prost-protovalidate-build

Compile-time code generator for Protocol Buffer validation using buf.validate rules, built for prost — direct field access at runtime, no reflection, no CEL interpreter on the hot path.

Generates impl prost_protovalidate::Validate for messages with standard buf.validate field constraints. Messages containing CEL expressions are excluded at build time and fall back to runtime evaluation via prost_protovalidate::Validator.

Usage

Add the dependency to your Cargo.toml:

[build-dependencies]
prost-build = "0.14"
prost-protovalidate-build = "0.4"

In your build.rs:

fn main() -> Result<(), Box<dyn std::error::Error>> {
    let descriptor_path = std::path::PathBuf::from(std::env::var("OUT_DIR")?)
        .join("file_descriptor_set.bin");

    prost_build::Config::new()
        .file_descriptor_set_path(&descriptor_path)
        .compile_protos(&["proto/service.proto"], &["proto/"])?;

    prost_protovalidate_build::Builder::new()
        .file_descriptor_set_path(&descriptor_path)?
        .compile()?;

    Ok(())
}

Include the generated validation code alongside prost output:

include!(concat!(env!("OUT_DIR"), "/validate_impl.rs"));

Supported Rules

• Scalar: bool const, numeric comparisons (gt/gte/lt/lte/const/in/not_in), string (min_len/max_len/pattern/prefix/suffix/contains/in/not_in/well-known formats), bytes (min_len/max_len/pattern/prefix/suffix/in/not_in/well-known formats), enum (const/in/not_in/defined_only)
• Repeated: min_items/max_items, per-item scalar constraints
• Map: min_pairs/max_pairs, per-key and per-value scalar constraints
• Message: required, nested recursive validation
• Duration/Timestamp: comparison constraints (gt/gte/lt/lte/const)
• FieldMask: const, in/not_in with prefix path matching
• Any: type_url in/not_in
• WKT wrappers: google.protobuf.*Value types unwrapped to inner scalar rules
• Oneof: required (must have a variant set)
• Ignore: IGNORE_ALWAYS, IGNORE_IF_ZERO_VALUE, IGNORE_UNSPECIFIED with presence semantics

Limitations

• CEL expressions are not evaluated at build time. Messages with CEL rules (field-level or message-level) are skipped with a cargo:warning.
• Predefined CEL constraints (custom rules) are not supported.
• Nested runtime-only dependencies (for example, nested CEL or unsupported nested rules) cause parent message codegen to be skipped to prevent partial validation.

License

MIT OR Apache-2.0