Auto merge of #120924 - xFrednet:rfc-2383-stabilization-party, r=Urgau,blyxyas

Let's `#[expect]` some lints: Stabilize `lint_reasons` (RFC 2383)

Let's give this another try! The [previous stabilization attempt](https://github.com/rust-lang/rust/pull/99063) was stalled by some unresolved questions. These have been discussed in a [lang team](https://github.com/rust-lang/lang-team/issues/191) meeting. The last open question, regarding the semantics of the `#[expect]` attribute was decided on in https://github.com/rust-lang/rust/issues/115980

I've just updated the [stabilization report](https://github.com/rust-lang/rust/issues/54503#issuecomment-1179563964) with the discussed questions and decisions. Luckily, the decision is inline with the current implementation.

This hopefully covers everything. Let's hope that the CI will be green like the spring.

fixes #115980
fixes #54503

---

r? `@wesleywiser`

Tacking Issue: https://github.com/rust-lang/rust/issues/54503
Stabilization Report: https://github.com/rust-lang/rust/issues/54503#issuecomment-1179563964
Documentation Update: https://github.com/rust-lang/reference/pull/1237

<!--
For Clippy:

changelog: [`allow_attributes`]: Is now available on stable, since the `lint_reasons` feature was stabilized
changelog: [`allow_attributes_without_reason`]: Is now available on stable, since the `lint_reasons` feature was stabilized
-->

---

Roses are red,
Violets are blue,
Let's expect lints,
With reason clues

9 files changed, 21 insertions, 41 deletions

diff --git a/compiler/rustc_builtin_macros/src/lib.rs b/compiler/rustc_builtin_macros/src/lib.rs
index 8ac59605bc1..f8d93666145 100644
--- a/compiler/rustc_builtin_macros/src/lib.rs
+++ b/compiler/rustc_builtin_macros/src/lib.rs
@@ -5,6 +5,7 @@
 #![allow(internal_features)]
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::untranslatable_diagnostic)]
+#![cfg_attr(bootstrap, feature(lint_reasons))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
@@ -12,7 +13,6 @@
 #![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(lint_reasons)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
 #![feature(rustdoc_internals)]
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index cddc67d1578..356ddf014be 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -10,6 +10,7 @@
 #![allow(internal_features)]
 #![allow(rustc::default_hash_types)]
 #![allow(rustc::potential_query_instability)]
+#![cfg_attr(bootstrap, feature(lint_reasons))]
 #![cfg_attr(not(parallel_compiler), feature(cell_leak))]
 #![deny(unsafe_op_in_unsafe_fn)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
@@ -24,7 +25,6 @@
 #![feature(extend_one)]
 #![feature(hash_raw_entry)]
 #![feature(hasher_prefixfree_extras)]
-#![feature(lint_reasons)]
 #![feature(macro_metavar_expr)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]
diff --git a/compiler/rustc_errors/src/json.rs b/compiler/rustc_errors/src/json.rs
index af82d8092c2..764134d5335 100644
--- a/compiler/rustc_errors/src/json.rs
+++ b/compiler/rustc_errors/src/json.rs
@@ -135,7 +135,12 @@ impl Emitter for JsonEmitter {
         let data: Vec<FutureBreakageItem<'_>> = diags
             .into_iter()
             .map(|mut diag| {
-                if diag.level == crate::Level::Allow {
+                // Allowed or expected lints don't normally (by definition) emit a lint
+                // but future incompat lints are special and are emitted anyway.
+                //
+                // So to avoid ICEs and confused users we "upgrade" the lint level for
+                // those `FutureBreakageItem` to warn.
+                if matches!(diag.level, crate::Level::Allow | crate::Level::Expect(..)) {
                     diag.level = crate::Level::Warning;
                 }
                 FutureBreakageItem {
diff --git a/compiler/rustc_feature/src/accepted.rs b/compiler/rustc_feature/src/accepted.rs
index 9beaa6b8d95..f082cc2b569 100644
--- a/compiler/rustc_feature/src/accepted.rs
+++ b/compiler/rustc_feature/src/accepted.rs
@@ -232,6 +232,8 @@ declare_features! (
     (accepted, label_break_value, "1.65.0", Some(48594)),
     /// Allows `let...else` statements.
     (accepted, let_else, "1.65.0", Some(87335)),
+    /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
+    (accepted, lint_reasons, "CURRENT_RUSTC_VERSION", Some(54503)),
     /// Allows `break {expr}` with a value inside `loop`s.
     (accepted, loop_break_value, "1.19.0", Some(37339)),
     /// Allows use of `?` as the Kleene "at most one" operator in macros.
diff --git a/compiler/rustc_feature/src/builtin_attrs.rs b/compiler/rustc_feature/src/builtin_attrs.rs
index c53bf965139..32a047a9363 100644
--- a/compiler/rustc_feature/src/builtin_attrs.rs
+++ b/compiler/rustc_feature/src/builtin_attrs.rs
@@ -369,9 +369,9 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         allow, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#),
         DuplicatesOk, EncodeCrossCrate::No,
     ),
-    gated!(
-        expect, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#), DuplicatesOk,
-        EncodeCrossCrate::No, lint_reasons, experimental!(expect)
+    ungated!(
+        expect, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#),
+        DuplicatesOk, EncodeCrossCrate::No,
     ),
     ungated!(
         forbid, Normal, template!(List: r#"lint1, lint2, ..., /*opt*/ reason = "...""#),
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 2dfaac8f6e7..f4e20328814 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -512,8 +512,6 @@ declare_features! (
     /// Allows using `#[link(kind = "link-arg", name = "...")]`
     /// to pass custom arguments to the linker.
     (unstable, link_arg_attribute, "1.76.0", Some(99427)),
-    /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.
-    (unstable, lint_reasons, "1.31.0", Some(54503)),
     /// Give access to additional metadata about declarative macro meta-variables.
     (unstable, macro_metavar_expr, "1.61.0", Some(83527)),
     /// Provides a way to concatenate identifiers using metavariable expressions.
diff --git a/compiler/rustc_lint/src/expect.rs b/compiler/rustc_lint/src/expect.rs
index 40db765da53..04c2ebf189f 100644
--- a/compiler/rustc_lint/src/expect.rs
+++ b/compiler/rustc_lint/src/expect.rs
@@ -3,7 +3,6 @@ use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::lint::builtin::UNFULFILLED_LINT_EXPECTATIONS;
 use rustc_session::lint::LintExpectationId;
-use rustc_span::symbol::sym;
 use rustc_span::Symbol;
 
 pub(crate) fn provide(providers: &mut Providers) {
@@ -11,10 +10,6 @@ pub(crate) fn provide(providers: &mut Providers) {
 }
 
 fn check_expectations(tcx: TyCtxt<'_>, tool_filter: Option<Symbol>) {
-    if !tcx.features().active(sym::lint_reasons) {
-        return;
-    }
-
     let lint_expectations = tcx.lint_expectations(());
     let fulfilled_expectations = tcx.dcx().steal_fulfilled_expectation_ids();
 
diff --git a/compiler/rustc_lint/src/levels.rs b/compiler/rustc_lint/src/levels.rs
index 1317af50a4a..0df34c32e38 100644
--- a/compiler/rustc_lint/src/levels.rs
+++ b/compiler/rustc_lint/src/levels.rs
@@ -37,7 +37,6 @@ use rustc_session::lint::{
     },
     Level, Lint, LintExpectationId, LintId,
 };
-use rustc_session::parse::feature_err;
 use rustc_session::Session;
 use rustc_span::symbol::{sym, Symbol};
 use rustc_span::{Span, DUMMY_SP};
@@ -788,15 +787,6 @@ impl<'s, P: LintLevelsProvider> LintLevelsBuilder<'s, P> {
                     ast::MetaItemKind::NameValue(ref name_value) => {
                         if item.path == sym::reason {
                             if let ast::LitKind::Str(rationale, _) = name_value.kind {
-                                if !self.features.lint_reasons {
-                                    feature_err(
-                                        &self.sess,
-                                        sym::lint_reasons,
-                                        item.span,
-                                        "lint reasons are experimental",
-                                    )
-                                    .emit();
-                                }
                                 reason = Some(rationale);
                             } else {
                                 sess.dcx().emit_err(MalformedAttribute {
diff --git a/compiler/rustc_lint_defs/src/builtin.rs b/compiler/rustc_lint_defs/src/builtin.rs
index a023d6161df..472e93d202d 100644
--- a/compiler/rustc_lint_defs/src/builtin.rs
+++ b/compiler/rustc_lint_defs/src/builtin.rs
@@ -608,13 +608,13 @@ declare_lint! {
 }
 
 declare_lint! {
-    /// The `unfulfilled_lint_expectations` lint detects lint trigger expectations
-    /// that have not been fulfilled.
+    /// The `unfulfilled_lint_expectations` lint detects when a lint expectation is
+    /// unfulfilled.
     ///
     /// ### Example
     ///
     /// ```rust
-    /// #![feature(lint_reasons)]
+    /// #![cfg_attr(bootstrap, feature(lint_reasons))]
     ///
     /// #[expect(unused_variables)]
     /// let x = 10;
@@ -625,24 +625,14 @@ declare_lint! {
     ///
     /// ### Explanation
     ///
-    /// It was expected that the marked code would emit a lint. This expectation
-    /// has not been fulfilled.
+    /// The `#[expect]` attribute can be used to create a lint expectation. The
+    /// expectation is fulfilled, if a `#[warn]` attribute at the same location
+    /// would result in a lint emission. If the expectation is unfulfilled,
+    /// because no lint was emitted, this lint will be emitted on the attribute.
     ///
-    /// The `expect` attribute can be removed if this is intended behavior otherwise
-    /// it should be investigated why the expected lint is no longer issued.
-    ///
-    /// In rare cases, the expectation might be emitted at a different location than
-    /// shown in the shown code snippet. In most cases, the `#[expect]` attribute
-    /// works when added to the outer scope. A few lints can only be expected
-    /// on a crate level.
-    ///
-    /// Part of RFC 2383. The progress is being tracked in [#54503]
-    ///
-    /// [#54503]: https://github.com/rust-lang/rust/issues/54503
     pub UNFULFILLED_LINT_EXPECTATIONS,
     Warn,
-    "unfulfilled lint expectation",
-    @feature_gate = rustc_span::sym::lint_reasons;
+    "unfulfilled lint expectation"
 }
 
 declare_lint! {