Rollup merge of #94670 - xFrednet:rfc-2383-expect-impl-after-party, r=flip1995,wesleywiser

Improve `expect` impl and handle `#[expect(unfulfilled_lint_expectations)]` (RFC 2383)

This PR updates unstable `ExpectationIds` in stashed diagnostics and adds some asserts to ensure that the stored expectations are really empty in the end. Additionally, it handles the `#[expect(unfulfilled_lint_expectations)]` case.

According to the [Errors and lints docs](https://rustc-dev-guide.rust-lang.org/diagnostics.html#diagnostic-levels) the `error` level should only be used _"when the compiler detects a problem that makes it unable to compile the program"_. As this isn't the case with `#[expect(unfulfilled_lint_expectations)]` I decided to only create a warning. To avoid adding a new lint only for this case, I simply emit a `unfulfilled_lint_expectations` diagnostic with an additional note.

---

r? `@wesleywiser` I'm requesting a review from you since you reviewed the previous PR https://github.com/rust-lang/rust/pull/87835. You are welcome to reassign it if you're busy :upside_down_face:

rfc: [RFC-2383](https://rust-lang.github.io/rfcs/2383-lint-reasons.html)

tracking issue: https://github.com/rust-lang/rust/issues/85549

cc: `@flip1995` In case you're also interested in this :)

2 files changed, 33 insertions, 12 deletions

diff --git a/compiler/rustc_lint/src/expect.rs b/compiler/rustc_lint/src/expect.rs
index e6c9d0b0ab0..74fef0be9e9 100644
--- a/compiler/rustc_lint/src/expect.rs
+++ b/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();
         },
     );
diff --git a/compiler/rustc_lint/src/levels.rs b/compiler/rustc_lint/src/levels.rs
index 7b018e7f75f..47899f8625d 100644
--- a/compiler/rustc_lint/src/levels.rs
+++ b/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::{add_feature_diagnostics, feature_err};
@@ -218,6 +218,14 @@ 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 {
             self.current_specs_mut().insert(id, (old_level, old_src));
         } else {
@@ -350,6 +358,22 @@ 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 {
+                            // The `unfulfilled_lint_expectations` lint is not part of any lint groups. Therefore. we
+                            // only need to check the slice if it contains a single lint.
+                            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,
@@ -360,10 +384,6 @@ impl<'s> LintLevelsBuilder<'s> {
                                 self.insert_spec(id, (level, src));
                             }
                         }
-                        if let Level::Expect(expect_id) = level {
-                            self.lint_expectations
-                                .push((expect_id, LintExpectation::new(reason, sp)));
-                        }
                     }
 
                     CheckLintNameResult::Tool(result) => {
@@ -381,7 +401,7 @@ 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)) => {
@@ -425,7 +445,7 @@ 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, _)) => {
@@ -531,7 +551,7 @@ 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);