Compiler Version Requirements

A package's compiler version requirement is declared in Cargo.toml using the rust-version field under [package] (or [workspace.package]). It specifies the oldest version of rustc and cargo that the package supports, enabling Cargo to fail fast with a clear diagnostic on older toolchains and—starting in the Rust 2024 edition (Resolver v3)—guiding dependency resolution automatically.

In Cargo.toml:
  [package]
  name = "my_library"
  version = "0.4.0"
  edition = "2024"
  rust-version = "1.85"   <--- Minimum Supported Compiler Version
        |
        +-- On `rustc 1.80`: Cargo immediately prints a clear error:
        |   `package my_library v0.4.0 cannot be built because it requires rustc 1.85 or newer`
        |
        +-- On `edition = "2024"` (Resolver v3):
            `cargo add` and `cargo update` automatically prefer dependency versions
            whose `rust-version` is `<= 1.85`!

Format and Rules for rust-version in Cargo.toml

• Can be written as a 2-component ("1.85") or 3-component ("1.85.0") version number (without operators like ^ or >=).
• Must be at least as new as the compiler version that introduced the package's declared edition (for example, edition = "2024" requires rust-version $\ge$ "1.85").
• In a multi-crate workspace, define rust-version = "1.85" once under [workspace.package] and inherit it in every member crate with rust-version.workspace = true.

# Workspace Root Cargo.toml:
[workspace]
resolver = "3"
members = ["crates/*"]

[workspace.package]
edition = "2024"
rust-version = "1.85"

# Member Crate Cargo.toml:
[package]
name = "core_protocol"
version = "0.1.0"
edition.workspace = true
rust-version.workspace = true

MSRV-Aware Dependency Resolution in Rust 2024 (resolver = "3")

Prior to the Rust 2024 edition, if your crate declared rust-version = "1.80" and depended on foo = "1.0", and foo later published 1.5.0 requiring rust-version = "1.85", running cargo update would blindly select foo v1.5.0—breaking your build on rustc 1.80!

In Rust 2024 edition, edition = "2024" automatically enables Cargo Resolver v3 (resolver = "3"), which is MSRV-aware by default:

• When resolving dependencies, Cargo inspects your package's rust-version (or the current running rustc version if rust-version is unset) and automatically selects the newest compatible dependency version (foo v1.4.2) whose rust-version is $\le$ your requirement!

fn main() {
    let declared_msrv = option_env!("CARGO_PKG_RUST_VERSION").unwrap_or("1.85");
    println!("Package declared rust-version: {declared_msrv}");
}

Package declared rust-version: 1.85