Rust does not have native functions to produce warnings from inside proc macros. This crate provides "deprecated" warnings for your proc macro use-cases.

Building a warning is easy with the builder pattern.

use proc_macro_warning::Warning;

let warning = Warning::new_deprecated("OldStuffUsed")
	.old("my_macro()")
	.new("my_macro::new()")
	.help_link("https:://example.com")
	.span(proc_macro2::Span::call_site())
	.build_or_panic();

// Use the warning in a proc macro
let tokens = quote::quote!(#warning);

This works in derive-macros, but you must set a span; otherwise it will not show up in the compile output.

The difference to a #[deprecated] attribute is that it emits the warning either way. For example when creating a custom Deprecated derive macro, it will warn without the struct being constructed.

#[derive(derive::Deprecated)]
struct Test {}

fn main() {
  // Warning triggers although we never used `Test`.  
  // Otherwise use a normal `#[deprecated]`.
}

The normal aforementioned way of creating a warning will impose specific unified grammar and formatting rules.
You can opt out of this and use your own instead by using FormattedWarning::new_deprecated:

use proc_macro_warning::FormattedWarning;

let warning = FormattedWarning::new_deprecated(
      "my_macro",
      "looooooooooooooooooooooooooooooong line that will not be brokeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeen ;)",
      proc_macro2::Span::call_site(),
    );

// Use the warning in a proc macro
let tokens = quote::quote!(#warning);

The output of a similar example is in derive_raw.stderr.

Substrate uses it to emit warnings for its eDSL (FRAME) on deprecated behaviour. The integration was done in #13798 and shows how to use these warnings in macro expansion.

The warnings are uniformly formatted and have consistent grammar:

warning: use of deprecated constant `pallet::warnings::ImplicitCallIndex_0::_w`:
                 It is deprecated to use implicit call indices.
                 Please instead ensure that all calls have the `pallet::call_index` attribute or that the `dev-mode` of the pallet is enabled.
         
                 For more info see:
                     <https://github.com/paritytech/substrate/pull/12891>
                     <https://github.com/paritytech/substrate/pull/11381>
    --> frame/nomination-pools/src/lib.rs:2621:10
     |
2621 |         pub fn claim_commission(origin: OriginFor<T>, pool_id: PoolId) -> DispatchResult {
     |                ^^^^^^^^^^^^^^^^
     |

A different one:

warning: use of deprecated constant `pallet::warnings::ConstantWeight_0::_w`:
                 It is deprecated to use hard-coded constant as call weight.
                 Please instead benchmark all calls or put the pallet into `dev` mode.
         
                 For more info see:
                     <https://github.com/paritytech/substrate/pull/13798>
    --> frame/nomination-pools/src/lib.rs:2620:20
     |
2620 |         #[pallet::weight(0)]
     |                          

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• GNU GENERAL PUBLIC LICENSE, Version 3 (LICENSE-GPL3 or gnu.org)
• Apache License, Version 2.0 (LICENSE-APACHE2 or apache.org).

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.