@ -24,8 +24,3 @@ thiserror.workspace = true
 
 bun_simdutf_sys.workspace = true
 bun_collections.workspace = true
-# `wyhash_url_safe` (CSS-modules / dependency placeholder hasher) — leaf crates,
-# no cycle.
-bun_wyhash.workspace = true
-bumpalo.workspace = true
-bun_alloc.workspace = true

src/base64/lib.rs

@@ -979,58 +979,3 @@ pub mod zig_base64 {
         }
     }
 }
-
-// ──────────────────────────────────────────────────────────────────────────
-// LAYERING: hoisted from `bun_css::css_modules::hash` so `bun_bundler` can
-// call the *same* implementation without taking a hard dep on `bun_css` (and
-// without re-implementing the hash, which would diverge — see review of
-// `LinkerContext.rs::css_modules_hash_shim`). `bun_css` re-exports this as
-// `css_modules::hash` for its in-crate callers.
-//
-// Behavior: wyhash(u64) of the formatted args,
-// truncated to u32, url-safe-base64-encoded into a bump-allocated slice. If
-// `at_start` and the first encoded byte is a digit, prefix `_` (CSS idents
-// can't start with a digit).
-// ──────────────────────────────────────────────────────────────────────────
-
-// TODO: replace with bun's hash
-pub fn wyhash_url_safe<'a>(
-    bump: &'a bun_alloc::Arena,
-    args: core::fmt::Arguments<'_>,
-    at_start: bool,
-) -> &'a [u8] {
-    use std::io::Write as _;
-
-    let mut hasher = bun_wyhash::Wyhash11::init(0);
-    // Write into a scratch Vec then hash; freed immediately.
-    let mut fmt_str: Vec<u8> = Vec::with_capacity(128);
-    write!(&mut fmt_str, "{}", args).expect("unreachable");
-    hasher.update(&fmt_str);
-
-    let h: u32 = hasher.final_() as u32; // @truncate
-    let h_bytes: [u8; 4] = h.to_le_bytes();
-
-    let encode_len = simdutf_encode_len_url_safe(h_bytes.len());
-
-    // Always alloc a fresh slice from the arena.
-    // PERF: no buffer reuse for large encode_len — profile if hot.
-    let slice_to_write: &mut [u8] =
-        bump.alloc_slice_fill_default(encode_len + usize::from(at_start));
-
-    let base64_encoded_hash_len = encode_url_safe(slice_to_write, &h_bytes);
-
-    let base64_encoded_hash = &slice_to_write[0..base64_encoded_hash_len];
-
-    if at_start
-        && !base64_encoded_hash.is_empty()
-        && base64_encoded_hash[0] >= b'0'
-        && base64_encoded_hash[0] <= b'9'
-    {
-        // Overlapping copy, dest > src → copy_within.
-        slice_to_write.copy_within(0..base64_encoded_hash_len, 1);
-        slice_to_write[0] = b'_';
-        return &slice_to_write[0..base64_encoded_hash_len + 1];
-    }
-
-    &slice_to_write[0..base64_encoded_hash_len]
-}

src/bundler/LinkerContext.rs

@@ -2397,7 +2397,7 @@ impl<'a> LinkerContext<'a> {
                         let original_name: &[u8] = symbol.original_name.slice();
                         // The hash itself is short-lived; use a scratch bump.
                         let scratch = ::bun_alloc::Arena::new();
-                        let path_hash = ::bun_base64::wyhash_url_safe(
+                        let path_hash = ::bun_css::css_modules::hash(
                             &scratch,
                             // use path relative to cwd for determinism
                             format_args!("{}", bstr::BStr::new(&source.path.pretty)),

src/css/css_modules.rs

@@ -1,4 +1,4 @@
-use core::fmt::Arguments;
+use core::fmt::{Arguments, Write as _};
 
 use bun_alloc::Arena as Bump;
 use bun_alloc::{ArenaVec as BumpVec, ArenaVecExt as _};
@@ -465,12 +465,62 @@ impl<'a> CssModuleReference<'a> {
     }
 }
 
-/// LAYERING: canonical implementation lives in `bun_base64::wyhash_url_safe`
-/// (a leaf crate) so `bun_bundler::LinkerContext::mangle_local_css` can call
-/// the *same* hasher without depending on `bun_css`. Re-export here so
-/// in-crate callers (`dependencies.rs`, `rules/import.rs`) keep the
-/// `css_modules::hash` path.
-#[inline]
+struct WyhashWriter<'a>(&'a mut bun_wyhash::Wyhash11);
+
+impl core::fmt::Write for WyhashWriter<'_> {
+    fn write_str(&mut self, s: &str) -> core::fmt::Result {
+        self.0.update(s.as_bytes());
+        Ok(())
+    }
+}
+
+/// Formats the arguments into the legacy Wyhash11 algorithm and encodes its
+/// low 32 bits as unpadded base64url. This output is embedded in generated CSS
+/// names and dependency placeholders, so the algorithm and byte order are
+/// compatibility-sensitive.
 pub fn hash<'a>(bump: &'a Bump, args: Arguments<'_>, at_start: bool) -> &'a [u8] {
-    bun_base64::wyhash_url_safe(bump, args, at_start)
+    let mut hasher = bun_wyhash::Wyhash11::init(0);
+    WyhashWriter(&mut hasher)
+        .write_fmt(args)
+        .expect("infallible: WyhashWriter::write_str always succeeds");
+
+    let hash_bytes = (hasher.final_() as u32).to_le_bytes();
+    let encoded_len = bun_base64::url_safe_encode_len(&hash_bytes);
+    let encoded_start = usize::from(at_start);
+    let output = bump.alloc_slice_fill_default(encoded_start + encoded_len);
+    let written = bun_base64::encode_url_safe(&mut output[encoded_start..], &hash_bytes);
+    debug_assert_eq!(written, encoded_len);
+
+    if at_start && output[encoded_start].is_ascii_digit() {
+        output[0] = b'_';
+        &output[..encoded_start + written]
+    } else {
+        &output[encoded_start..encoded_start + written]
+    }
+}
+
+#[cfg(test)]
+mod hash_tests {
+    use super::*;
+
+    #[test]
+    fn formatted_fragments_match_one_shot_hash() {
+        const INPUT: &[u8] = b"0123456789abcdefghijklmnopqrstu_more-than-one-wyhash-block";
+
+        let bump = Bump::new();
+        let actual = hash(
+            &bump,
+            format_args!(
+                "{}_{}",
+                bstr::BStr::new(b"0123456789abcdefghijklmnopqrstu"),
+                bstr::BStr::new(b"more-than-one-wyhash-block")
+            ),
+            false,
+        );
+
+        let hash_bytes = (bun_wyhash::Wyhash11::hash(0, INPUT) as u32).to_le_bytes();
+        let mut expected = [0; 6];
+        let written = bun_base64::encode_url_safe(&mut expected, &hash_bytes);
+        assert_eq!(actual, &expected[..written]);
+    }
 }