feat(compress): move static CCtx gating into Rust

Move public ZSTD_initStaticCCtx null, alignment, minimum-size, and callback
dispatch into Rust. Keep private workspace reservations, block-state buffers,
and CPU initialization in the C callback.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release init_static_cctx -- --nocapture
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --release --all-targets -- -D warnings
- ulimit -v 41943040; make -B -C programs -j1 zstd
- ulimit -v 41943040; make -C tests -j1 test-zstream ZSTREAM_TESTTIME=-T1s

rust/src/zstd_compress.rs

@@ -1324,6 +1324,51 @@ pub unsafe extern "C" fn ZSTD_rust_createCCtx(
     cctx
 }
 
+type InitStaticCCtxFn = unsafe extern "C" fn(*mut c_void) -> *mut c_void;
+const ZSTD_STATIC_WORKSPACE_ALIGNMENT: usize = 8;
+
+/// Explicit projection for the public `ZSTD_initStaticCCtx` wrapper.
+///
+/// Rust owns workspace pointer/alignment validation, minimum-size validation,
+/// and callback ordering. C retains the private static-workspace layout and
+/// context initialization behind one callback.
+#[repr(C)]
+pub struct ZSTD_rust_initStaticCCtxState {
+    callback_context: *mut c_void,
+    workspace: *mut c_void,
+    workspace_size: usize,
+    cctx_size: usize,
+    init: InitStaticCCtxFn,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_initStaticCCtxState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_initStaticCCtxState, workspace) == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initStaticCCtxState, workspace_size) == 2 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initStaticCCtxState, cctx_size) == 3 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initStaticCCtxState, init) == 4 * size_of::<usize>());
+    assert!(size_of::<ZSTD_rust_initStaticCCtxState>() == size_of::<[usize; 5]>());
+};
+
+/// Validate static CCtx workspace ownership before entering the C leaf.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_initStaticCCtx(
+    state: *const ZSTD_rust_initStaticCCtxState,
+) -> *mut c_void {
+    if state.is_null() {
+        return ptr::null_mut();
+    }
+    let state = unsafe { &*state };
+    if state.callback_context.is_null()
+        || state.workspace.is_null()
+        || (state.workspace as usize) & (ZSTD_STATIC_WORKSPACE_ALIGNMENT - 1) != 0
+        || state.workspace_size <= state.cctx_size
+    {
+        return ptr::null_mut();
+    }
+    unsafe { (state.init)(state.callback_context) }
+}
+
 type FreeCCtxContentFn = unsafe extern "C" fn(*mut c_void);
 type FreeCCtxObjectFn = unsafe extern "C" fn(*mut c_void);
 
@@ -12045,6 +12090,103 @@ mod tests {
         assert!(context.initialized.is_null());
     }
 
+    #[derive(Default)]
+    struct InitStaticCCtxTestContext {
+        events: Vec<&'static str>,
+        result: *mut c_void,
+    }
+
+    unsafe extern "C" fn init_static_cctx_test_init(context: *mut c_void) -> *mut c_void {
+        let context = unsafe { &mut *context.cast::<InitStaticCCtxTestContext>() };
+        context.events.push("init");
+        context.result
+    }
+
+    fn init_static_cctx_test_state(
+        context: &mut InitStaticCCtxTestContext,
+        workspace: *mut c_void,
+        workspace_size: usize,
+        cctx_size: usize,
+    ) -> ZSTD_rust_initStaticCCtxState {
+        ZSTD_rust_initStaticCCtxState {
+            callback_context: (context as *mut InitStaticCCtxTestContext).cast(),
+            workspace,
+            workspace_size,
+            cctx_size,
+            init: init_static_cctx_test_init,
+        }
+    }
+
+    #[test]
+    fn init_static_cctx_rejects_an_unaligned_workspace_before_callback() {
+        let mut context = InitStaticCCtxTestContext {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let mut workspace = [0usize; 2];
+        let unaligned_workspace = workspace.as_mut_ptr().cast::<u8>().wrapping_add(1).cast();
+        let state = init_static_cctx_test_state(&mut context, unaligned_workspace, usize::MAX, 0);
+
+        let result = unsafe { ZSTD_rust_initStaticCCtx(&state) };
+
+        assert!(result.is_null());
+        assert!(context.events.is_empty());
+    }
+
+    #[test]
+    fn init_static_cctx_rejects_a_small_workspace_before_callback() {
+        let mut context = InitStaticCCtxTestContext {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let mut workspace = [0usize; 1];
+        let state = init_static_cctx_test_state(
+            &mut context,
+            workspace.as_mut_ptr().cast(),
+            size_of_val(&workspace),
+            size_of_val(&workspace),
+        );
+
+        let result = unsafe { ZSTD_rust_initStaticCCtx(&state) };
+
+        assert!(result.is_null());
+        assert!(context.events.is_empty());
+    }
+
+    #[test]
+    fn init_static_cctx_calls_the_initializer_after_workspace_validation() {
+        let mut context = InitStaticCCtxTestContext {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let mut workspace = [0usize; 1];
+        let state = init_static_cctx_test_state(
+            &mut context,
+            workspace.as_mut_ptr().cast(),
+            size_of_val(&workspace),
+            size_of_val(&workspace) - 1,
+        );
+
+        let result = unsafe { ZSTD_rust_initStaticCCtx(&state) };
+
+        assert_eq!(result, context.result);
+        assert_eq!(context.events, ["init"]);
+    }
+
+    #[test]
+    fn init_static_cctx_rejects_a_null_workspace() {
+        let mut context = InitStaticCCtxTestContext {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let state = init_static_cctx_test_state(&mut context, ptr::null_mut(), usize::MAX, 0);
+
+        let result = unsafe { ZSTD_rust_initStaticCCtx(&state) };
+
+        assert!(result.is_null());
+        assert!(context.events.is_empty());
+    }
+
     #[derive(Default)]
     struct FreeCCtxTestContext {
         events: Vec<&'static str>,