proc-macro-warning 0.4.2

Emit warnings from inside proc macros.
Documentation 
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<h1 align="center"><br>
    Proc Macro Warning
<br></h1>

<h4 align="center">Emit warnings from inside proc macros.</h4>

<p align="center">
  <a href="https://www.gnu.org/licenses/gpl-3.0">
    <img src="https://img.shields.io/crates/l/proc-macro-warning" alt="License: (GPL-3.0 or Apache-2.0)">
  </a>
  <a href="https://crates.io/crates/proc-macro-warning">
    <img src="https://img.shields.io/crates/v/proc-macro-warning"/>
  </a>
  <a href="https://github.com/ggwpez/proc-macro-warning/actions/workflows/rust.yml">
    <img src="https://github.com/ggwpez/proc-macro-warning/actions/workflows/rust.yml/badge.svg"/>
  </a>
</p>

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

## Example

Building a warning is easy with the builder pattern.

```rust
use proc_macro_warning::Warning;

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

// 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.

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

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

## Un-opinionated Formatting

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`:  

```rust
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](ui-tests/derive/src/lib.rs) is in [derive_raw.stderr](ui-tests/ui/src/warn/derive_raw.stderr).

## Used In 

Substrate uses it to emit warnings for its eDSL (FRAME) on deprecated behaviour. The integration was done in [#13798](https://github.com/paritytech/substrate/pull/13798) and shows how to use these warnings in macro expansion.

The warnings are uniformly formatted and have consistent grammar:
```pre
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:
```pre
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)]
     |                          
```

## License

Licensed under either of at your own choice:

* GNU GENERAL PUBLIC LICENSE, Version 3 ([LICENSE-GPL3](./LICENSE-GPL3) or [gnu.org](https://www.gnu.org/licenses/gpl-3.0.txt>))
* Apache License, Version 2.0 ([LICENSE-APACHE2](/LICENSE-APACHE2) or [apache.org](https://www.apache.org/licenses/LICENSE-2.0.txt>)).

### Contribution

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.