macros: add #[no_arg] to skip set_arg call

A call to `set_arg` is generated for every field of a
`SessionDiagnostic` struct without attributes, but not all types support
being an argument, so `#[no_arg]` is introduced to skip these fields.

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

compiler/rustc_macros/src/lib.rs

@@ -67,6 +67,7 @@ decl_derive!(
         warning,
         error,
         // field attributes
+        skip_arg,
         primary_span,
         label,
         suggestion,

compiler/rustc_macros/src/session_diagnostic.rs

@@ -216,7 +216,12 @@ impl<'a> SessionDiagnosticDerive<'a> {
                     if field.attrs.is_empty() {
                         let diag = &builder.diag;
                         let ident = field_binding.ast().ident.as_ref().unwrap();
-                        quote! { #diag.set_arg(stringify!(#ident), #field_binding.into_diagnostic_arg()); }
+                        quote! {
+                            #diag.set_arg(
+                                stringify!(#ident),
+                                #field_binding.into_diagnostic_arg()
+                            );
+                        }
                     } else {
                         quote! {}
                     }
@@ -566,6 +571,11 @@ impl<'a> SessionDiagnosticDeriveBuilder<'a> {
         let meta = attr.parse_meta()?;
         match meta {
             syn::Meta::Path(_) => match name {
+                "skip_arg" => {
+                    // Don't need to do anything - by virtue of the attribute existing, the
+                    // `set_arg` call will not be generated.
+                    Ok(quote! {})
+                }
                 "primary_span" => {
                     if type_matches_path(&info.ty, &["rustc_span", "Span"]) {
                         return Ok(quote! {

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

@@ -311,3 +311,23 @@ struct ErrorWithLifetime<'a> {
     span: Span,
     name: &'a str,
 }
+
+#[derive(SessionDiagnostic)]
+//~^ ERROR no method named `into_diagnostic_arg` found for struct `Hello` in the current scope
+#[error(code = "E0123", slug = "foo")]
+struct ArgFieldWithoutSkip {
+    #[primary_span]
+    span: Span,
+    other: Hello,
+}
+
+#[derive(SessionDiagnostic)]
+#[error(code = "E0123", slug = "foo")]
+struct ArgFieldWithSkip {
+    #[primary_span]
+    span: Span,
+    // `Hello` does not implement `IntoDiagnosticArg` so this would result in an error if
+    // not for `#[skip_arg]`.
+    #[skip_arg]
+    other: Hello,
+}

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

@@ -274,5 +274,17 @@ error: cannot find attribute `nonsense` in this scope
 LL |     #[nonsense]
    |       ^^^^^^^^
 
-error: aborting due to 34 previous errors
+error[E0599]: no method named `into_diagnostic_arg` found for struct `Hello` in the current scope
+  --> $DIR/session-derive-errors.rs:315:10
+   |
+LL | struct Hello {}
+   | ------------ method `into_diagnostic_arg` not found for this
+...
+LL | #[derive(SessionDiagnostic)]
+   |          ^^^^^^^^^^^^^^^^^ method not found in `Hello`
+   |
+   = note: this error originates in the derive macro `SessionDiagnostic` (in Nightly builds, run with -Z macro-backtrace for more info)
 
+error: aborting due to 35 previous errors
+
+For more information about this error, try `rustc --explain E0599`.