Add support for removed library features

[nilotpal-n7]

PR: Add support for removed library features

Overview

This PR extends the Rust stability system to handle removed library features. The new attribute #[unstable_removed] can be applied to APIs that have been permanently removed from the standard library. This ensures removed items always produce an error when referenced.

The approach follows the existing stability infrastructure (#[unstable], #[stable]), introducing a new Removed state with dedicated compiler diagnostics.

---

Implementation details

• New attribute:

  #[unstable_removed(
      feature = "removed_example",
      since = "1.89.0",
      issue = "123456",
      reason = "testing removed feature"
  )]

• Library integration:

  • Fake removed items are defined in library/core/src/removed_lib_features.rs.
  • These serve as test cases and examples of usage.

• Compiler changes:

  • Extended stability handling to include StabilityLevel::Removed.

  • Updated checks in:

    • rustc_const_eval → reports errors on evaluation of removed consts.
    • rustc_hir_analysis → enforces removed status during analysis.
    • rustc_passes → detects duplicates and unused/invalid declarations.
    • rustc_resolve → reports errors for removed macros and intrinsic resolution.

• Validation: Missing fields (since, issue) are rejected with compiler errors.

---

Flow

1. A library item is annotated with #[unstable_removed].
2. During HIR lowering and stability passes, it is recognized as Removed.
3. Any reference to such an item triggers a hard error (E0658).
4. Compiler ensures consistency across all passes and diagnostics.

---

Testing

• Location: UI tests under tests/ui/stability-attribute/.

• Coverage:

  • Direct use of removed items.
  • Removed trait functions and associated constants.
  • Intrinsic wrappers.
  • Invalid attributes (missing since, issue).
  • Duplicate stability declarations (removed vs stable/unstable).

---

What’s left / To Fix

• Error codes: All new diagnostics currently reuse existing errors (e.g., E0658). Explicit, dedicated error codes should be added for clarity.
• Lang/intrinsics distinction: Real intrinsics and lang items aren’t yet cleanly separated in the removed-feature handling. Right now, only simulated intrinsics are tested.
• Polish: Some duplicated error emissions (e.g., trait fn + trait impl both firing) could be made cleaner.

Issue: #141617

[rustbot]

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to constck

cc @fee1-dead

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

Some changes occurred in exhaustiveness checking

cc @Nadrieril

[rustbot]

r? @nnethercote

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[RalfJung]

Note that this is not the design that was proposed at #141617 (comment). The proposal was to have a crate-level attribute (#![unstable_removed(feature = "...", reason = "...", issue = "...", since = "...")]) -- basically a bit of metadata attached to a crate that says "there also used to be the following features here".

What is the point of marking an existing item as being removed? The item clearly still exists, otherwise it couldn't carry the attribute.^^

I already noted on your previous PR that having a new StabilityLevel for this makes little sense.

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

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fmt: checked 6372 files
tidy check
Checking tidy rustdoc_json...
No error code explanation was removed!
tidy error: /checkout/compiler/rustc_const_eval/messages.ftl: message `The intrinsic `` starts with an uppercase letter. Fix it or add it to `ALLOWED_CAPITALIZED_WORDS`
tidy: Skipping binary file check, read-only filesystem
removing old virtual environment
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'venv'
creating virtual environment at '/checkout/obj/build/venv' using 'python3.10' and 'virtualenv'
Requirement already satisfied: pip in ./build/venv/lib/python3.10/site-packages (25.2)
---
info: ES-Check: there were no ES version matching errors!  🎉
typechecking javascript files
some tidy checks failed
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1556:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1281:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:03:24
  local time: Sun Sep 14 20:26:23 UTC 2025
  network time: Sun, 14 Sep 2025 20:26:24 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[RalfJung]

Sorry, I don't have the time to give a lot of detailed guidance here. You can always ask on Zulip.