LibUnicode/Rust: Expose IDNA to_ascii to Rust

Add a thin FFI wrapper for Unicode::IDNA::to_ascii.

Libraries/LibUnicode/IDNA.cpp

@@ -7,6 +7,7 @@
 
 #include <LibUnicode/ICU.h>
 #include <LibUnicode/IDNA.h>
+#include <LibUnicode/RustFFI.h>
 
 #include <unicode/idna.h>
 
@@ -63,3 +64,27 @@ ErrorOr<String> to_ascii(Utf8View domain_name, ToAsciiOptions const& options)
 }
 
 }
+
+namespace Unicode::FFI {
+
+extern "C" void unicode_rust_idna_to_ascii(uint8_t const* domain, size_t domain_length, ToAsciiOptions const* options, void* ctx, FfiIdnaResultFn on_success)
+{
+    Unicode::IDNA::ToAsciiOptions const cpp_options {
+        options->check_hyphens ? Unicode::IDNA::CheckHyphens::Yes : Unicode::IDNA::CheckHyphens::No,
+        options->check_bidi ? Unicode::IDNA::CheckBidi::Yes : Unicode::IDNA::CheckBidi::No,
+        options->check_joiners ? Unicode::IDNA::CheckJoiners::Yes : Unicode::IDNA::CheckJoiners::No,
+        options->use_std3_ascii_rules ? Unicode::IDNA::UseStd3AsciiRules::Yes : Unicode::IDNA::UseStd3AsciiRules::No,
+        options->transitional_processing ? Unicode::IDNA::TransitionalProcessing::Yes : Unicode::IDNA::TransitionalProcessing::No,
+        options->verify_dns_length ? Unicode::IDNA::VerifyDnsLength::Yes : Unicode::IDNA::VerifyDnsLength::No,
+        options->ignore_invalid_punycode ? Unicode::IDNA::IgnoreInvalidPunycode::Yes : Unicode::IDNA::IgnoreInvalidPunycode::No,
+    };
+
+    auto result = Unicode::IDNA::to_ascii(Utf8View { StringView { domain, domain_length } }, cpp_options);
+    if (result.is_error())
+        return;
+
+    auto const ascii_bytes = result.value().bytes_as_string_view();
+    on_success(ctx, reinterpret_cast<uint8_t const*>(ascii_bytes.characters_without_null_termination()), ascii_bytes.length());
+}
+
+}

Libraries/LibUnicode/Rust/src/idna.rs

@@ -0,0 +1,64 @@
+/*
+ * Copyright (c) 2026, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+//! Rust bindings for the LibUnicode IDNA API.
+
+use std::ffi::c_void;
+
+/// Options controlling IDNA `to_ascii` processing (C++ `UnicodeRustToAsciiOptions`).
+#[repr(C)]
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub struct ToAsciiOptions {
+    pub check_hyphens: bool,
+    pub check_bidi: bool,
+    pub check_joiners: bool,
+    pub use_std3_ascii_rules: bool,
+    pub transitional_processing: bool,
+    pub verify_dns_length: bool,
+    pub ignore_invalid_punycode: bool,
+}
+
+type FfiIdnaResultFn = unsafe extern "C" fn(*mut c_void, *const u8, usize);
+
+unsafe extern "C" {
+    fn unicode_rust_idna_to_ascii(
+        domain: *const u8,
+        domain_length: usize,
+        options: *const ToAsciiOptions,
+        ctx: *mut c_void,
+        on_success: FfiIdnaResultFn,
+    );
+}
+
+/// Called by C++ on success with the ASCII result bytes.
+///
+/// # Safety
+/// `ctx` must be a valid `*mut Option<String>` live for the duration of the
+/// enclosing `idna_to_ascii` call. `data` must point to `len` valid ASCII bytes.
+unsafe extern "C" fn on_idna_result(ctx: *mut c_void, data: *const u8, len: usize) {
+    // SAFETY: `ctx` was set to `addr_of_mut!(result)` in `idna_to_ascii`.
+    let out = unsafe { &mut *(ctx as *mut Option<String>) };
+    // SAFETY: `data` is valid for `len` bytes; IDNA to_ascii guarantees ASCII output.
+    let bytes = unsafe { std::slice::from_raw_parts(data, len) };
+    debug_assert!(bytes.is_ascii(), "unicode_rust_idna_to_ascii must return ASCII");
+    // SAFETY: ASCII is a strict subset of UTF-8.
+    *out = Some(unsafe { String::from_utf8_unchecked(bytes.to_vec()) });
+}
+
+pub fn idna_to_ascii(domain: &str, options: ToAsciiOptions) -> Option<String> {
+    let mut result: Option<String> = None;
+    // SAFETY: `domain` is valid for its length; `options` and `result` are valid for the call.
+    unsafe {
+        unicode_rust_idna_to_ascii(
+            domain.as_ptr(),
+            domain.len(),
+            &raw const options,
+            std::ptr::addr_of_mut!(result) as *mut c_void,
+            on_idna_result,
+        );
+    }
+    result
+}

Libraries/LibUnicode/Rust/src/lib.rs

@@ -10,3 +10,4 @@ mod rust_allocator;
 
 pub mod calendar;
 pub mod character_types;
+pub mod idna;