Rust 1.66: What's New, Release History & Support Lifecycle

What Is New in Rust 1.66

How can I assign explicit discriminants to enum variants that have fields?

Rust 1.66 lets you give a numeric discriminant to any enum variant, even when the variant carries data.

#[repr(u8)]
enum Foo {
    A(u8),
    B(i8),
    C(bool) = 42,
}

In practice this is useful for FFI where the exact layout must match a C enum. Variants without an explicit value keep the automatically incremented discriminants, so you can mix explicit and implicit values without inserting dummy variants.

How do I prevent the compiler from optimizing away code in benchmarks?

The newly stabilized std::hint::black_box acts as an opaque barrier that tells the optimizer it cannot assume anything about the value.

use std::hint::black_box;

fn push_cap(v: &mut Vec<i32>) {
    for i in 0..4 {
        v.push(i);
        black_box(v.as_ptr());
    }
}

This matters if you need to measure the cost of a loop or a function call; without black_box the compiler may elide the entire body.

How can I remove a dependency from Cargo.toml via the command line?

Rust 1.66 adds the cargo remove subcommand, mirroring the earlier cargo add workflow.

cargo remove serde

It edits Cargo.toml in place, removes the entry from the [dependencies] table, and updates the lock file, saving a manual edit.

What new pattern matching capabilities does Rust 1.66 introduce?

You can now use inclusive range patterns with the ..=X syntax directly in match arms.

match value {
    0..=10 => println!("small"),
    11..=100 => println!("medium"),
    _ => println!("large"),
}

This improves readability and eliminates the need for separate guard clauses for simple numeric ranges.

How does Rust 1.66 improve Linux build performance?

Linux builds now enable Link-Time Optimization (LTO) for the rustc frontend and BOLT for the LLVM backend.

In practice this reduces both runtime execution time and the memory footprint of the generated binaries, especially for large crates with many generic instantiations.