Enable diagnostic attributes for Rust 1.78+

[jplatte]

Stabilized for 1.78 in rust-lang/rust#119888.

@tisonkun does this fix your issue? You can try it by adding

[patch.crates-io]
axum = { git = "https://github.com/tokio-rs/axum", branch = "jplatte/diagnostic-ver" }
axum-core = { git = "https://github.com/tokio-rs/axum", branch = "jplatte/diagnostic-ver" }

to your top-level Cargo.toml (the workspace file if you use a Cargo workspace).

[tisonkun]

Confirmed.

With this patch, it compiles; otherwise, it can't.

Thank you!

[seanmonstar]

Does adding this new proc macro slow down compilation?

[jplatte]

We were already using rustversion, just in a build script. By using it in the crate directly and deleting the build script, I would expect things to become slightly faster since every build script has cargo spawn a separate compiler process, then the build script process.