Rewrite on_unimplemented format string parser.

compiler/rustc_span/src/hygiene.rs

@@ -1270,6 +1270,25 @@ impl DesugaringKind {
             DesugaringKind::PatTyRange => "pattern type",
         }
     }
+
+    /// For use with `rustc_unimplemented` to support conditions
+    /// like `from_desugaring = "QuestionMark"`
+    pub fn matches(&self, value: &str) -> bool {
+        match self {
+            DesugaringKind::CondTemporary => value == "CondTemporary",
+            DesugaringKind::Async => value == "Async",
+            DesugaringKind::Await => value == "Await",
+            DesugaringKind::QuestionMark => value == "QuestionMark",
+            DesugaringKind::TryBlock => value == "TryBlock",
+            DesugaringKind::YeetExpr => value == "YeetExpr",
+            DesugaringKind::OpaqueTy => value == "OpaqueTy",
+            DesugaringKind::ForLoop => value == "ForLoop",
+            DesugaringKind::WhileLoop => value == "WhileLoop",
+            DesugaringKind::BoundModifier => value == "BoundModifier",
+            DesugaringKind::Contract => value == "Contract",
+            DesugaringKind::PatTyRange => value == "PatTyRange",
+        }
+    }
 }
 
 #[derive(Default)]

compiler/rustc_span/src/symbol.rs

@@ -372,6 +372,7 @@ symbols! {
         SyncUnsafeCell,
         T,
         Target,
+        This,
         ToOwned,
         ToString,
         TokenStream,

compiler/rustc_trait_selection/src/error_reporting/traits/mod.rs

@@ -2,6 +2,8 @@ pub mod ambiguity;
 pub mod call_kind;
 mod fulfillment_errors;
 pub mod on_unimplemented;
+pub mod on_unimplemented_condition;
+pub mod on_unimplemented_format;
 mod overflow;
 pub mod suggestions;