Objective-Rust
What I wanted was for Objective-C’s messaging syntax, or something like it, to be valid anywhere in Rust code. It didn’t have to be exactly Objective-C, but I quickly realized the advantage of Objective-C’s syntax: it’s delimited, i.e. it’s a self-contained expression that can be dropped into something larger without changing how that larger thing parses. (That’s probably why it’s bracketed in C as well.)
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If I were just matching that, I could use Rust’s original pattern-matching macros. But to take a whole block of code, and replace everything that looks like a message lock in that block…well, it might be possible with pattern-matching and quite a bit of recursion, but it’s going to be a lot easier to use procedural macros, a Rust macro interface written in Rust and used as a compiler plugin. (Swift folks, basically what SwiftSyntax allows you to do, but invoked on-demand during compilation.)
Update (2020-08-31): See also: Hacker News.
