`unused_dependencies` can't be granularly disabled/expected

Problem

So, the specific use case I'm running into here is allocator-api2 for the hashbrown crate, which is enabled by default for MSRV reasons: even though allocator_api was just stabilised on nightly, we still need it for most users.

However, with the nightly feature, we explicitly do not use this dependency at all, and thus it gets reported as unused. The only way to fix this would be to use --no-default-features alongside the nightly feature, plus every feature except allocator-api2, to avoid the unused dependency.

It would be nice if, in this case, we could simply mark this one case as being acceptable as an unused dependency, rather than completely disabling the lint. Or even expect(...) the lint somehow and acknowledge it.

Steps

No response

Possible Solution(s)

Since the resolver is additive, we should not have some way to disable certain feature combinations, but we could have a way to accept unused dependencies when certain features are enabled. For example, in this case, specifically configure the lint to state that, whenever the nightly feature is enabled, allocator-api2 will be unused. In this case, cargo should be able to determine that, since allocator-api2 is an optional dependency, it will only be unused if it's explicitly enabled.

Notes

No response

Version

cargo 1.100.0-nightly (98a09e7e7 2026-09-21)
release: 1.100.0-nightly
commit-hash: 98a09e7e7d62850f14e5b6132101fc1edd19a16f
commit-date: 2026-09-21
host: x86_64-unknown-linux-gnu
libgit2: 1.9.6 (sys:0.21.0 vendored)
libcurl: 8.21.0-DEV (sys:0.4.90+curl-8.21.0 vendored ssl:OpenSSL/3.6.3)
ssl: OpenSSL 3.6.3 9 Jun 2026
os: Arch Linux Rolling Release [64-bit]