feat(fileio): move buffer helpers to Rust

Route file-I/O input/output buffer construction and the LZ4 block-size mapping through Rust ABI leaves while preserving the existing C helper names and public buffer layouts. Add layout, field, and formula coverage.

Test Plan: cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,decompression,benchmark; cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets --no-default-features --features cli,compression,decompression,benchmark; make -B -C programs -j2 zstd zstd-small zstd-frugal; make -B -C tests -j2 test-cli-tests

rust/src/fileio_prefs.rs

@@ -20,6 +20,20 @@ const FIO_ZSTD_COMPRESSION: c_int = 0;
 const FIO_OVERLAP_LOG_NOTSET: c_int = 9999;
 const FIO_LDM_PARAM_NOTSET: c_int = 9999;
 
+#[repr(C)]
+pub struct FIO_inBuffer {
+    src: *const c_void,
+    size: usize,
+    pos: usize,
+}
+
+#[repr(C)]
+pub struct FIO_outBuffer {
+    dst: *mut c_void,
+    size: usize,
+    pos: usize,
+}
+
 #[cfg(target_vendor = "apple")]
 const ZSTD_SPARSE_DEFAULT: c_int = 0;
 #[cfg(not(target_vendor = "apple"))]
@@ -517,6 +531,53 @@ pub unsafe extern "C" fn FIO_rust_getLargestFileSize(
     )
 }
 
+/// Fill a C-compatible input buffer through an output pointer.
+///
+/// The pointer form avoids relying on a cross-language struct return ABI while
+/// retaining the public `ZSTD_inBuffer` field order and values.
+#[no_mangle]
+pub unsafe extern "C" fn FIO_rust_setInBuffer(
+    output: *mut FIO_inBuffer,
+    buf: *const c_void,
+    size: usize,
+    pos: usize,
+) {
+    unsafe {
+        output.write(FIO_inBuffer {
+            src: buf,
+            size,
+            pos,
+        });
+    }
+}
+
+/// Fill a C-compatible output buffer through an output pointer.
+#[no_mangle]
+pub unsafe extern "C" fn FIO_rust_setOutBuffer(
+    output: *mut FIO_outBuffer,
+    buf: *mut c_void,
+    size: usize,
+    pos: usize,
+) {
+    unsafe {
+        output.write(FIO_outBuffer {
+            dst: buf,
+            size,
+            pos,
+        });
+    }
+}
+
+#[inline]
+fn lz4_block_size_from_block_id(id: c_int) -> c_int {
+    1 << (8 + 2 * id)
+}
+
+#[no_mangle]
+pub extern "C" fn FIO_rust_LZ4_GetBlockSize_FromBlockId(id: c_int) -> c_int {
+    lz4_block_size_from_block_id(id)
+}
+
 #[inline]
 fn filename_path_separator() -> u8 {
     if cfg!(windows) {
@@ -650,6 +711,76 @@ mod tests {
         );
     }
 
+    #[test]
+    fn buffer_shims_preserve_fields_and_c_layout() {
+        let input_word = size_of::<*const c_void>();
+        let output_word = size_of::<*mut c_void>();
+
+        assert_eq!(offset_of!(FIO_inBuffer, src), 0);
+        assert_eq!(offset_of!(FIO_inBuffer, size), input_word);
+        assert_eq!(
+            offset_of!(FIO_inBuffer, pos),
+            input_word + size_of::<usize>()
+        );
+        assert_eq!(
+            size_of::<FIO_inBuffer>(),
+            input_word + 2 * size_of::<usize>()
+        );
+        assert_eq!(offset_of!(FIO_outBuffer, dst), 0);
+        assert_eq!(offset_of!(FIO_outBuffer, size), output_word);
+        assert_eq!(
+            offset_of!(FIO_outBuffer, pos),
+            output_word + size_of::<usize>()
+        );
+        assert_eq!(
+            size_of::<FIO_outBuffer>(),
+            output_word + 2 * size_of::<usize>()
+        );
+
+        let source = [1u8, 2, 3, 4];
+        let mut destination = [0u8; 8];
+        let mut input = FIO_inBuffer {
+            src: ptr::null(),
+            size: 0,
+            pos: 0,
+        };
+        let mut output = FIO_outBuffer {
+            dst: ptr::null_mut(),
+            size: 0,
+            pos: 0,
+        };
+
+        unsafe {
+            FIO_rust_setInBuffer(
+                &mut input,
+                source.as_ptr().cast::<c_void>(),
+                source.len(),
+                1,
+            );
+            FIO_rust_setOutBuffer(
+                &mut output,
+                destination.as_mut_ptr().cast::<c_void>(),
+                destination.len(),
+                2,
+            );
+        }
+
+        assert_eq!(input.src, source.as_ptr().cast::<c_void>());
+        assert_eq!(input.size, source.len());
+        assert_eq!(input.pos, 1);
+        assert_eq!(output.dst, destination.as_mut_ptr().cast::<c_void>());
+        assert_eq!(output.size, destination.len());
+        assert_eq!(output.pos, 2);
+    }
+
+    #[test]
+    fn lz4_block_size_shim_preserves_the_block_id_formula() {
+        assert_eq!(FIO_rust_LZ4_GetBlockSize_FromBlockId(0), 1 << 8);
+        assert_eq!(FIO_rust_LZ4_GetBlockSize_FromBlockId(1), 1 << 10);
+        assert_eq!(FIO_rust_LZ4_GetBlockSize_FromBlockId(3), 1 << 14);
+        assert_eq!(FIO_rust_LZ4_GetBlockSize_FromBlockId(4), 1 << 16);
+    }
+
     #[test]
     fn c_layouts_match_the_headers() {
         let word = size_of::<usize>();