Add U32string_view wrapper by Jennycj · Pull Request #71 · rustfoundation/overloading-macros

Jennycj · 2026-11-04T12:20:40Z

Added a wrapper for std::U32string_view with 4 overloads including constructor, and methods: compare, find and startwith
Implements #43

Write Rust wrappers for that U32string_view, for 3-5 different overloaded method names:
- Create a new file named after your type in overloading-macros/cpp-overload-test/src/stdcpp
- Wrap constructors first, writing tests to make sure the correct constructor is called
- Explain any potential compatibility issues in a Limitations doc section
- Explain usability issues in an Ergonomics doc section
- Check that you have implemented all the overloads for each method name, or documented why you can't
- Push your changes and wait for a review
Add a separate example with some failing overloads
- Create a new file named after your type in overloading-macros/cpp-overload-test/examples
- Wrap accessor/modifier methods
- Explain how they fail in an Incompatibilities doc section
- Explain how the overload failures could be resolved in a Workarounds doc section
- Optional: If there's a C++ feature with no equivalent in Rust, explain that in an Inexpressible Overloads doc section
- Push your changes and wait for a review
Update the overload macro compatibility table Markdown doc
- If you haven't checked some kinds of overloads, put a ？in those squares
- Push your changes and wait for a review

teor2345

Hi, thank you for this PR!

Please submit steps 2 & 3 immediately, without waiting for reviews. Outreachy applications are due on Monday 9 November, and I want you to be able to get one or two reviews of everything in your PR.

Please remember to submit an Outreachy application! You can include any Rust work during the application period.

I added a merge commit, feel free to keep it or force-push over it.

teor2345 · 2026-11-05T00:14:49Z

+//!
+//! Callers can only build a view from static data, such as string-literal constants, constant
+//! arrays, or a leaked `Box`. A view of a local array is a compile error. Supporting borrowed
+//! data needs generics (and lifetime) support in `overload!`.


Please move all this documentation next to the types or functions it documents. That way, when we fix these issues, we will see the documentation and update it.

Noted, I will do this.

teor2345 · 2026-11-05T00:15:39Z

+///
+/// `std::basic_string_view<CharT>` is a template, so this wrapper is hardcoded to `char32_t`.
+#[cfg_attr(not(test), expect(dead_code, reason = "Only used in tests"))]
+pub struct StdU32StringView(*mut c_void);


Rust requires a marker type to show what this type contains behind the pointer.

Suggested change

pub struct StdU32StringView(*mut c_void);

pub struct StdU32StringView(*mut c_void, PhantomData<CharT>);

Thanks for this suggestion, I will apply it.

teor2345 · 2026-11-05T00:20:19Z

+#[cfg_attr(not(test), expect(dead_code, reason = "Only used in tests"))]
+pub struct StdU32StringView(*mut c_void);
+
+/// The C++ `char32_t` type is used as `u32` in Rust since it has the same size and alignment.


Unfortunately, this is not guaranteed, even if most C++ platforms do it that way:
https://en.cppreference.com/c/string/multibyte/char32_t

This is a portability limitation, which could result in a compiler failure on weird platforms (the cpp! macro checks size and alignment match). Please update the docs here, document this limitation, and link to the reference.

It is true Rust does not have a corresponding type in its standard library, but it could in future.

Thanks fpr pointing this out.
I'll update the documentation to explain that u32 is the Rust-side representation, but compatibility with C++ char32_t depends on the target platform.
I will also document the limitation and add the reference link.

teor2345 · 2026-11-05T00:22:32Z

+impl<'a> NullTerminated<'a> {
+    /// Returns `None` if `s` has no zero terminator, because C++ would read past the end.
+    pub fn new(s: &'a [CharT]) -> Option<NullTerminated<'a>> {
+        s.contains(&0).then_some(NullTerminated(s))


What happens if the null terminator is not at the end of the slice?

Characters after the null terminator will be ignored even if they are not at the end of the slice since the C++ constructor stops reading at the first null terminator.

This might be worth documenting?

teor2345 · 2026-11-05T00:28:56Z

+        StdU32StringView(view)
+    }
+
+    fn copy_from(other: &StdU32StringView) -> StdU32StringView {


This constructor is only sound because all the other constructors take static data. Otherwise, it could extend the lifetime of the new string view beyond the lifetime of its contents.

I agree.
The copy constructor only copies the std::u32string_view and does not extend the lifetime of the underlying character storage.

The current implementation relies on all constructors restricting the underlying character data to 'static, which makes this safe. I'll document this invariant alongside the copy constructor.

teor2345 · 2026-11-05T00:29:45Z

+//! Wrapped overloaded names: constructors, `compare`, `find`, `starts_with`.
+//!
+//! The `basic_string_view(nullptr_t) = delete` constructor (C++23) is not wrapped.
+//! It exists only to reject `nullptr` at compile time, and Rust raw pointers


Suggested change

//! It exists only to reject `nullptr` at compile time, and Rust raw pointers

//! It exists only to reject `nullptr` at compile time, and Rust references

Noted, thank you!

Jennycj · 2026-11-05T17:53:20Z

Hi, thank you for this PR!

Please submit steps 2 & 3 immediately, without waiting for reviews. Outreachy applications are due on Monday 9 November, and I want you to be able to get one or two reviews of everything in your PR.

Please remember to submit an Outreachy application! You can include any Rust work during the application period.

I added a merge commit, feel free to keep it or force-push over it.

Thank you @teor2345 for the reminder!
I will keep the merge commit and send in 2 & 3 as soon as possible

teor2345 · 2026-11-06T09:41:42Z

Just reminding you that the Outreachy final application deadline is this Monday 9 November. You can include any Rust PR or ticket work you've done, it doesn't have to be merged (or reviewed) yet.

Please submit a final application on the Outreachy website before Monday. The exact deadline is 4pm UTC, but it is better to submit early. You can edit your submitted application up until the deadline (and you can keep working on PRs after the deadline).

Jennycj and others added 3 commits November 4, 2026 13:07

Add u32string_view wrapper and its constructor overloads

4b9e567

Add method overloads and tests

94cd3b1

Merge branch 'main' into u32string-view-overloads

e511335

teor2345 reviewed Nov 5, 2026

View reviewed changes

teor2345 assigned teor2345 and Jennycj Nov 5, 2026

	pub struct StdU32StringView(*mut c_void);
	pub struct StdU32StringView(*mut c_void, PhantomData<CharT>);

	//! It exists only to reject `nullptr` at compile time, and Rust raw pointers
	//! It exists only to reject `nullptr` at compile time, and Rust references

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Jennycj commented Nov 5, 2026

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teor2345 commented Nov 6, 2026

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Jennycj commented Nov 4, 2026 •

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