private_macro_use warning needs more information

[axos88]

Related: #120192 Tracking Issue for private_macro_use lint

The warning needs more information on why a macro is considered private, possibly with full module path. The following produces a warning with very little information to go on to figure out that it's picking up the Serialize coming from serde_with, instead of serde. Having the definition of Foo in another module makes it magic on how the Serialize is not imported. (but I would guess that's an issue with #[macro_use]

Interestingly the #[macro_use] is considered unused by the compiler as well, but removing that produces an import error (correctly).

#[macro_use]
extern crate serde_with;


#[derive(Serialize)]
pub struct Foo;

pub fn main() {}

My Cargo.toml contains:

[dependencies]
serde = { version = "1.0.192", features = ["derive"] }
serde_with = "1.10.0"

warning: macro `Serialize` is private
 --> src/main.rs:5:10
  |
5 | #[derive(Serialize)]
  |          ^^^^^^^^^
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
  = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
  = note: `#[warn(private_macro_use)]` on by default

warning: unused `#[macro_use]` import
 --> src/main.rs:1:1
  |
1 | #[macro_use]
  | ^^^^^^^^^^^^
  |
  = note: `#[warn(unused_imports)]` on by default

warning: `private-macro-test` (bin "private-macro-test") generated 2 warnings

[tgrushka]

This lint is a major problem waiting to happen. It is likely to break a bunch of public crates (e.g. rsa depends on num-bigint-dig which no longer compiles, does that raise enough 🚩red flags 🚩?). One should not have to pub use alloc::vec in order to use the vec![] macro in a library crate in order to avoid this warning (now an error), because encouraging the re-exporting of std/alloc/rust internals is an anti-pattern.

Who in the world let this through without serious vetting? What problem is it solving? If a downstream crate tries to use a private macro, it won't compile anyway, right? So why is this lint needed outside of use by rustc itself?

This problem does not show up in cargo check in the "offending" crate either; it only shows when compiling a dependent crate.

To be even usable, this lint:

1. should not lint on standard rust macros that are public by definition;
2. should raise on the offending crate, if a privately-imported macro is used in a public function (that doesn't even make sense, as plenty of implementations use private internals, but I guess rust cannot expand privately-exported macros?);
3. should be easily opted out of via -Aprivate-macro-use or #![allow(private_macro_use)]. These don't even seem to work today.

For example, num-bigint-dig now throws a compile warning on nightly, but its claim that "macro vec is private" is completely and utterly false:

warning: the following packages contain code that will be rejected by a future version of Rust: num-bigint-dig v0.8.4
note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1

The following warnings were discovered during the build. These warnings are an
indication that the packages contain code that will become an error in a
future release of Rust. These warnings typically cover changes to close
soundness problems, unintended or undocumented behavior, or critical problems
that cannot be fixed in a backwards-compatible fashion, and are not expected
to be in wide use.

Each warning should contain a link for more information on what the warning
means and how to resolve it.


To solve this problem, you can try the following approaches:

- If the issue is not solved by updating the dependencies, a fix has to be
implemented by those dependencies. You can help with that by notifying the
maintainers of this problem (e.g. by creating a bug report) or by proposing a
fix to the maintainers (e.g. by creating a pull request):

  - num-bigint-dig@0.8.4
  - Repository: https://github.com/dignifiedquire/num-bigint
  - Detailed warning command: `cargo report future-incompatibilities --id 1 --package num-bigint-dig@0.8.4`

- If waiting for an upstream fix is not an option, you can use the `[patch]`
section in `Cargo.toml` to use your own version of the dependency. For more
information, see:
https://doc.rust-lang.org/cargo/reference/overriding-dependencies.html#the-patch-section

The package `num-bigint-dig v0.8.4` currently triggers the following future incompatibility lints:
> warning: macro `vec` is private
>    --> /Users/tom/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/num-bigint-dig-0.8.4/src/biguint.rs:490:22
>     |
> 490 |         BigUint::new(vec![1])
>     |                      ^^^
>     |
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>     = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
>
> warning: macro `vec` is private
>     --> /Users/tom/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/num-bigint-dig-0.8.4/src/biguint.rs:2005:9
>      |
> 2005 |         vec![0]
>      |         ^^^
>      |
>      = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>      = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
>
> warning: macro `vec` is private
>     --> /Users/tom/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/num-bigint-dig-0.8.4/src/biguint.rs:2027:16
>      |
> 2027 |         return vec![b'0'];
>      |                ^^^
>      |
>      = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>      = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
>
> warning: macro `vec` is private
>     --> /Users/tom/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/num-bigint-dig-0.8.4/src/biguint.rs:2313:13
>      |
> 2313 |             vec![0]
>      |             ^^^
>      |
>      = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>      = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
>
> warning: macro `vec` is private
>    --> /Users/tom/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/num-bigint-dig-0.8.4/src/prime.rs:138:22
>     |
> 138 |     let mut moduli = vec![BigUint::zero(); prime_limit];
>     |                      ^^^
>     |
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>     = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>
>
> warning: macro `vec` is private
>    --> /Users/tom/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/num-bigint-dig-0.8.4/src/bigrand.rs:319:25
>     |
> 319 |         let mut bytes = vec![0u8; bytes_len];
>     |                         ^^^
>     |
>     = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
>     = note: for more information, see issue #120192 <https://github.com/rust-lang/rust/issues/120192>