Handle #[expect(unfulfilled_lint_expectations)] with a lint message

compiler/rustc_errors/src/lib.rs

@@ -970,6 +970,7 @@ impl Handler {
 
     /// This methods steals all [`LintExpectationId`]s that are stored inside
     /// [`HandlerInner`] and indicate that the linked expectation has been fulfilled.
+    #[must_use]
     pub fn steal_fulfilled_expectation_ids(&self) -> FxHashSet<LintExpectationId> {
         assert!(
             self.inner.borrow().unstable_expect_diagnostics.is_empty(),

compiler/rustc_lint/src/expect.rs

@@ -30,10 +30,6 @@ fn emit_unfulfilled_expectation_lint(
     hir_id: HirId,
     expectation: &LintExpectation,
 ) {
-    // FIXME: The current implementation doesn't cover cases where the
-    // `unfulfilled_lint_expectations` is actually expected by another lint
-    // expectation. This can be added here by checking the lint level and
-    // retrieving the `LintExpectationId` if it was expected.
     tcx.struct_span_lint_hir(
         builtin::UNFULFILLED_LINT_EXPECTATIONS,
         hir_id,
@@ -43,6 +39,11 @@ fn emit_unfulfilled_expectation_lint(
             if let Some(rationale) = expectation.reason {
                 diag.note(&rationale.as_str());
             }
+
+            if expectation.is_unfulfilled_lint_expectations {
+                diag.note("the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message");
+            }
+
             diag.emit();
         },
     );

compiler/rustc_lint/src/levels.rs

@@ -14,7 +14,7 @@ use rustc_middle::lint::{
 use rustc_middle::ty::query::Providers;
 use rustc_middle::ty::{RegisteredTools, TyCtxt};
 use rustc_session::lint::{
-    builtin::{self, FORBIDDEN_LINT_GROUPS},
+    builtin::{self, FORBIDDEN_LINT_GROUPS, UNFULFILLED_LINT_EXPECTATIONS},
     Level, Lint, LintExpectationId, LintId,
 };
 use rustc_session::parse::feature_err;
@@ -212,6 +212,14 @@ impl<'s> LintLevelsBuilder<'s> {
                 }
             }
         }
+
+        // The lint `unfulfilled_lint_expectations` can't be expected, as it would suppress itself.
+        // Handling expectations of this lint would add additional complexity with little to no
+        // benefit. The expect level for this lint will therefore be ignored.
+        if let Level::Expect(_) = level && id == LintId::of(UNFULFILLED_LINT_EXPECTATIONS) {
+            return;
+        }
+
         if let Level::ForceWarn = old_level {
             specs.insert(id, (old_level, old_src));
         } else {
@@ -344,6 +352,20 @@ impl<'s> LintLevelsBuilder<'s> {
                     self.store.check_lint_name(&name, tool_name, self.registered_tools);
                 match &lint_result {
                     CheckLintNameResult::Ok(ids) => {
+                        // This checks for instances where the user writes `#[expect(unfulfilled_lint_expectations)]`
+                        // in that case we want to avoid overriding the lint level but instead add an expectation that
+                        // can't be fulfilled. The lint message will include an explanation, that the
+                        // `unfulfilled_lint_expectations` lint can't be expected.
+                        if let Level::Expect(expect_id) = level {
+                            let is_unfulfilled_lint_expectations = match ids {
+                                [lint] => *lint == LintId::of(UNFULFILLED_LINT_EXPECTATIONS),
+                                _ => false,
+                            };
+                            self.lint_expectations.push((
+                                expect_id,
+                                LintExpectation::new(reason, sp, is_unfulfilled_lint_expectations),
+                            ));
+                        }
                         let src = LintLevelSource::Node(
                             meta_item.path.segments.last().expect("empty lint name").ident.name,
                             sp,
@@ -353,10 +375,6 @@ impl<'s> LintLevelsBuilder<'s> {
                             self.check_gated_lint(id, attr.span);
                             self.insert_spec(&mut specs, id, (level, src));
                         }
-                        if let Level::Expect(expect_id) = level {
-                            self.lint_expectations
-                                .push((expect_id, LintExpectation::new(reason, sp)));
-                        }
                     }
 
                     CheckLintNameResult::Tool(result) => {
@@ -374,7 +392,7 @@ impl<'s> LintLevelsBuilder<'s> {
                                 }
                                 if let Level::Expect(expect_id) = level {
                                     self.lint_expectations
-                                        .push((expect_id, LintExpectation::new(reason, sp)));
+                                        .push((expect_id, LintExpectation::new(reason, sp, false)));
                                 }
                             }
                             Err((Some(ids), ref new_lint_name)) => {
@@ -414,7 +432,7 @@ impl<'s> LintLevelsBuilder<'s> {
                                 }
                                 if let Level::Expect(expect_id) = level {
                                     self.lint_expectations
-                                        .push((expect_id, LintExpectation::new(reason, sp)));
+                                        .push((expect_id, LintExpectation::new(reason, sp, false)));
                                 }
                             }
                             Err((None, _)) => {
@@ -511,7 +529,7 @@ impl<'s> LintLevelsBuilder<'s> {
                         }
                         if let Level::Expect(expect_id) = level {
                             self.lint_expectations
-                                .push((expect_id, LintExpectation::new(reason, sp)));
+                                .push((expect_id, LintExpectation::new(reason, sp, false)));
                         }
                     } else {
                         panic!("renamed lint does not exist: {}", new_name);

compiler/rustc_middle/src/lint.rs

@@ -204,11 +204,19 @@ pub struct LintExpectation {
     pub reason: Option<Symbol>,
     /// The [`Span`] of the attribute that this expectation originated from.
     pub emission_span: Span,
+    /// Lint messages for the `unfulfilled_lint_expectations` lint will be
+    /// adjusted to include an additional note. Therefore, we have to track if
+    /// the expectation is for the lint.
+    pub is_unfulfilled_lint_expectations: bool,
 }
 
 impl LintExpectation {
-    pub fn new(reason: Option<Symbol>, attr_span: Span) -> Self {
-        Self { reason, emission_span: attr_span }
+    pub fn new(
+        reason: Option<Symbol>,
+        emission_span: Span,
+        is_unfulfilled_lint_expectations: bool,
+    ) -> Self {
+        Self { reason, emission_span, is_unfulfilled_lint_expectations }
     }
 }
 

src/test/ui/lint/rfc-2383-lint-reason/expect_unfulfilled_expectation.rs

@@ -0,0 +1,39 @@
+// check-pass
+// ignore-tidy-linelength
+
+#![feature(lint_reasons)]
+#![warn(unused_mut)]
+
+#![expect(unfulfilled_lint_expectations, reason = "idk why you would expect this")]
+//~^ WARNING this lint expectation is unfulfilled
+//~| NOTE `#[warn(unfulfilled_lint_expectations)]` on by default
+//~| NOTE idk why you would expect this
+//~| NOTE the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
+
+#[expect(unfulfilled_lint_expectations, reason = "a local: idk why you would expect this")]
+//~^ WARNING this lint expectation is unfulfilled
+//~| NOTE a local: idk why you would expect this
+//~| NOTE the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
+pub fn normal_test_fn() {
+    #[expect(unused_mut, reason = "this expectation will create a diagnostic with the default lint level")]
+    //~^ WARNING this lint expectation is unfulfilled
+    //~| NOTE this expectation will create a diagnostic with the default lint level
+    let mut v = vec![1, 1, 2, 3, 5];
+    v.sort();
+
+    // Check that lint lists including `unfulfilled_lint_expectations` are also handled correctly
+    #[expect(unused, unfulfilled_lint_expectations, reason = "the expectation for `unused` should be fulfilled")]
+    //~^ WARNING this lint expectation is unfulfilled
+    //~| NOTE the expectation for `unused` should be fulfilled
+    //~| NOTE the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
+    let value = "I'm unused";
+}
+
+#[expect(warnings, reason = "this suppresses all warnings and also suppresses itself. No warning will be issued")]
+pub fn expect_warnings() {
+    // This lint trigger will be suppressed
+    #[warn(unused_mut)]
+    let mut v = vec![1, 1, 2, 3, 5];
+}
+
+fn main() {}

src/test/ui/lint/rfc-2383-lint-reason/expect_unfulfilled_expectation.stderr

@@ -0,0 +1,38 @@
+warning: this lint expectation is unfulfilled
+  --> $DIR/expect_unfulfilled_expectation.rs:7:11
+   |
+LL | #![expect(unfulfilled_lint_expectations, reason = "idk why you would expect this")]
+   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(unfulfilled_lint_expectations)]` on by default
+   = note: idk why you would expect this
+   = note: the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
+
+warning: this lint expectation is unfulfilled
+  --> $DIR/expect_unfulfilled_expectation.rs:13:10
+   |
+LL | #[expect(unfulfilled_lint_expectations, reason = "a local: idk why you would expect this")]
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: a local: idk why you would expect this
+   = note: the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
+
+warning: this lint expectation is unfulfilled
+  --> $DIR/expect_unfulfilled_expectation.rs:18:14
+   |
+LL |     #[expect(unused_mut, reason = "this expectation will create a diagnostic with the default lint level")]
+   |              ^^^^^^^^^^
+   |
+   = note: this expectation will create a diagnostic with the default lint level
+
+warning: this lint expectation is unfulfilled
+  --> $DIR/expect_unfulfilled_expectation.rs:25:22
+   |
+LL |     #[expect(unused, unfulfilled_lint_expectations, reason = "the expectation for `unused` should be fulfilled")]
+   |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: the expectation for `unused` should be fulfilled
+   = note: the `unfulfilled_lint_expectations` lint can't be expected and will always produce this message
+
+warning: 4 warnings emitted
+