improve debug message by eagerly translating

compiler/rustc_const_eval/src/errors.rs

@@ -1,3 +1,5 @@
+use std::fmt;
+
 use rustc_errors::{
     DiagnosticArgValue, DiagnosticBuilder, DiagnosticMessage, EmissionGuarantee, Handler,
     IntoDiagnostic,
@@ -8,7 +10,7 @@ use rustc_middle::mir::interpret::{
     CheckInAllocMsg, ExpectedKind, InterpError, InvalidMetaKind, InvalidProgramInfo, PointerKind,
     ResourceExhaustionInfo, UndefinedBehaviorInfo, UnsupportedOpInfo, ValidationErrorInfo,
 };
-use rustc_middle::ty::Ty;
+use rustc_middle::ty::{self, Ty};
 use rustc_span::Span;
 use rustc_target::abi::call::AdjustForForeignAbiError;
 use rustc_target::abi::{Size, WrappingRange};
@@ -425,6 +427,24 @@ pub struct UndefinedBehavior {
     pub raw_bytes: RawBytesNote,
 }
 
+pub struct DebugExt<T>(T);
+
+impl<T: ReportErrorExt> fmt::Debug for DebugExt<T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let s = ty::tls::with(|tcx| {
+            let mut builder = tcx.sess.struct_allow("");
+            let handler = &tcx.sess.parse_sess.span_diagnostic;
+            let message = self.0.diagnostic_message();
+            self.0.add_args(handler, &mut builder);
+            let s = handler.eagerly_translate_to_string(message, builder.args());
+            builder.cancel();
+            s
+        });
+
+        f.write_str(&s)
+    }
+}
+
 pub trait ReportErrorExt {
     /// Returns the diagnostic message for this error.
     fn diagnostic_message(&self) -> DiagnosticMessage;
@@ -433,6 +453,13 @@ pub trait ReportErrorExt {
         handler: &Handler,
         builder: &mut DiagnosticBuilder<'_, G>,
     );
+
+    fn debug(self) -> DebugExt<Self>
+    where
+        Self: Sized,
+    {
+        DebugExt(self)
+    }
 }
 
 fn bad_pointer_message(msg: CheckInAllocMsg, handler: &Handler) -> String {