errors: implement IntoDiagnosticArg for &T

Implement `IntoDiagnosticArg` for `&'a T` when `T` implements
`IntoDiagnosticArg` and `Clone`. Makes it easier to write diagnostic
structs that borrow something which implements `IntoDiagnosticArg`.

Signed-off-by: David Wood <david.wood@huawei.com>

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -187,6 +187,12 @@ impl Display for RegionName {
     }
 }
 
+impl rustc_errors::IntoDiagnosticArg for RegionName {
+    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
+        self.to_string().into_diagnostic_arg()
+    }
+}
+
 impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
     pub(crate) fn mir_def_id(&self) -> hir::def_id::LocalDefId {
         self.body.source.def_id().expect_local()

compiler/rustc_borrowck/src/session_diagnostics.rs

@@ -1,4 +1,4 @@
-use rustc_errors::{IntoDiagnosticArg, MultiSpan};
+use rustc_errors::MultiSpan;
 use rustc_macros::{Diagnostic, LintDiagnostic, Subdiagnostic};
 use rustc_middle::ty::{GenericArg, Ty};
 use rustc_span::Span;
@@ -128,18 +128,6 @@ pub(crate) enum LifetimeReturnCategoryErr<'a> {
     },
 }
 
-impl IntoDiagnosticArg for &RegionName {
-    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
-        format!("{}", self).into_diagnostic_arg()
-    }
-}
-
-impl IntoDiagnosticArg for RegionName {
-    fn into_diagnostic_arg(self) -> rustc_errors::DiagnosticArgValue<'static> {
-        format!("{}", self).into_diagnostic_arg()
-    }
-}
-
 #[derive(Subdiagnostic)]
 pub(crate) enum RequireStaticErr {
     #[note(borrowck_used_impl_require_static)]

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -36,6 +36,12 @@ impl<'a, T: fmt::Display> From<&'a T> for DiagnosticArgFromDisplay<'a> {
     }
 }
 
+impl<'a, T: Clone + IntoDiagnosticArg> IntoDiagnosticArg for &'a T {
+    fn into_diagnostic_arg(self) -> DiagnosticArgValue<'static> {
+        self.clone().into_diagnostic_arg()
+    }
+}
+
 macro_rules! into_diagnostic_arg_using_display {
     ($( $ty:ty ),+ $(,)?) => {
         $(