WIP: Lint unused features

[mu001999]

Fixes #44232
Fixes #151752

[jyn514]

this is neat! in the future, it would be easier to review if you split the rename into a separate commit from the change in behavior.

could you add some tests for this please?

View changes since this review

[mu001999]

Initial implementation completed. I plan to add more tests and check whether features in rustc are reported as unused correctly or not.

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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    |
206 | #![feature(riscv_target_feature)]
    |            ^^^^^^^^^^^^^^^^^^^^

warning: feature `rtm_target_feature` is declared but not used
   --> library/core/src/lib.rs:207:12
    |
207 | #![feature(rtm_target_feature)]
    |            ^^^^^^^^^^^^^^^^^^

warning: feature `s390x_target_feature` is declared but not used
   --> library/core/src/lib.rs:208:12
    |
---
   Compiling unwind v0.0.0 (/checkout/library/unwind)
warning: feature `libc` is declared but not used
 --> library/unwind/src/lib.rs:7:47
  |
7 | #![cfg_attr(not(target_env = "msvc"), feature(libc))]
  |                                               ^^^^
  |
  = note: `#[warn(unused_features)]` (part of `#[warn(unused)]`) on by default

[RUSTC-TIMING] unwind test:false 0.078
---

warning: feature `string_replace_in_place` is declared but not used
  --> library/alloc/src/lib.rs:88:50
   |
88 | #![cfg_attr(not(no_global_oom_handling), feature(string_replace_in_place))]
   |                                                  ^^^^^^^^^^^^^^^^^^^^^^^

warning: feature `assert_matches` is declared but not used
  --> library/alloc/src/lib.rs:92:12
   |