track the version and PR in which a nightly feature was removed

[jyn514]

Code

#![feature(external_doc)]
#![doc(include("README.md"))]

Current output

error[E0557]: feature has been removed
 --> src/lib.rs:1:12
  |
1 | #![feature(external_doc)]
  |            ^^^^^^^^^^^^ feature has been removed
  |
  = note: use #[doc = include_str!("filename")] instead, which handles macro invocations

error: unknown `doc` attribute `include`
 --> src/lib.rs:2:8
  |
2 | #![doc(include("README.md"))]
  |        ^^^^^^^^^^^^^^^^^^^^
  |
  = note: `#[deny(invalid_doc_attributes)]` on by default

Desired output

error[E0557]: feature has been removed
 --> src/lib.rs:1:12
  |
1 | #![feature(external_doc)]
  |            ^^^^^^^^^^^^ feature has been removed
  |
  = note: removed in 1.54 (you are using 1.89-nightly); see <https://github.com/rust-lang/rust/pull/85457>
  = note: use #[doc = include_str!("filename")] instead, which handles macro invocations

Rationale and extra context

this makes it more clear what is going on, and also gives people enough breadcrumbs to find more information. right now it is very hard to find info about removed features without trawling through the code (it took me about 15 minutes just now, and i am both the person who removed this feature and the one who stabilized its replacement).

saying the version also makes it more clear that "removed" is specifically about versions, and tells the person compiling the code that using an older toolchain will likely work. this matters because the person seeing this error is likely not the one who wrote the code, and possibly knows very little about rust's stability policy.

Other cases

Rust Version

1.89.0-nightly

(2025-05-25 283db70ace62a0ae704a)

Anything else?

No response

[jyn514]

oh neat, we already track version:

rust/compiler/rustc_feature/src/removed.rs

Line 43 in 30721b0

(removed, abi_amdgpu_kernel, "1.77.0", Some(51575), None),

that makes the version part of this quite simple, and we can add pr info later.

mentoring instructions: in

rust/compiler/rustc_expand/src/config.rs

Lines 82 to 89 in 30721b0

	// If the enabled feature has been removed, issue an error.
	if let Some(f) = REMOVED_LANG_FEATURES.iter().find(|f| name == f.feature.name) {
	sess.dcx().emit_err(FeatureRemoved {
	span: mi.span(),
	reason: f.reason.map(|reason| FeatureRemovedReason { reason }),
	});
	continue;
	}

, add a note about when the feature was removed.

@rustbot label E-help-wanted E-easy

[xizheyin]

@rustbot claim

[xizheyin]

For removed feature, the version may be not accurate, is this some kind of historical problem?

rust/compiler/rustc_feature/src/lib.rs

Lines 33 to 41 in 30721b0

	pub struct Feature {
	pub name: Symbol,
	/// For unstable features: the version the feature was added in.
	/// For accepted features: the version the feature got stabilized in.
	/// For removed features we are inconsistent; sometimes this is the
	/// version it got added, sometimes the version it got removed.
	pub since: &'static str,
	issue: Option<NonZero<u32>>,
	}

[jyn514]

hm. ideally we would not give wrong info in the diagnostic. are you willing to go through the git history for each feature one by one and see when it was removed? i think it will not be as bad as it sounds, a github blame view will get you 80% of the info you need because each commit links to the PR it's from (and the PR has a version number). it might also be possible to script this.

if you are willing, please note down the PR number as you do this, that will save someone from having to do the work again.

[xizheyin]

No problem, I've just submitted #141642 to finish this for now, not sure if we want to merge this PR first before we go back to gradually fixing the diagnostic info.