Crate venial

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§Lightweight parsing for Rust proc macros

Venial is a small parser for Rust proc macros.

When writing proc macros that need to parse Rust code (such as attribute and derive macros), the most common solution is to use the syn crate. Syn can parse arbitrary valid Rust code, and even Rust-based DSLs, and return versatile data structures that can inspected and mutated in powerful ways.

It’s also extremely heavy. In one analysis of lqd’s early 2022 benchmark collection, the author estimates that syn is reponsible for 8% of compile times of the benchmark, which accounts for Rust’s most popular crates. There are subtleties (eg this isn’t necessarily critical path time, but syn is often in the critical path anyway), but the overall takeaway is clear: syn is expensive.

And yet, a lot of the power of syn is often unneeded. If we look at the crates that depend on syn, we can see that the 5 most downloaded are:

  • serde_derive
  • proc-macro-hack
  • pin-project-internal
  • anyhow
  • thiserror-impl

Of these, proc-macro-hack is deprecated, and the other four only need to parse basic information on a type.

Other popular reverse-dependencies of syn (such as futures-macro, tokios-macros, async-trait, etc) do use syn’s more advanced features, but there’s still room for a lightweight parser in proc-macros.

Venial is that parser.

§Example

use venial::{parse_item, Item};
use quote::quote;

let enum_type = parse_item(quote!(
    enum Shape {
        Square(Square),
        Circle(Circle),
        Triangle(Triangle),
    }
));

let enum_type = match enum_type {
    Ok(Item::Enum(enum_type)) => enum_type,
    _ => unreachable!(),
};

assert_eq!(enum_type.variants[0].0.name, "Square");
assert_eq!(enum_type.variants[1].0.name, "Circle");
assert_eq!(enum_type.variants[2].0.name, "Triangle");

§Spans

Spans mark the beginning and end of tokens in the source code and can be used to generate precise error messages, highlighting certain parts. Item types provide a span() method that returns their respective span.

Note that Rust’s proc_macro crate does not support span joining in stable. That means that if you want items to hold the full full span of the input, you’ll need a nightly compiler. Otherwise, the span will typically only include the first token-tree in the sequence.

For example, if you have a NamedField you’d like to highlight in an error message, you can use named_field.span(). On nightly, a custom error could be rendered as follows:

error: my_attribute cannot be applied in this context
  --> crate/rust/src/file.rs:58:5
   |
58 | /     #[my_attribute]
59 | |     field: i32,
   | |______________^

However, on stable, the span might only refer to the first token tree (here #):

error: my_attribute cannot be applied in this context
  --> crate/rust/src/file.rs:58:5
   |
58 |      #[my_attribute]
   |      ^

You can however control this behavior more precisely by choosing which sub-tokens to highlight. On stable compilers, you might want to put focus on the field name named_field.name.span() instead of the entire field named_field.span(). This would yield:

error: my_attribute cannot be applied in this context
 --> crate/rust/src/file.rs:58:5
   |
59 |     field: i32,
   |     ^^^^^

Re-exports§

Structs§

Enums§

Functions§

  • consume_item
    Consume an item declaration from a token stream.
  • parse_item
    Parses the token stream of an item declaration.