feat(rust): export compression bound from Rust

Move the context-free public ZSTD_compressBound ABI into Rust while leaving
the stateful compressor context in C. The Rust implementation preserves the
size-width-specific input limit, macro arithmetic, and srcSize_wrong error
contract used by C callers.

Test Plan:
- cargo clippy
- cargo clippy --benches
- cargo clippy --tests
- cargo +nightly fmt
- cargo test zstd_compress_api::tests -- --nocapture
- cargo test --target i686-unknown-linux-gnu zstd_compress_api::tests
- make -B -C lib -j2 libzstd.a
- fuzzer -s5346 -i1 --no-big-tests

Refs: public ZSTD_COMPRESSBOUND macro in lib/zstd.h

lib/compress/zstd_compress.c

@@ -59,19 +59,9 @@
 /*-*************************************
 *  Helper functions
 ***************************************/
-/* ZSTD_compressBound()
- * Note that the result from this function is only valid for
- * the one-pass compression functions.
- * When employing the streaming mode,
- * if flushes are frequently altering the size of blocks,
- * the overhead from block headers can make the compressed data larger
- * than the return value of ZSTD_compressBound().
- */
-size_t ZSTD_compressBound(size_t srcSize) {
-    size_t const r = ZSTD_COMPRESSBOUND(srcSize);
-    if (r==0) return ERROR(srcSize_wrong);
-    return r;
-}
+/* ZSTD_compressBound() lives in rust/src/zstd_compress_api.rs.  Keep this
+ * translation unit for the context implementation, which still calls the
+ * exported ABI symbol. */
 
 
 /*-*************************************

rust/src/lib.rs

@@ -22,6 +22,8 @@ pub mod threading;
 pub mod xxhash;
 pub mod zstd_common;
 #[cfg(feature = "compression")]
+pub mod zstd_compress_api;
+#[cfg(feature = "compression")]
 pub mod zstd_compress_literals;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_sequences;

rust/src/zstd_compress_api.rs

@@ -0,0 +1,45 @@
+//! Public, context-free compression API helpers.
+//!
+//! The stateful compression context remains in the C translation unit for
+//! now.  These helpers have no context layout dependency, so they can expose
+//! the public ABI directly from Rust.
+
+use crate::errors::{ZstdErrorCode, ERROR};
+
+#[cfg(target_pointer_width = "64")]
+const ZSTD_MAX_INPUT_SIZE: usize = 0xFF00_FF00_FF00_FF00;
+#[cfg(target_pointer_width = "32")]
+const ZSTD_MAX_INPUT_SIZE: usize = 0xFF00_FF00;
+
+const SMALL_INPUT_THRESHOLD: usize = 128 << 10;
+
+/// Returns the maximum one-pass compressed size, matching
+/// `ZSTD_COMPRESSBOUND()` and its public error contract.
+#[no_mangle]
+pub extern "C" fn ZSTD_compressBound(src_size: usize) -> usize {
+    if src_size >= ZSTD_MAX_INPUT_SIZE {
+        return ERROR(ZstdErrorCode::SrcSizeWrong);
+    }
+
+    let margin = if src_size < SMALL_INPUT_THRESHOLD {
+        (SMALL_INPUT_THRESHOLD - src_size) >> 11
+    } else {
+        0
+    };
+    src_size.wrapping_add(src_size >> 8).wrapping_add(margin)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::errors::ERR_isError;
+
+    #[test]
+    fn compress_bound_matches_the_public_macro_edges() {
+        assert_eq!(ZSTD_compressBound(0), 64);
+        assert_eq!(ZSTD_compressBound(1), 64);
+        assert_eq!(ZSTD_compressBound(SMALL_INPUT_THRESHOLD - 1), 131_582);
+        assert_eq!(ZSTD_compressBound(SMALL_INPUT_THRESHOLD), 131_584);
+        assert!(ERR_isError(ZSTD_compressBound(ZSTD_MAX_INPUT_SIZE)));
+    }
+}