Does rustfmt format macro calls?

[calebcartwright]

Can/does rustfmt format macro calls?

[calebcartwright]

The short answer is: sometimes, but only in certain cases

rustfmt is a Rust code formatter which operates as a pretty-printer of the parsed AST (Abstract Syntax Tree) representation of a Rust program. rustfmt has rules defined for each type of AST node it could encounter, such as a match expression, or a function definition, and as rustfmt walks the AST of a program it emits the formatted result for each node it encounters according to the corresponding rules.

However, macros can be, and often are, used to extend the language with syntax that isn't directly valid Rust syntax, for example the html! macro provided by the yew crate. rustfmt sees these macro calls in their raw form with arg tokens just as they were written in the source *.rs file (the potentially non-valid Rust syntax) because it's working with the AST prior to any macro expansion. Since rustfmt is only a formatter for valid Rust syntax, it doesn't have any rules and doesn't know how to process invalid Rust syntax and as such there are limits to how much macro formatting rustfmt can currently perform. Furthermore, there are also additional constraints compared to formatting regular Rust code because some things that aren't semantically important in normal Rust code can actually be semantically important for macros (e.g. trailing commas).

However, there has been a sizeable desire from the community over the years for rustfmt to try to support formatting macro calls, and there are plenty of macros that are called with args that are valid Rust syntax which rustfmt could format. As such rustfmt currently:

• essentially ignores calls that use brace delimiters, { ... }
• attempts to format calls that use parentheses or brackets for delimiters, ( ... ) or [ ... ]. However, rustfmt will only attempt to format these calls if all of the args can be parsed as valid Rust syntax (like an item or an expression), and if so, then those args are formatted according to the corresponding rules for that type of Rust syntax.

We hope to continue improving macro support over time, and there are various open discussions on the topic which can be found by searching in the issue tracker.