feat(compress): move static CDict gating into Rust

Move public ZSTD_initStaticCDict alignment, null, workspace-size, and callback
dispatch into Rust. Keep C static workspace construction and dictionary
initialization behind a private callback.

Test Plan:
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --release init_static_cdict -- --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_dictionary.rs

@@ -42,6 +42,7 @@ const ZSTD_MAGIC_DICTIONARY: u32 = 0xEC30_A437;
 const ZSTD_DCT_AUTO: c_int = 0;
 const ZSTD_DCT_RAW_CONTENT: c_int = 1;
 const ZSTD_DCT_FULL_DICT: c_int = 2;
+const ZSTD_STATIC_WORKSPACE_ALIGNMENT: usize = 8;
 
 /// C keeps match-state/content insertion private because it depends on the
 /// configuration-sensitive `ZSTD_MatchState_t`, `ldmState_t`, workspace, and
@@ -479,6 +480,50 @@ pub unsafe extern "C" fn ZSTD_rust_freeCDict(state: *const ZSTD_rust_freeCDictSt
     0
 }
 
+type InitStaticCDictFn = unsafe extern "C" fn(*mut c_void) -> *mut c_void;
+
+/// Explicit projection for the public `ZSTD_initStaticCDict` wrapper.
+///
+/// Rust owns workspace pointer/alignment validation, minimum-size validation,
+/// and callback ordering. C retains the private static-workspace layout and
+/// dictionary initialization behind one callback.
+#[repr(C)]
+pub struct ZSTD_rust_initStaticCDictState {
+    callback_context: *mut c_void,
+    workspace: *mut c_void,
+    workspace_size: usize,
+    needed_size: usize,
+    init: InitStaticCDictFn,
+}
+
+const _: () = {
+    assert!(offset_of!(ZSTD_rust_initStaticCDictState, callback_context) == 0);
+    assert!(offset_of!(ZSTD_rust_initStaticCDictState, workspace) == size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initStaticCDictState, workspace_size) == 2 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initStaticCDictState, needed_size) == 3 * size_of::<usize>());
+    assert!(offset_of!(ZSTD_rust_initStaticCDictState, init) == 4 * size_of::<usize>());
+    assert!(size_of::<ZSTD_rust_initStaticCDictState>() == size_of::<[usize; 5]>());
+};
+
+/// Validate static CDict workspace ownership before entering the C leaf.
+#[no_mangle]
+pub unsafe extern "C" fn ZSTD_rust_initStaticCDict(
+    state: *const ZSTD_rust_initStaticCDictState,
+) -> *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.needed_size
+    {
+        return ptr::null_mut();
+    }
+    unsafe { (state.init)(state.callback_context) }
+}
+
 type CompressBeginUsingDictBeginFn =
     unsafe extern "C" fn(*mut c_void, *const c_void, usize, *const c_void, u64) -> usize;
 
@@ -2939,6 +2984,103 @@ mod tests {
         assert_eq!(result, 0);
     }
 
+    #[derive(Default)]
+    struct InitStaticCDictProbe {
+        events: Vec<&'static str>,
+        result: *mut c_void,
+    }
+
+    unsafe extern "C" fn init_static_cdict_test_init(context: *mut c_void) -> *mut c_void {
+        let probe = unsafe { &mut *context.cast::<InitStaticCDictProbe>() };
+        probe.events.push("init");
+        probe.result
+    }
+
+    fn init_static_cdict_test_state(
+        probe: &mut InitStaticCDictProbe,
+        workspace: *mut c_void,
+        workspace_size: usize,
+        needed_size: usize,
+    ) -> ZSTD_rust_initStaticCDictState {
+        ZSTD_rust_initStaticCDictState {
+            callback_context: (probe as *mut InitStaticCDictProbe).cast(),
+            workspace,
+            workspace_size,
+            needed_size,
+            init: init_static_cdict_test_init,
+        }
+    }
+
+    #[test]
+    fn init_static_cdict_rejects_an_unaligned_workspace_before_callback() {
+        let mut probe = InitStaticCDictProbe {
+            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_cdict_test_state(&mut probe, unaligned_workspace, usize::MAX, 0);
+
+        let result = unsafe { ZSTD_rust_initStaticCDict(&state) };
+
+        assert!(result.is_null());
+        assert!(probe.events.is_empty());
+    }
+
+    #[test]
+    fn init_static_cdict_rejects_insufficient_workspace_before_callback() {
+        let mut probe = InitStaticCDictProbe {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let mut workspace = [0usize; 1];
+        let state = init_static_cdict_test_state(
+            &mut probe,
+            workspace.as_mut_ptr().cast(),
+            size_of_val(&workspace),
+            size_of_val(&workspace) + 1,
+        );
+
+        let result = unsafe { ZSTD_rust_initStaticCDict(&state) };
+
+        assert!(result.is_null());
+        assert!(probe.events.is_empty());
+    }
+
+    #[test]
+    fn init_static_cdict_calls_the_initializer_after_workspace_validation() {
+        let mut probe = InitStaticCDictProbe {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let mut workspace = [0usize; 1];
+        let state = init_static_cdict_test_state(
+            &mut probe,
+            workspace.as_mut_ptr().cast(),
+            size_of_val(&workspace),
+            size_of_val(&workspace),
+        );
+
+        let result = unsafe { ZSTD_rust_initStaticCDict(&state) };
+
+        assert_eq!(result, probe.result);
+        assert_eq!(probe.events, ["init"]);
+    }
+
+    #[test]
+    fn init_static_cdict_rejects_a_null_workspace() {
+        let mut probe = InitStaticCDictProbe {
+            result: ptr::dangling_mut(),
+            ..Default::default()
+        };
+        let state = init_static_cdict_test_state(&mut probe, ptr::null_mut(), usize::MAX, 0);
+
+        let result = unsafe { ZSTD_rust_initStaticCDict(&state) };
+
+        assert!(result.is_null());
+        assert!(probe.events.is_empty());
+    }
+
     #[derive(Default)]
     struct CompressBeginUsingDictProbe {
         events: Vec<&'static str>,