Exploring Crate Graph Build Times with `cargo build -Ztimings`

[dtolnay]

As you flagged in serde-rs/serde#1146, procedural macros currently never get pipelined with their dependencies. Would it be reasonable for Cargo to automatically apply the transformation of turning all dependency-having proc macro crates into small shims around an underlying non proc macro impl crate to get better pipelining?

#[proc_macro_derive(Deserialize, attributes(serde))]
pub fn derive_deserialize(input: TokenStream) -> TokenStream {
    serde_derive_impl::derive_deserialize(input)
}

Before:

After: