errors: set_arg takes IntoDiagnosticArg

Manual implementors of translatable diagnostics will need to call
`set_arg`, not just the derive, so make this function a bit more
ergonomic by taking `IntoDiagnosticArg` rather than
`DiagnosticArgValue`.

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

compiler/rustc_errors/src/diagnostic.rs

@@ -821,9 +821,9 @@ impl Diagnostic {
     pub fn set_arg(
         &mut self,
         name: impl Into<Cow<'static, str>>,
-        arg: DiagnosticArgValue<'static>,
+        arg: impl IntoDiagnosticArg,
     ) -> &mut Self {
-        self.args.push((name.into(), arg));
+        self.args.push((name.into(), arg.into_diagnostic_arg()));
         self
     }
 

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -1,4 +1,4 @@
-use crate::diagnostic::DiagnosticArgValue;
+use crate::diagnostic::IntoDiagnosticArg;
 use crate::{Diagnostic, DiagnosticId, DiagnosticMessage, DiagnosticStyledString, ErrorGuaranteed};
 use crate::{Handler, Level, MultiSpan, StashKey};
 use rustc_lint_defs::Applicability;
@@ -528,7 +528,7 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
     forward!(pub fn set_arg(
         &mut self,
         name: impl Into<Cow<'static, str>>,
-        arg: DiagnosticArgValue<'static>,
+        arg: impl IntoDiagnosticArg,
     ) -> &mut Self);
 
     forward!(pub fn subdiagnostic(

compiler/rustc_macros/src/diagnostics/diagnostic.rs

@@ -113,7 +113,7 @@ impl<'a> SessionDiagnosticDerive<'a> {
                         quote! {
                             #diag.set_arg(
                                 stringify!(#ident),
-                                #field_binding.into_diagnostic_arg()
+                                #field_binding
                             );
                         }
                     } else {

compiler/rustc_macros/src/diagnostics/subdiagnostic.rs

@@ -349,7 +349,7 @@ impl<'a> SessionSubdiagnosticDeriveBuilder<'a> {
         let generated = quote! {
             #diag.set_arg(
                 stringify!(#ident),
-                #binding.into_diagnostic_arg()
+                #binding
             );
         };
 

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.rs

@@ -327,7 +327,7 @@ struct ErrorWithDefaultLabelAttr<'a> {
 }
 
 #[derive(SessionDiagnostic)]
-//~^ ERROR no method named `into_diagnostic_arg` found for struct `Hello` in the current scope
+//~^ ERROR the trait bound `Hello: IntoDiagnosticArg` is not satisfied
 #[error(code = "E0123", slug = "foo")]
 struct ArgFieldWithoutSkip {
     #[primary_span]

src/test/ui-fulldeps/session-diagnostic/diagnostic-derive.stderr

@@ -349,17 +349,26 @@ error: cannot find attribute `nonsense` in this scope
 LL |     #[nonsense]
    |       ^^^^^^^^
 
-error[E0599]: no method named `into_diagnostic_arg` found for struct `Hello` in the current scope
+error[E0277]: the trait bound `Hello: IntoDiagnosticArg` is not satisfied
   --> $DIR/diagnostic-derive.rs:329:10
    |
-LL | struct Hello {}
-   | ------------ method `into_diagnostic_arg` not found for this
-...
 LL | #[derive(SessionDiagnostic)]
-   |          ^^^^^^^^^^^^^^^^^ method not found in `Hello`
+   |          ^^^^^^^^^^^^^^^^^ the trait `IntoDiagnosticArg` is not implemented for `Hello`
    |
+   = help: the following other types implement trait `IntoDiagnosticArg`:
+             &'a str
+             Ident
+             String
+             Symbol
+             rustc_middle::ty::Ty<'tcx>
+             usize
+note: required by a bound in `DiagnosticBuilder::<'a, G>::set_arg`
+  --> $COMPILER_DIR/rustc_errors/src/diagnostic_builder.rs:531:19
+   |
+LL |         arg: impl IntoDiagnosticArg,
+   |                   ^^^^^^^^^^^^^^^^^ required by this bound in `DiagnosticBuilder::<'a, G>::set_arg`
    = note: this error originates in the derive macro `SessionDiagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
 error: aborting due to 43 previous errors
 
-For more information about this error, try `rustc --explain E0599`.
+For more information about this error, try `rustc --explain E0277`.