Don't eagerly convert principal to string

compiler/rustc_lint/src/deref_into_dyn_supertrait.rs

@@ -78,7 +78,7 @@ impl<'tcx> LateLintPass<'tcx> for DerefIntoDynSupertrait {
             });
             cx.emit_spanned_lint(DEREF_INTO_DYN_SUPERTRAIT, cx.tcx.def_span(item.owner_id.def_id), SupertraitAsDerefTarget {
                 t,
-                target_principal: target_principal.to_string(),
+                target_principal,
                 label,
             });
         }

compiler/rustc_lint/src/lints.rs

@@ -8,7 +8,7 @@ use rustc_errors::{
 };
 use rustc_hir::def_id::DefId;
 use rustc_macros::{LintDiagnostic, Subdiagnostic};
-use rustc_middle::ty::{Predicate, Ty, TyCtxt};
+use rustc_middle::ty::{PolyExistentialTraitRef, Predicate, Ty, TyCtxt};
 use rustc_session::parse::ParseSess;
 use rustc_span::{edition::Edition, sym, symbol::Ident, Span, Symbol};
 
@@ -556,8 +556,7 @@ pub struct BuiltinUnexpectedCliConfigValue {
 #[diag(lint_supertrait_as_deref_target)]
 pub struct SupertraitAsDerefTarget<'a> {
     pub t: Ty<'a>,
-    pub target_principal: String,
+    pub target_principal: PolyExistentialTraitRef<'a>,
-    // pub target_principal: Binder<'a, ExistentialTraitRef<'b>>,
     #[subdiagnostic]
     pub label: Option<SupertraitAsDerefTargetLabel>,
 }

compiler/rustc_middle/src/ty/sty.rs

@@ -933,6 +933,12 @@ impl<'tcx> PolyExistentialTraitRef<'tcx> {
     }
 }
 
+impl rustc_errors::IntoDiagnosticArg for PolyExistentialTraitRef<'_> {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        self.to_string().into_diagnostic_arg()
+    }
+}
+
 #[derive(Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash, Debug, TyEncodable, TyDecodable)]
 #[derive(HashStable)]
 pub enum BoundVariableKind {

tests/ui/lint/issue-108155.rs

@@ -0,0 +1,15 @@
+// check-pass
+// check that `deref_into_dyn_supertrait` doesn't cause ICE by eagerly converting
+// a cancelled lint
+
+#![allow(deref_into_dyn_supertrait)]
+
+trait Trait {}
+impl std::ops::Deref for dyn Trait + Send + Sync {
+    type Target = dyn Trait;
+    fn deref(&self) -> &Self::Target {
+        self
+    }
+}
+
+fn main() {}