refactor(compress): move stable buffer policy to Rust

Move the stable input and output expectation update into the Rust compression
module. Keep the private CCtx layout and debug logging in the C adapter while
passing only the buffer-mode scalars and projected buffer fields across the
ABI. Preserve C size_t wrapping for the output remainder and leave buffered
modes unchanged.

Test Plan:
- rustfmt --edition 2021 --check rust/src/zstd_compress.rs
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --all-targets
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets
- ulimit -v 41943040; make -j1 -C tests test

lib/compress/zstd_compress.c

@@ -1792,6 +1792,10 @@ size_t ZSTD_rust_checkBufferStability(
         const void* expectedInSrc, size_t expectedInPos,
         const void* inputSrc, size_t inputPos,
         size_t expectedOutBufferSize, size_t outputSize, size_t outputPos);
+void ZSTD_rust_setBufferExpectations(
+        int inBufferMode, int outBufferMode,
+        ZSTD_inBuffer* expectedInBuffer, size_t* expectedOutBufferSize,
+        const ZSTD_outBuffer* output, const ZSTD_inBuffer* input);
 
 /* Context-free compression-parameter selection and sizing leaves live in
  * Rust (rust/src/zstd_compress_params.rs).  This file retains
@@ -7690,12 +7694,13 @@ static void
 ZSTD_setBufferExpectations(ZSTD_CCtx* cctx, const ZSTD_outBuffer* output, const ZSTD_inBuffer* input)
 {
     DEBUGLOG(5, "ZSTD_setBufferExpectations (for advanced stable in/out modes)");
-    if (cctx->appliedParams.inBufferMode == ZSTD_bm_stable) {
-        cctx->expectedInBuffer = *input;
-    }
-    if (cctx->appliedParams.outBufferMode == ZSTD_bm_stable) {
-        cctx->expectedOutBufferSize = output->size - output->pos;
-    }
+    ZSTD_rust_setBufferExpectations(
+            (int)cctx->appliedParams.inBufferMode,
+            (int)cctx->appliedParams.outBufferMode,
+            &cctx->expectedInBuffer,
+            &cctx->expectedOutBufferSize,
+            output,
+            input);
 }
 
 /* Validate that the input/output buffers match the expectations set by

rust/src/zstd_compress.rs

@@ -6169,6 +6169,53 @@ pub extern "C" fn ZSTD_rust_checkBufferStability(
     )
 }
 
+#[inline]
+unsafe fn set_buffer_expectations(
+    in_buffer_mode: c_int,
+    out_buffer_mode: c_int,
+    expected_in_buffer: *mut ZSTD_inBuffer,
+    expected_out_buffer_size: *mut usize,
+    output: *const ZSTD_outBuffer,
+    input: *const ZSTD_inBuffer,
+) {
+    if in_buffer_mode == ZSTD_BM_STABLE {
+        unsafe {
+            *expected_in_buffer = ptr::read(input);
+        }
+    }
+    if out_buffer_mode == ZSTD_BM_STABLE {
+        let output = unsafe { &*output };
+        unsafe {
+            *expected_out_buffer_size = output.size.wrapping_sub(output.pos);
+        }
+    }
+}
+
+/// Update the stable input/output expectations after a compression call.
+///
+/// C retains the private `ZSTD_CCtx` fields and passes only their projections;
+/// Rust owns the buffer-mode policy and C-size arithmetic.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_setBufferExpectations(
+    in_buffer_mode: c_int,
+    out_buffer_mode: c_int,
+    expected_in_buffer: *mut ZSTD_inBuffer,
+    expected_out_buffer_size: *mut usize,
+    output: *const ZSTD_outBuffer,
+    input: *const ZSTD_inBuffer,
+) {
+    unsafe {
+        set_buffer_expectations(
+            in_buffer_mode,
+            out_buffer_mode,
+            expected_in_buffer,
+            expected_out_buffer_size,
+            output,
+            input,
+        );
+    }
+}
+
 #[inline]
 fn select_sequence_copier(mode: c_int) -> c_int {
     debug_assert!(
@@ -14597,6 +14644,75 @@ mod tests {
         );
     }
 
+    #[test]
+    fn set_buffer_expectations_updates_only_enabled_stable_modes() {
+        let expected_src = b"expected".as_ptr().cast::<c_void>();
+        let input_src = b"input".as_ptr().cast::<c_void>();
+        let mut expected_input = ZSTD_inBuffer {
+            src: expected_src,
+            size: 3,
+            pos: 1,
+        };
+        let input = ZSTD_inBuffer {
+            src: input_src,
+            size: 11,
+            pos: 7,
+        };
+        let output = ZSTD_outBuffer {
+            dst: ptr::null_mut(),
+            size: 40,
+            pos: 9,
+        };
+        let mut expected_output_size = 5;
+
+        unsafe {
+            ZSTD_rust_setBufferExpectations(
+                ZSTD_BM_STABLE,
+                ZSTD_BM_STABLE,
+                &mut expected_input,
+                &mut expected_output_size,
+                &output,
+                &input,
+            );
+        }
+
+        assert_eq!(expected_input.src, input.src);
+        assert_eq!(expected_input.size, input.size);
+        assert_eq!(expected_input.pos, input.pos);
+        assert_eq!(expected_output_size, 31);
+
+        let original_input_src = expected_input.src;
+        let original_input_size = expected_input.size;
+        let original_input_pos = expected_input.pos;
+        let original_output_size = expected_output_size;
+        let other_input = ZSTD_inBuffer {
+            src: expected_src,
+            size: 99,
+            pos: 22,
+        };
+        let other_output = ZSTD_outBuffer {
+            dst: ptr::null_mut(),
+            size: 3,
+            pos: 8,
+        };
+
+        unsafe {
+            ZSTD_rust_setBufferExpectations(
+                ZSTD_BM_BUFFERED,
+                ZSTD_BM_BUFFERED,
+                &mut expected_input,
+                &mut expected_output_size,
+                &other_output,
+                &other_input,
+            );
+        }
+
+        assert_eq!(expected_input.src, original_input_src);
+        assert_eq!(expected_input.size, original_input_size);
+        assert_eq!(expected_input.pos, original_input_pos);
+        assert_eq!(expected_output_size, original_output_size);
+    }
+
     #[test]
     fn sequence_api_plan_places_input_and_frame_checksums_around_blocks() {
         assert_eq!(