[red-knot] Add support for the LSP diagnostic tag (#17657)

Co-authored-by: Micha Reiser <micha@reiser.io>

crates/ruff_db/src/diagnostic/mod.rs

@@ -227,6 +227,11 @@ impl Diagnostic {
     pub fn primary_span(&self) -> Option<Span> {
         self.primary_annotation().map(|ann| ann.span.clone())
     }
+
+    /// Returns the tags from the primary annotation of this diagnostic if it exists.
+    pub fn primary_tags(&self) -> Option<&[DiagnosticTag]> {
+        self.primary_annotation().map(|ann| ann.tags.as_slice())
+    }
 }
 
 #[derive(Debug, Clone, Eq, PartialEq)]
@@ -338,6 +343,8 @@ pub struct Annotation {
     /// Whether this annotation is "primary" or not. When it isn't primary, an
     /// annotation is said to be "secondary."
     is_primary: bool,
+    /// The diagnostic tags associated with this annotation.
+    tags: Vec<DiagnosticTag>,
 }
 
 impl Annotation {
@@ -355,6 +362,7 @@ impl Annotation {
             span,
             message: None,
             is_primary: true,
+            tags: Vec::new(),
         }
     }
 
@@ -370,6 +378,7 @@ impl Annotation {
             span,
             message: None,
             is_primary: false,
+            tags: Vec::new(),
         }
     }
 
@@ -412,6 +421,36 @@ impl Annotation {
     pub fn get_span(&self) -> &Span {
         &self.span
     }
+
+    /// Returns the tags associated with this annotation.
+    pub fn get_tags(&self) -> &[DiagnosticTag] {
+        &self.tags
+    }
+
+    /// Attaches this tag to this annotation.
+    ///
+    /// It will not replace any existing tags.
+    pub fn tag(mut self, tag: DiagnosticTag) -> Annotation {
+        self.tags.push(tag);
+        self
+    }
+
+    /// Attaches an additional tag to this annotation.
+    pub fn push_tag(&mut self, tag: DiagnosticTag) {
+        self.tags.push(tag);
+    }
+}
+
+/// Tags that can be associated with an annotation.
+///
+/// These tags are used to provide additional information about the annotation.
+/// and are passed through to the language server protocol.
+#[derive(Debug, Clone, Eq, PartialEq)]
+pub enum DiagnosticTag {
+    /// Unused or unnecessary code. Used for unused parameters, unreachable code, etc.
+    Unnecessary,
+    /// Deprecated or obsolete code.
+    Deprecated,
 }
 
 /// A string identifier for a lint rule.

crates/ty_python_semantic/src/types/context.rs

@@ -1,6 +1,7 @@
 use std::fmt;
 
 use drop_bomb::DebugDropBomb;
+use ruff_db::diagnostic::DiagnosticTag;
 use ruff_db::{
     diagnostic::{Annotation, Diagnostic, DiagnosticId, IntoDiagnosticMessage, Severity, Span},
     files::File,
@@ -259,6 +260,21 @@ impl LintDiagnosticGuard<'_, '_> {
         let ann = self.primary_annotation_mut().unwrap();
         ann.set_message(message);
     }
+
+    /// Adds a tag on the primary annotation for this diagnostic.
+    ///
+    /// This tag is associated with the primary annotation created
+    /// for every `Diagnostic` that uses the `LintDiagnosticGuard` API.
+    /// Specifically, the annotation is derived from the `TextRange` given to
+    /// the `InferContext::report_lint` API.
+    ///
+    /// Callers can add additional primary or secondary annotations via the
+    /// `DerefMut` trait implementation to a `Diagnostic`.
+    #[expect(dead_code)]
+    pub(super) fn add_primary_tag(&mut self, tag: DiagnosticTag) {
+        let ann = self.primary_annotation_mut().unwrap();
+        ann.push_tag(tag);
+    }
 }
 
 impl std::ops::Deref for LintDiagnosticGuard<'_, '_> {

crates/ty_server/src/server/api/requests/diagnostic.rs

@@ -2,9 +2,9 @@ use std::borrow::Cow;
 
 use lsp_types::request::DocumentDiagnosticRequest;
 use lsp_types::{
-    Diagnostic, DiagnosticSeverity, DocumentDiagnosticParams, DocumentDiagnosticReport,
-    DocumentDiagnosticReportResult, FullDocumentDiagnosticReport, NumberOrString, Range,
-    RelatedFullDocumentDiagnosticReport, Url,
+    Diagnostic, DiagnosticSeverity, DiagnosticTag, DocumentDiagnosticParams,
+    DocumentDiagnosticReport, DocumentDiagnosticReportResult, FullDocumentDiagnosticReport,
+    NumberOrString, Range, RelatedFullDocumentDiagnosticReport, Url,
 };
 
 use crate::document::ToRangeExt;
@@ -92,10 +92,22 @@ fn to_lsp_diagnostic(
         Severity::Error | Severity::Fatal => DiagnosticSeverity::ERROR,
     };
 
+    let tags = diagnostic
+        .primary_tags()
+        .map(|tags| {
+            tags.iter()
+                .map(|tag| match tag {
+                    ruff_db::diagnostic::DiagnosticTag::Unnecessary => DiagnosticTag::UNNECESSARY,
+                    ruff_db::diagnostic::DiagnosticTag::Deprecated => DiagnosticTag::DEPRECATED,
+                })
+                .collect::<Vec<DiagnosticTag>>()
+        })
+        .filter(|mapped_tags| !mapped_tags.is_empty());
+
     Diagnostic {
         range,
         severity: Some(severity),
-        tags: None,
+        tags,
         code: Some(NumberOrString::String(diagnostic.id().to_string())),
         code_description: None,
         source: Some("ty".into()),