Try Trait Support (Nightly Feature)

Stillwater provides experimental support for Rust’s ? operator with Validation on nightly Rust.

Enabling

Add to Cargo.toml:

[dependencies]
stillwater = { version = "0.1", features = ["try_trait"] }

Use nightly Rust and opt into the unstable implementation explicitly:

RUSTFLAGS="--cfg try_trait_nightly" cargo +nightly test --features try_trait

Applications that use ? with Validation must also add #![feature(try_trait_v2)] to their crate root.

Using ? with Validation

#![allow(unused)]
#![feature(try_trait_v2)]

fn main() {
use stillwater::Validation;

fn validate_user(email: &str, age: u8) -> Validation<User, Vec<Error>> {
    let email = validate_email(email)?;  // ⚠️ Short-circuits!
    let age = validate_age(age)?;
    Ok(User { email, age })
}
}

Warning: Using ? with Validation short-circuits on first error, defeating the purpose of error accumulation!

Recommendation: Don’t use ? with Validation. Use Validation::all() instead:

#![allow(unused)]
fn main() {
// ✓ Better: accumulates all errors
Validation::all((
    validate_email(email),
    validate_age(age),
))
.map(|(email, age)| User { email, age })
}

Should You Use This Feature?

Pros:

• Familiar ? syntax
• Cleaner for sequential operations

Cons:

• Requires nightly Rust
• Requires RUSTFLAGS="--cfg try_trait_nightly"
• Defeats Validation’s purpose
• and_then() is just as readable

Recommendation: Wait for stable Rust support before using in production. The feature is mainly experimental.

Migration Path

When try_trait_v2 stabilizes:

1. Update to stable Rust
2. Remove #![feature(try_trait_v2)]
3. Keep using Validation::all() for accumulation
4. Use ? with Effect where it makes sense

Next Steps

• Check the FAQ for common questions
• See Patterns for practical recipes
• Read Comparison vs other libraries