Add FCW for derive helper attributes that will conflict with built-in attributes

[nik-rev]

Adds a future-compatibility-warning deny-by-default lint that helps catch invalid derive helper attribute names early.

This issues the lint, saying that ignore helper will clash with the built-in ignore attribute.

#![crate_type = "proc-macro"]
#![deny(ambiguous_derive_helpers)] 
use proc_macro::TokenStream; 

#[proc_macro_derive(Trait, attributes(ignore))]
pub fn example(input: TokenStream) -> TokenStream {
    TokenStream::new()
}

If you actually tried to use that ignore helper attribute, you won't be able to due to the ambiguity:

#[derive(Trait)]
struct Foo {
    #[ignore]
    field: (),
}

Produces:

error[E0659]: `ignore` is ambiguous
 --> src/lib.rs:5:7
  |
5 |     #[ignore]
  |       ^^^^^^ ambiguous name
  |
  = note: ambiguous because of a name conflict with a builtin attribute
  = note: `ignore` could refer to a built-in attribute
note: `ignore` could also refer to the derive helper attribute defined here
 --> src/lib.rs:3:10
  |
3 | #[derive(Trait)]
  |          ^^^^^

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @chenyukang

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[chenyukang]

@bors r=chenyukang

[rust-bors]

📌 Commit 4dda363 has been approved by chenyukang

It is now in the queue for this repository.

[petrochenkov]

New deprecation lints go through language team process, and the lint name also doesn't follow naming conventions.

[chenyukang]

@bors r-

[rust-bors]

Commit 4dda363 has been unapproved.

[nik-rev]

@rustbot label +I-lang-nominated

I am nominating this for lang-team attention.

What name should this lint have? The current name doesn't follow the naming conventions

My main idea is allow(derive_helper_attr_with_same_name_as_a_builtin_attr), but that's quite long

[traviscross]

Looks reasonable to me.

View changes since this review

[traviscross]

Thanks.

@rfcbot resolve name

[traviscross]

@rfcbot reviewed

[rust-rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[rust-rfcbot]

The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[chenyukang]

Thanks!
@bors r=chenyukang

[rust-bors]

📌 Commit f4eca01 has been approved by chenyukang

It is now in the queue for this repository.

[JonathanBrouwer]

@bors r-
#152421 (comment)

[rust-bors]

Commit f4eca01 has been unapproved.

This PR was contained in a rollup (#152421), which was also unapproved.