Diags `unused_crate_dependencies`&`unused_extern_crates` "errneously" issued w/ `-Z build-std`

[LebedevRI]

Code

mkdir /tmp/repro
cd /tmp/repro
tee -a Cargo.toml > /dev/null <<EOF

[lints.rust]
unused_crate_dependencies = { level = "warn", priority = -1 }
unused_extern_crates = { level = "warn", priority = -1 }
EOF
cargo +nightly -Z build-std build

Current output

$ cargo +nightly -Z build-std build
   Compiling compiler_builtins v0.1.160 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/compiler-builtins/compiler-builtins)
   Compiling core v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core)
   Compiling libc v0.2.178
   Compiling object v0.37.3
   Compiling std v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std)
   Compiling rustc-std-workspace-core v1.99.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-core)
   Compiling alloc v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc)
   Compiling adler2 v2.0.1
   Compiling memchr v2.7.6
   Compiling rustc-demangle v0.1.27
   Compiling panic_abort v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/panic_abort)
   Compiling cfg-if v1.0.4
   Compiling rustc-literal-escaper v0.0.7
   Compiling unwind v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/unwind)
   Compiling rustc-std-workspace-alloc v1.99.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/rustc-std-workspace-alloc)
   Compiling panic_unwind v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/panic_unwind)
   Compiling gimli v0.32.3
   Compiling std_detect v0.1.5 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/std_detect)
   Compiling miniz_oxide v0.8.9
   Compiling hashbrown v0.16.1
   Compiling addr2line v0.25.1
   Compiling proc_macro v0.0.0 (/home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/proc_macro)
   Compiling repro v0.1.0 (/tmp/repro)
warning: extern crate `alloc` is unused in crate `repro`
  |
  = help: remove the dependency or add `use alloc as _;` to the crate root
  = note: requested on the command line with `-W unused-crate-dependencies`

warning: extern crate `compiler_builtins` is unused in crate `repro`
  |
  = help: remove the dependency or add `use compiler_builtins as _;` to the crate root

warning: extern crate `core` is unused in crate `repro`
  |
  = help: remove the dependency or add `use core as _;` to the crate root

warning: extern crate `proc_macro` is unused in crate `repro`
  |
  = help: remove the dependency or add `use proc_macro as _;` to the crate root

warning: `repro` (bin "repro") generated 4 warnings
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 15.77s

Desired output

No Diags `unused_crate_dependencies`/`unused_extern_crates` should be issued.

Rationale and extra context

Those crates aren't listed in Cargo.toml, they were implicitly added by -Z build-std,
therefore it would be great if they weren't diagnosed.

Other cases

Rust Version

$ /home/rust/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc --version --verbose
rustc 1.95.0-nightly (7057231bd 2026-02-11)
binary: rustc
commit-hash: 7057231bd78d6c7893f905ea1832365d4c5efe17
commit-date: 2026-02-11
host: x86_64-unknown-linux-gnu
release: 1.95.0-nightly
LLVM version: 22.1.0

Anything else?

No response

[biscuitrescue]

Hi I'd like to work on this. @rustbot claim