Auto merge of #120924 - xFrednet:rfc-2383-stabilization-party, r=Urga…

…u,blyxyas

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

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

I've just updated the [stabilization report](#54503 (comment)) 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: #54503
Stabilization Report: #54503 (comment)
Documentation Update: rust-lang/reference#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

compiler/rustc_builtin_macros/src/lib.rs

@@ -5,14 +5,14 @@
 #![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)]
 #![feature(box_patterns)]
 #![feature(decl_macro)]
 #![feature(if_let_guard)]
 #![feature(let_chains)]
-#![feature(lint_reasons)]
 #![feature(proc_macro_internals)]
 #![feature(proc_macro_quote)]
 #![feature(rustdoc_internals)]

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)]

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 {

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.

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 = "...""#),

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.

compiler/rustc_lint/src/expect.rs

@@ -3,18 +3,13 @@ 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) {
     *providers = Providers { check_expectations, ..*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();
 

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 {

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! {

src/doc/rustc/src/lints/levels.md

@@ -1,12 +1,13 @@
 # Lint Levels
 
-In `rustc`, lints are divided into five *levels*:
+In `rustc`, lints are divided into six *levels*:
 
 1. allow
-2. warn
-3. force-warn
-4. deny
-5. forbid
+2. expect
+3. warn
+4. force-warn
+5. deny
+6. forbid
 
 Each lint has a default level (explained in the lint listing later in this
 chapter), and the compiler has a default warning level. First, let's explain
@@ -33,6 +34,40 @@ But this code violates the `missing_docs` lint.
 These lints exist mostly to be manually turned on via configuration, as we'll
 talk about later in this section.
 
+## expect
+
+Sometimes, it can be helpful to suppress lints, but at the same time ensure that
+the code in question still emits them. The 'expect' level does exactly this. If
+the lint in question is not emitted, the `unfulfilled_lint_expectation` lint
+triggers on the `expect` attribute, notifying you that the expectation is no
+longer fulfilled.
+
+```rust
+fn main() {
+    #[expect(unused_variables)]
+    let unused = "Everyone ignores me";
+
+    #[expect(unused_variables)] // `unused_variables` lint is not emitted
+    let used = "I'm useful";    // the expectation is therefore unfulfilled
+    println!("The `used` value is equal to: {:?}", used);
+}
+```
+
+This will produce the following warning:
+
+```txt
+warning: this lint expectation is unfulfilled
+ --> src/main.rs:7:14
+  |
+7 |     #[expect(unused_variables)]
+  |              ^^^^^^^^^^^^^^^^
+  |
+  = note: `#[warn(unfulfilled_lint_expectations)]` on by default
+```
+
+This level can only be defined via the `#[expect]` attribute, there is no equivalent
+flag. Lints with the special 'force-warn' level will still be emitted as usual.
+
 ## warn
 
 The 'warn' lint level will produce a warning if you violate the lint. For example,
@@ -240,6 +275,21 @@ And use multiple attributes together:
 pub fn foo() {}
 ```
 
+All lint attributes support an additional `reason` parameter, to give context why
+a certain attribute was added. This reason will be displayed as part of the lint
+message, if the lint is emitted at the defined level.
+
+```rust
+use std::path::PathBuf;
+pub fn get_path() -> PathBuf {
+    #[allow(unused_mut, reason = "this is only modified on some platforms")]
+    let mut file_name = PathBuf::from("git");
+    #[cfg(target_os = "windows")]
+    file_name.set_extension("exe");
+    file_name
+}
+```
+
 ### Capping lints
 
 `rustc` supports a flag, `--cap-lints LEVEL` that sets the "lint cap level."

src/tools/clippy/CHANGELOG.md

@@ -1943,7 +1943,7 @@ Released 2022-05-19
   [#8218](https://github.com/rust-lang/rust-clippy/pull/8218)
 * [`needless_match`]
   [#8471](https://github.com/rust-lang/rust-clippy/pull/8471)
-* [`allow_attributes_without_reason`] (Requires `#![feature(lint_reasons)]`)
+* [`allow_attributes_without_reason`]
   [#8504](https://github.com/rust-lang/rust-clippy/pull/8504)
 * [`print_in_format_impl`]
   [#8253](https://github.com/rust-lang/rust-clippy/pull/8253)

src/tools/clippy/book/src/lint_configuration.md

@@ -669,6 +669,8 @@ The minimum rust version that the project supports. Defaults to the `rust-versio
 
 ---
 **Affected lints:**
+* [`allow_attributes`](https://rust-lang.github.io/rust-clippy/master/index.html#allow_attributes)
+* [`allow_attributes_without_reason`](https://rust-lang.github.io/rust-clippy/master/index.html#allow_attributes_without_reason)
 * [`almost_complete_range`](https://rust-lang.github.io/rust-clippy/master/index.html#almost_complete_range)
 * [`approx_constant`](https://rust-lang.github.io/rust-clippy/master/index.html#approx_constant)
 * [`assigning_clones`](https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones)

src/tools/clippy/clippy_config/src/conf.rs

@@ -265,7 +265,7 @@ define_Conf! {
     ///
     /// Suppress lints whenever the suggested change would cause breakage for other crates.
     (avoid_breaking_exported_api: bool = true),
-    /// Lint: MANUAL_SPLIT_ONCE, MANUAL_STR_REPEAT, CLONED_INSTEAD_OF_COPIED, REDUNDANT_FIELD_NAMES, OPTION_MAP_UNWRAP_OR, REDUNDANT_STATIC_LIFETIMES, FILTER_MAP_NEXT, CHECKED_CONVERSIONS, MANUAL_RANGE_CONTAINS, USE_SELF, MEM_REPLACE_WITH_DEFAULT, MANUAL_NON_EXHAUSTIVE, OPTION_AS_REF_DEREF, MAP_UNWRAP_OR, MATCH_LIKE_MATCHES_MACRO, MANUAL_STRIP, MISSING_CONST_FOR_FN, UNNESTED_OR_PATTERNS, FROM_OVER_INTO, PTR_AS_PTR, IF_THEN_SOME_ELSE_NONE, APPROX_CONSTANT, DEPRECATED_CFG_ATTR, INDEX_REFUTABLE_SLICE, MAP_CLONE, BORROW_AS_PTR, MANUAL_BITS, ERR_EXPECT, CAST_ABS_TO_UNSIGNED, UNINLINED_FORMAT_ARGS, MANUAL_CLAMP, MANUAL_LET_ELSE, UNCHECKED_DURATION_SUBTRACTION, COLLAPSIBLE_STR_REPLACE, SEEK_FROM_CURRENT, SEEK_REWIND, UNNECESSARY_LAZY_EVALUATIONS, TRANSMUTE_PTR_TO_REF, ALMOST_COMPLETE_RANGE, NEEDLESS_BORROW, DERIVABLE_IMPLS, MANUAL_IS_ASCII_CHECK, MANUAL_REM_EUCLID, MANUAL_RETAIN, TYPE_REPETITION_IN_BOUNDS, TUPLE_ARRAY_CONVERSIONS, MANUAL_TRY_FOLD, MANUAL_HASH_ONE, ITER_KV_MAP, MANUAL_C_STR_LITERALS, ASSIGNING_CLONES, LEGACY_NUMERIC_CONSTANTS.
+    /// Lint: MANUAL_SPLIT_ONCE, MANUAL_STR_REPEAT, CLONED_INSTEAD_OF_COPIED, REDUNDANT_FIELD_NAMES, OPTION_MAP_UNWRAP_OR, REDUNDANT_STATIC_LIFETIMES, FILTER_MAP_NEXT, CHECKED_CONVERSIONS, MANUAL_RANGE_CONTAINS, USE_SELF, MEM_REPLACE_WITH_DEFAULT, MANUAL_NON_EXHAUSTIVE, OPTION_AS_REF_DEREF, MAP_UNWRAP_OR, MATCH_LIKE_MATCHES_MACRO, MANUAL_STRIP, MISSING_CONST_FOR_FN, UNNESTED_OR_PATTERNS, FROM_OVER_INTO, PTR_AS_PTR, IF_THEN_SOME_ELSE_NONE, APPROX_CONSTANT, DEPRECATED_CFG_ATTR, INDEX_REFUTABLE_SLICE, MAP_CLONE, BORROW_AS_PTR, MANUAL_BITS, ERR_EXPECT, CAST_ABS_TO_UNSIGNED, UNINLINED_FORMAT_ARGS, MANUAL_CLAMP, MANUAL_LET_ELSE, UNCHECKED_DURATION_SUBTRACTION, COLLAPSIBLE_STR_REPLACE, SEEK_FROM_CURRENT, SEEK_REWIND, UNNECESSARY_LAZY_EVALUATIONS, TRANSMUTE_PTR_TO_REF, ALMOST_COMPLETE_RANGE, NEEDLESS_BORROW, DERIVABLE_IMPLS, MANUAL_IS_ASCII_CHECK, MANUAL_REM_EUCLID, MANUAL_RETAIN, TYPE_REPETITION_IN_BOUNDS, TUPLE_ARRAY_CONVERSIONS, MANUAL_TRY_FOLD, MANUAL_HASH_ONE, ITER_KV_MAP, MANUAL_C_STR_LITERALS, ASSIGNING_CLONES, LEGACY_NUMERIC_CONSTANTS, ALLOW_ATTRIBUTES, ALLOW_ATTRIBUTES_WITHOUT_REASON.
     ///
     /// The minimum rust version that the project supports. Defaults to the `rust-version` field in `Cargo.toml`
     #[default_text = ""]

src/tools/clippy/clippy_config/src/msrvs.rs

@@ -17,6 +17,7 @@ macro_rules! msrv_aliases {
 
 // names may refer to stabilized feature flags or library items
 msrv_aliases! {
+    1,81,0  { LINT_REASONS_STABILIZATION }
     1,77,0 { C_STR_LITERALS }
     1,76,0 { PTR_FROM_REF }
     1,71,0 { TUPLE_ARRAY_CONVERSIONS, BUILD_HASHER_HASH_ONE }

src/tools/clippy/clippy_lints/src/attrs/allow_attributes.rs

@@ -0,0 +1,26 @@
+use clippy_utils::diagnostics::span_lint_and_sugg;
+use clippy_utils::is_from_proc_macro;
+use rustc_ast::{AttrStyle, Attribute};
+use rustc_errors::Applicability;
+use rustc_lint::{LateContext, LintContext};
+use rustc_middle::lint::in_external_macro;
+use super::ALLOW_ATTRIBUTES;
+
+// Separate each crate's features.
+pub fn check<'cx>(cx: &LateContext<'cx>, attr: &'cx Attribute) {
+    if !in_external_macro(cx.sess(), attr.span)
+        && let AttrStyle::Outer = attr.style
+        && let Some(ident) = attr.ident()
+        && !is_from_proc_macro(cx, &attr)
+    {
+        span_lint_and_sugg(
+            cx,
+            ALLOW_ATTRIBUTES,
+            ident.span,
+            "#[allow] attribute found",
+            "replace it with",
+            "expect".into(),
+            Applicability::MachineApplicable,
+        );
+    }
+}

src/tools/clippy/clippy_lints/src/attrs/allow_attributes_without_reason.rs

@@ -8,11 +8,6 @@ use rustc_span::sym;
 use rustc_span::symbol::Symbol;
 
 pub(super) fn check<'cx>(cx: &LateContext<'cx>, name: Symbol, items: &[NestedMetaItem], attr: &'cx Attribute) {
-    // Check for the feature
-    if !cx.tcx.features().lint_reasons {
-        return;
-    }
-
     // Check if the reason is present
     if let Some(item) = items.last().and_then(NestedMetaItem::meta_item)
         && let MetaItemKind::NameValue(_) = &item.kind