`cargo check` fires when `rustc` lints use `-` instead of `_` on `nightly`

[zacknewman]

Problem

All the names of lints and lint groups use - instead of _ in the output of rustc -Whelp:

[zack@laptop ~]$ rustc --version
rustc 1.99.0-nightly (1a98b1e13 2026-08-07)
[zack@laptop ~]$ rustc -Whelp

Available lint options:
    -W <foo>           Warn about <foo>
    -A <foo>           Allow <foo>
    -D <foo>           Deny <foo>
    -F <foo>           Forbid <foo> (deny <foo> and all attempts to override)


Lint checks provided by rustc:

                                                       name  default  meaning
                                                       ----  -------  -------
                     absolute-paths-not-starting-with-crate  allow    fully qualified paths that start with a module name instead of `crate`, `self`, or an extern crate name
⋮

Despite the "official names" using - instead of _ at least per rustc in -Whelp and diagnostic messages (e.g., note: requested on the command line with `-D unused-results`), cargo check complains that the use of such names is deprecated and won't work in a future edition:

[zack@laptop example]$ cargo check
warning: Cargo.toml: `lints.rust.ambiguous-negative-literals` is deprecated in favor of `lints.rust.ambiguous_negative_literals` and will not work in a future edition
warning: Cargo.toml: `lints.rust.closure-returning-async-block` is deprecated in favor of `lints.rust.closure_returning_async_block` and will not work in a future edition
warning: Cargo.toml: `lints.rust.dead-code-pub-in-binary` is deprecated in favor of `lints.rust.dead_code_pub_in_binary` and will not work in a future edition
warning: Cargo.toml: `lints.rust.deprecated-in-future` is deprecated in favor of `lints.rust.deprecated_in_future` and will not work in a future edition
warning: Cargo.toml: `lints.rust.deprecated-safe` is deprecated in favor of `lints.rust.deprecated_safe` and will not work in a future edition
warning: Cargo.toml: `lints.rust.deref-into-dyn-supertrait` is deprecated in favor of `lints.rust.deref_into_dyn_supertrait` and will not work in a future edition
warning: Cargo.toml: `lints.rust.ffi-unwind-calls` is deprecated in favor of `lints.rust.ffi_unwind_calls` and will not work in a future edition
warning: Cargo.toml: `lints.rust.future-incompatible` is deprecated in favor of `lints.rust.future_incompatible` and will not work in a future edition
warning: Cargo.toml: `lints.rust.impl-trait-redundant-captures` is deprecated in favor of `lints.rust.impl_trait_redundant_captures` and will not work in a future edition
warning: Cargo.toml: `lints.rust.keyword-idents` is deprecated in favor of `lints.rust.keyword_idents` and will not work in a future edition
warning: Cargo.toml: `lints.rust.let-underscore` is deprecated in favor of `lints.rust.let_underscore` and will not work in a future edition
warning: Cargo.toml: `lints.rust.linker-info` is deprecated in favor of `lints.rust.linker_info` and will not work in a future edition
warning: Cargo.toml: `lints.rust.macro-use-extern-crate` is deprecated in favor of `lints.rust.macro_use_extern_crate` and will not work in a future edition
warning: Cargo.toml: `lints.rust.meta-variable-misuse` is deprecated in favor of `lints.rust.meta_variable_misuse` and will not work in a future edition
warning: Cargo.toml: `lints.rust.missing-copy-implementations` is deprecated in favor of `lints.rust.missing_copy_implementations` and will not work in a future edition
warning: Cargo.toml: `lints.rust.missing-debug-implementations` is deprecated in favor of `lints.rust.missing_debug_implementations` and will not work in a future edition
warning: Cargo.toml: `lints.rust.missing-docs` is deprecated in favor of `lints.rust.missing_docs` and will not work in a future edition
warning: Cargo.toml: `lints.rust.non-ascii-idents` is deprecated in favor of `lints.rust.non_ascii_idents` and will not work in a future edition
warning: Cargo.toml: `lints.rust.nonstandard-style` is deprecated in favor of `lints.rust.nonstandard_style` and will not work in a future edition
warning: Cargo.toml: `lints.rust.redundant-imports` is deprecated in favor of `lints.rust.redundant_imports` and will not work in a future edition
warning: Cargo.toml: `lints.rust.redundant-lifetimes` is deprecated in favor of `lints.rust.redundant_lifetimes` and will not work in a future edition
warning: Cargo.toml: `lints.rust.refining-impl-trait` is deprecated in favor of `lints.rust.refining_impl_trait` and will not work in a future edition
warning: Cargo.toml: `lints.rust.rust-2018-compatibility` is deprecated in favor of `lints.rust.rust_2018_compatibility` and will not work in a future edition
warning: Cargo.toml: `lints.rust.rust-2018-idioms` is deprecated in favor of `lints.rust.rust_2018_idioms` and will not work in a future edition
warning: Cargo.toml: `lints.rust.rust-2021-compatibility` is deprecated in favor of `lints.rust.rust_2021_compatibility` and will not work in a future edition
warning: Cargo.toml: `lints.rust.rust-2024-compatibility` is deprecated in favor of `lints.rust.rust_2024_compatibility` and will not work in a future edition
warning: Cargo.toml: `lints.rust.single-use-lifetimes` is deprecated in favor of `lints.rust.single_use_lifetimes` and will not work in a future edition
warning: Cargo.toml: `lints.rust.trivial-casts` is deprecated in favor of `lints.rust.trivial_casts` and will not work in a future edition
warning: Cargo.toml: `lints.rust.trivial-numeric-casts` is deprecated in favor of `lints.rust.trivial_numeric_casts` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unit-bindings` is deprecated in favor of `lints.rust.unit_bindings` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unknown-or-malformed-diagnostic-attributes` is deprecated in favor of `lints.rust.unknown_or_malformed_diagnostic_attributes` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unnameable-types` is deprecated in favor of `lints.rust.unnameable_types` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unreachable-pub` is deprecated in favor of `lints.rust.unreachable_pub` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unsafe-code` is deprecated in favor of `lints.rust.unsafe_code` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unstable-features` is deprecated in favor of `lints.rust.unstable_features` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unused-crate-dependencies` is deprecated in favor of `lints.rust.unused_crate_dependencies` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unused-import-braces` is deprecated in favor of `lints.rust.unused_import_braces` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unused-lifetimes` is deprecated in favor of `lints.rust.unused_lifetimes` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unused-qualifications` is deprecated in favor of `lints.rust.unused_qualifications` and will not work in a future edition
warning: Cargo.toml: `lints.rust.unused-results` is deprecated in favor of `lints.rust.unused_results` and will not work in a future edition
warning: `example` (manifest) generated 40 warnings

Proposed Solution

Lints should be allowed to have - instead of _ in [lints.rust] or rustc -Whelp and diagnostic messages reported from rustc should be updated to use _ instead of -.

Notes

I believe #17298 is the sudden cause of these warnings. I have been informed this is due to #17051.

[weihanglo]

In #17298 (comment):

Even the link to the "Lint naming" guidelines uses sentences like:

The basic rule is: the lint name should make sense when read as “allow lint-name” or “allow lint-name items”.

Notice the use of - instead of _. This seems overly opinionated to be warn-by-default.

However, down a few lines:

https://rustc-dev-guide.rust-lang.org/diagnostics.html#lint-naming

Use snake case in the same way you would for function names.

See also the rationale: #13943 (comment)

[zacknewman]

Hm, I don't find that very convincing though. I understand Cargo wants to require _ for its own lints, but I fail to see how that applies to a different table like lints.rust. Additionally, it seems inconsistent that on a package level no hard requirement is made for packages to only contain one or the other; and even more bizarre is the fact that if such a decision were made it'd likely be in favor of - over _ based on my experience with package names at least.

While I realize one can technically have two rustc lints that only differ by - or _, that's also true for dependencies. Just like dependencies, Cargo transforms the - one to the _ one when there are no "duplicates". When there are "duplicates", it doesn't complain (unless they are truly identical) and doesn't transform the - one to the _ one forcing one to handle the - one manually by giving it an alias. Because the most popular registry, crates.io, treats packages as the same if they only differ by - and _, it's rare when one has two dependencies in Cargo.toml that differ by - and _ alone. It can still happen though especially with local crates.

One difference I imagine is that the use of - in package names is far more common than people using - for rustc lints, so this breaking change is not as impactful. Still sucks that I have to update every crate I've ever written since all of them contain at least one rustc lint or lint group with a name that contains -. I chose to reference such lints with - because I thought it would never matter, and I assumed the one with - was "official" seeing how it's the version you always see in diagnostic messages and -Whelp. Contrast this with Clippy lints where online documentation and diagnostic messages use the _ version; thus I reference such lints using _.