move DiagnosticArgFromDisplay into rustc_errors

compiler/rustc_errors/src/diagnostic.rs

@@ -40,6 +40,26 @@ pub trait IntoDiagnosticArg {
     fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static>;
 }
 
+pub struct DiagnosticArgFromDisplay<'a>(pub &'a dyn fmt::Display);
+
+impl IntoDiagnosticArg for DiagnosticArgFromDisplay<'_> {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        self.0.to_string().into_diagnostic_arg()
+    }
+}
+
+impl<'a> From<&'a dyn fmt::Display> for DiagnosticArgFromDisplay<'a> {
+    fn from(t: &'a dyn fmt::Display) -> Self {
+        DiagnosticArgFromDisplay(t)
+    }
+}
+
+impl<'a, T: fmt::Display> From<&'a T> for DiagnosticArgFromDisplay<'a> {
+    fn from(t: &'a T) -> Self {
+        DiagnosticArgFromDisplay(t)
+    }
+}
+
 macro_rules! into_diagnostic_arg_using_display {
     ($( $ty:ty ),+ $(,)?) => {
         $(

compiler/rustc_errors/src/lib.rs

@@ -371,8 +371,8 @@ impl fmt::Display for ExplicitBug {
 impl error::Error for ExplicitBug {}
 
 pub use diagnostic::{
-    AddSubdiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId,
-    DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
+    AddSubdiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgFromDisplay,
+    DiagnosticArgValue, DiagnosticId, DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
 };
 pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee, LintDiagnosticBuilder};
 use std::backtrace::Backtrace;