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Implementation of the `expect` attribute (RFC 2383) This is an implementation of the `expect` attribute as described in [RFC-2383](https://rust-lang.github.io/rfcs/2383-lint-reasons.html). The attribute allows the suppression of lint message by expecting them. Unfulfilled lint expectations (meaning no expected lint was caught) will emit the `unfulfilled_lint_expectations` lint at the `expect` attribute. ### Example #### input ```rs // required feature flag #![feature(lint_reasons)] #[expect(unused_mut)] // Will warn about an unfulfilled expectation #[expect(unused_variables)] // Will be fulfilled by x fn main() { let x = 0; } ``` #### output ```txt warning: this lint expectation is unfulfilled --> $DIR/trigger_lint.rs:3:1 | LL | #[expect(unused_mut)] // Will warn about an unfulfilled expectation | ^^^^^^^^^^ | = note: `#[warn(unfulfilled_lint_expectations)]` on by default ``` ### Implementation This implementation introduces `Expect` as a new lint level for diagnostics, which have been expected. All lint expectations marked via the `expect` attribute are collected in the [`LintLevelsBuilder`] and assigned an ID that is stored in the new lint level. The `LintLevelsBuilder` stores all found expectations and the data needed to emit the `unfulfilled_lint_expectations` in the [`LintLevelsMap`] which is the result of the [`lint_levels()`] query. The [`rustc_errors::HandlerInner`] is the central error handler in rustc and handles the emission of all diagnostics. Lint message with the level `Expect` are suppressed during this emission, while the expectation ID is stored in a set which marks them as fulfilled. The last step is then so simply check if all expectations collected by the [`LintLevelsBuilder`] in the [`LintLevelsMap`] have been marked as fulfilled in the [`rustc_errors::HandlerInner`]. Otherwise, a new lint message will be emitted. The implementation of the `LintExpectationId` required some special handling to make it stable between sessions. Lints can be emitted during [`EarlyLintPass`]es. At this stage, it's not possible to create a stable identifier. The level instead stores an unstable identifier, which is later converted to a stable `LintExpectationId`. ### Followup TO-DOs All open TO-DOs have been marked with `FIXME` comments in the code. This is the combined list of them: * [ ] The current implementation doesn't cover cases where the `unfulfilled_lint_expectations` lint is actually expected by another `expect` attribute. * This should be easily possible, but I wanted to get some feedback before putting more work into this. * This could also be done in a new PR to not add to much more code to this one * [ ] Update unstable documentation to reflect this change. * [ ] Update unstable expectation ids in [`HandlerInner::stashed_diagnostics`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.HandlerInner.html#structfield.stashed_diagnostics) ### Open questions I also have a few open questions where I would like to get feedback on: 1. The RFC discussion included a suggestion to change the `expect` attribute to something else. (Initiated by `@Ixrec` [here](https://github.com/rust-lang/rfcs/pull/2383#issuecomment-378424091), suggestion from `@scottmcm` to use `#[should_lint(...)]` [here](https://github.com/rust-lang/rfcs/pull/2383#issuecomment-378648877)). No real conclusion was drawn on that point from my understanding. Is this still open for discussion, or was this discarded with the merge of the RFC? 2. How should the expect attribute deal with the new `force-warn` lint level? --- This approach was inspired by a discussion with `@LeSeulArtichaut.` RFC tracking issue: #54503 Mentoring/Implementation issue: #85549 [`LintLevelsBuilder`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/levels/struct.LintLevelsBuilder.html [`LintLevelsMap`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/struct.LintLevelMap.html [`lint_levels()`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/context/struct.TyCtxt.html#method.lint_levels [`rustc_errors::HandlerInner`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.HandlerInner.html [`EarlyLintPass`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_lint/trait.EarlyLintPass.html |
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