New format args template

[m-ou-se]

This PR is a template for new experimental fmt::Arguments implementations. If you want to try out a new fmt::Arguments implementation, this branch is a useful place to start from.

$ git fetch https://github.com/rust-lang/rust refs/pull/101272/head
$ git switch -c new-fmt-arguments FETCH_HEAD

To implement a new fmt::Arguments, you need to:

• Implement the part of rustc that expands format_args!(), in compiler/rustc_builtin_macros/src/format/expand.rs.

  • The expand_parsed_format_args function takes a FormatArgs struct representing the already parsed/processed format_args!(), and you need to return the expanded ast expression that will create a new fmt::Arguments at runtime.

• Implement the new fmt::Arguments struct in library/core/src/fmt/mod.rs.

  • You can't delete the old code here, since the beta/bootstrap compiler will be used to compile this for stage 0, with the old format_args!() macro implementation. Instead, you 'remove' old code by adding #[cfg(boostrap)] and add new code with #[cfg(not(bootstrap))]. (Old code is removed when the bootstrap/beta compiler is bumped to the next version.)
  • Look for the TODO comments, which point out the Arguments struct and the methods you need to implement. I've already put all the old code under #[cfg(bootstrap)] and created new empty placeholders for the new code.
  • Specifically, you need to implement:
    • struct Arguments<'a>: the structure that represents a complete format_args!() at runtime.
    • Arguments::new: the function used in the expansion of format_args!() to create this struct. Which arguments this takes is up to you, as it only needs to match your macro expansion.
    • Arguments::from_static_str: this should create an Arguments struct that represents exactly the given string.
    • Arguments::as_str: this should return the string in trivial cases like format_args!("hello") or Arguments::from_static_str("hello"), or None otherwise.
    • Arguments::estimated_capacity: this should return the estimated size of the complete output when displayed. It doesn't have to be very accurate. See the old implementation for an example.
    • write: This function takes your fmt::Arguments and should write/display it to a &mut dyn Write.

If you have something ready for sharing or testing, feel free to open a draft PR and drop a comment in #99012.

[bors]

☔ The latest upstream changes (presumably #101558) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #106745) made this pull request unmergeable. Please resolve the merge conflicts.

[m-ou-se]

Now that #106745 is merged, this is outdated.