Fix empty-set symbol in comments

compiler/rustc_ast/src/token.rs

@@ -51,7 +51,7 @@ pub enum Delimiter {
     Brace,
     /// `[ ... ]`
     Bracket,
-    /// `Ø ... Ø`
+    /// `∅ ... ∅`
     /// An invisible delimiter, that may, for example, appear around tokens coming from a
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.

compiler/rustc_parse/src/parser/item.rs

@@ -1063,7 +1063,7 @@ impl<'a> Parser<'a> {
     /// Parses a `UseTreeKind::Nested(list)`.
     ///
     /// ```text
-    /// USE_TREE_LIST = Ø | (USE_TREE `,`)* USE_TREE [`,`]
+    /// USE_TREE_LIST = ∅ | (USE_TREE `,`)* USE_TREE [`,`]
     /// ```
     fn parse_use_tree_list(&mut self) -> PResult<'a, ThinVec<(UseTree, ast::NodeId)>> {
         self.parse_delim_comma_seq(Delimiter::Brace, |p| {

library/proc_macro/src/lib.rs

@@ -811,7 +811,7 @@ pub enum Delimiter {
     /// `[ ... ]`
     #[stable(feature = "proc_macro_lib2", since = "1.29.0")]
     Bracket,
-    /// `Ø ... Ø`
+    /// `∅ ... ∅`
     /// An invisible delimiter, that may, for example, appear around tokens coming from a
     /// "macro variable" `$var`. It is important to preserve operator priorities in cases like
     /// `$var * 3` where `$var` is `1 + 2`.