stable 1.87.0 builds clap 2.34.0 cleanly, stable 1.88.0 issues a warning, 1.90.0-nightly (9748d87dc 2025-07-21) errors!

[nabijaczleweli]

I tried this code:

RUSTC_BOOTSTRAP=1 cargo install -f --git https://github.com/thecoshman/http

I expected to see this happen: it working

Instead, this happened: (https://github.com/thecoshman/http d3cf0f2ead63a2d9082f719671978bcc97d96599):

installed cleanly with stable 1.87.0

installed with a warning with stable 1.88.0

warning: implicit autoref creates a reference to the dereference of a raw pointer
   --> src/options.rs:130:21
    |
130 |             .author(crate_authors!("\n"))
    |                     ^^^^^^^^^^^^^^^^^^^^
    |
    = note: creating a reference requires the pointer target to be valid and imposes aliasing requirements
    = note: `#[warn(dangerous_implicit_autorefs)]` on by default
help: try using a raw pointer method instead; or if this reference is intentional, make it explicit
   --> /home/nabijaczleweli/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/clap-2.34.0/src/macros.rs:493:22
    |
493 |                     &(&(*VALUE))[..]
    |                      ++        +

failed to install with 1.90.0-nightly (9748d87 2025-07-21) (2)

error: implicit autoref creates a reference to the dereference of a raw pointer
   --> src/options.rs:130:21
    |
130 |             .author(crate_authors!("\n"))
    |                     ^^^^^^^^^^^^^^^^^^^^ this raw pointer has type `*const std::string::String`
    |
    = note: creating a reference requires the pointer target to be valid and imposes aliasing requirements
note: autoref is being applied to this expression, resulting in: `&std::string::String`
   --> src/options.rs:130:21
    |
130 |             .author(crate_authors!("\n"))
    |                     ^^^^^^^^^^^^^^^^^^^^
    = note: `#[deny(dangerous_implicit_autorefs)]` on by default
    = note: this error originates in the macro `crate_authors` (in Nightly builds, run with -Z macro-backtrace for more info)
help: try using a raw pointer method instead; or if this reference is intentional, make it explicit
   --> /home/nabijaczleweli/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/clap-2.34.0/src/macros.rs:493:22
    |
493 |                     &(&(*VALUE))[..]
    |                      ++        +

Meta

(Previous stable that worked: rustc 1.87.0 (17067e9ac 2025-05-09))

rustc --version --verbose:

rustc 1.88.0 (6b00bc388 2025-06-23)
binary: rustc
commit-hash: 6b00bc3880198600130e1cf62b8f8a93494488cc
commit-date: 2025-06-23
host: x86_64-unknown-linux-gnu
release: 1.88.0
LLVM version: 20.1.5

rustc +nightly --version --verbose:

rustc 1.90.0-nightly (9748d87dc 2025-07-21)
binary: rustc
commit-hash: 9748d87dc70a9a6725c5dbd76ce29d04752b4f90
commit-date: 2025-07-21
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8

[nabijaczleweli]

Inlined:

macro_rules! crate_authors {
    ($sep:expr) => {{
        use std::ops::Deref;
        #[allow(deprecated)]
        use std::sync::{Once, ONCE_INIT};

        #[allow(missing_copy_implementations)]
        #[allow(dead_code)]
        struct CargoAuthors {
            __private_field: (),
        };

        impl Deref for CargoAuthors {
            type Target = str;

            #[allow(unsafe_code)]
            fn deref(&self) -> &'static str {
                #[allow(deprecated)]
                static ONCE: Once = ONCE_INIT;
                static mut VALUE: *const String = 0 as *const String;

                unsafe {
                    ONCE.call_once(|| {
                        let s = env!("CARGO_PKG_AUTHORS").replace(':', $sep);
                        VALUE = Box::into_raw(Box::new(s));
                    });

                    &(*VALUE)[..]
                }
            }
        }

        &*CargoAuthors {
            __private_field: (),
        }
    }};
    () => {
        env!("CARGO_PKG_AUTHORS")
    };
}

fn main() {
    dbg!(crate_authors!(":"));
}

$ CARGO_PKG_AUTHORS= rustc --edition 2018  a.rs
warning: unnecessary trailing semicolon
  --> a.rs:11:10
   |
11 |         };
   |          ^ help: remove this semicolon
...
43 |     dbg!(crate_authors!(":"));
   |          ------------------- in this macro invocation
   |
   = note: `#[warn(redundant_semicolons)]` on by default
   = note: this warning originates in the macro `crate_authors` (in Nightly builds, run with -Z macro-backtrace for more info)

warning: implicit autoref creates a reference to the dereference of a raw pointer
  --> a.rs:43:10
   |
43 |     dbg!(crate_authors!(":"));
   |          ^^^^^^^^^^^^^^^^^^^
   |
   = note: creating a reference requires the pointer target to be valid and imposes aliasing requirements
   = note: `#[warn(dangerous_implicit_autorefs)]` on by default
help: try using a raw pointer method instead; or if this reference is intentional, make it explicit
   |
28 |                     &(&(*VALUE))[..]
   |                      ++        +

warning: 2 warnings emitted

$ CARGO_PKG_AUTHORS= rustc +nightly --edition 2018  a.rs
warning: unnecessary trailing semicolon
  --> a.rs:11:10
   |
11 |         };
   |          ^
...
43 |     dbg!(crate_authors!(":"));
   |          ------------------- in this macro invocation
   |
   = note: `#[warn(redundant_semicolons)]` on by default
   = note: this warning originates in the macro `crate_authors` (in Nightly builds, run with -Z macro-backtrace for more info)

error: implicit autoref creates a reference to the dereference of a raw pointer
  --> a.rs:43:10
   |
28 |                     &(*VALUE)[..]
   |                        ----- this raw pointer has type `*const String`
...
43 |     dbg!(crate_authors!(":"));
   |          ^^^^^^^^^^^^^^^^^^^
   |
   = note: creating a reference requires the pointer target to be valid and imposes aliasing requirements
note: autoref is being applied to this expression, resulting in: `&String`
  --> a.rs:28:22
   |
28 |                     &(*VALUE)[..]
   |                      ^^^^^^^^
...
43 |     dbg!(crate_authors!(":"));
   |          ------------------- in this macro invocation
   = note: `#[deny(dangerous_implicit_autorefs)]` on by default
   = note: this error originates in the macro `crate_authors` (in Nightly builds, run with -Z macro-backtrace for more info)
help: try using a raw pointer method instead; or if this reference is intentional, make it explicit
   |
28 |                     &(&(*VALUE))[..]
   |                      ++        +

error: aborting due to 1 previous error; 1 warning emitted

[nabijaczleweli]

Reduced:

fn main() {
    let value = 0 as *const String;
    unsafe {
        let _ = &(*value)[..];
    }
}

$ rustc --edition 2018 a.rs
warning: implicit autoref creates a reference to the dereference of a raw pointer
 --> a.rs:4:18
  |
4 |         let _ = &(*value)[..];
  |                  ^^^^^^^^^^^^
  |
  = note: creating a reference requires the pointer target to be valid and imposes aliasing requirements
  = note: `#[warn(dangerous_implicit_autorefs)]` on by default
help: try using a raw pointer method instead; or if this reference is intentional, make it explicit
  |
4 |         let _ = &(&(*value))[..];
  |                  ++        +

warning: 1 warning emitted

$ rustc +nightly --edition 2018 a.rs
error: implicit autoref creates a reference to the dereference of a raw pointer
 --> a.rs:4:18
  |
4 |         let _ = &(*value)[..];
  |                  ^^-----^^^^^
  |                    |
  |                    this raw pointer has type `*const String`
  |
  = note: creating a reference requires the pointer target to be valid and imposes aliasing requirements
note: autoref is being applied to this expression, resulting in: `&String`
 --> a.rs:4:18
  |
4 |         let _ = &(*value)[..];
  |                  ^^^^^^^^
  = note: `#[deny(dangerous_implicit_autorefs)]` on by default
help: try using a raw pointer method instead; or if this reference is intentional, make it explicit
  |
4 |         let _ = &(&(*value))[..];
  |                  ++        +

error: aborting due to 1 previous error

[hanna-kruppe]

New lints get introduced and default levels get increased, that’s normal and generally not considered a regression. Usually Cargo capping lints in non-path dependencies keeps it actionable (developers only get warnings about their own code where they can easily fix or silence the lint). The interesting complications in this case are (1) the lint firing in a macro from a dependency expanded in the main crate so it’s not capped, and (2) cargo install not capping lints in the main crate even though it’s generally used by end users rather than developers.

[jieyouxu]

As far as I know, this is fully intentional: see #123239, #141661 and #142172 (in fact, it only being a warning in 1.88.0 is the unintentional bit).

EDIT: this was explicitly called out in 1.88.0's release notes.

• Add warn-by-default dangerous_implicit_autorefs lint against implicit autoref of raw pointer dereference.
  The lint will be bumped to deny-by-default in the next version of Rust.

I'm going to close this as "working as intended", but if you have any specific concerns about e.g. the lint should or should not fire against a specific case, feel free to reopen.

[Noratrieb]

as a workaround you can cap lints: RUSTFLAGS=--cap-lints allow. but an issue should be opened against the crate to make its macro follow the lint