Add OnceHelp lint level (same as OnceNote, except for help)

compiler/rustc_errors/src/annotate_snippet_emitter_writer.rs

@@ -91,7 +91,7 @@ fn annotation_type_for_level(level: Level) -> AnnotationType {
         }
         Level::Warning(_) => AnnotationType::Warning,
         Level::Note | Level::OnceNote => AnnotationType::Note,
-        Level::Help => AnnotationType::Help,
+        Level::Help | Level::OnceHelp => AnnotationType::Help,
         // FIXME(#59346): Not sure how to map this level
         Level::FailureNote => AnnotationType::Error,
         Level::Allow => panic!("Should not call with Allow"),

compiler/rustc_errors/src/diagnostic.rs

@@ -270,6 +270,7 @@ impl Diagnostic {
             | Level::Note
             | Level::OnceNote
             | Level::Help
+            | Level::OnceHelp
             | Level::Allow
             | Level::Expect(_) => false,
         }
@@ -532,6 +533,13 @@ impl Diagnostic {
         self
     }
 
+    /// Prints the span with a help above it.
+    /// This is like [`Diagnostic::help()`], but it gets its own span.
+    pub fn help_once(&mut self, msg: impl Into<SubdiagnosticMessage>) -> &mut Self {
+        self.sub(Level::OnceHelp, msg, MultiSpan::new(), None);
+        self
+    }
+
     /// Add a help message attached to this diagnostic with a customizable highlighted message.
     pub fn highlighted_help(&mut self, msg: Vec<(String, Style)>) -> &mut Self {
         self.sub_with_highlights(Level::Help, msg, MultiSpan::new(), None);

compiler/rustc_errors/src/lib.rs

@@ -1390,7 +1390,7 @@ impl HandlerInner {
                 debug!(?self.emitted_diagnostics);
                 let already_emitted_sub = |sub: &mut SubDiagnostic| {
                     debug!(?sub);
-                    if sub.level != Level::OnceNote {
+                    if sub.level != Level::OnceNote && sub.level != Level::OnceHelp {
                         return false;
                     }
                     let mut hasher = StableHasher::new();
@@ -1792,6 +1792,8 @@ pub enum Level {
     /// A note that is only emitted once.
     OnceNote,
     Help,
+    /// A help that is only emitted once.
+    OnceHelp,
     FailureNote,
     Allow,
     Expect(LintExpectationId),
@@ -1816,7 +1818,7 @@ impl Level {
             Note | OnceNote => {
                 spec.set_fg(Some(Color::Green)).set_intense(true);
             }
-            Help => {
+            Help | OnceHelp => {
                 spec.set_fg(Some(Color::Cyan)).set_intense(true);
             }
             FailureNote => {}
@@ -1831,7 +1833,7 @@ impl Level {
             Fatal | Error { .. } => "error",
             Warning(_) => "warning",
             Note | OnceNote => "note",
-            Help => "help",
+            Help | OnceHelp => "help",
             FailureNote => "failure-note",
             Allow => panic!("Shouldn't call on allowed error"),
             Expect(_) => panic!("Shouldn't call on expected error"),