From 70ee0c96fc29c57f64a6ccfeb66cccf6fac951f7 Mon Sep 17 00:00:00 2001
From: David Wood <david.wood@huawei.com>
Date: Thu, 31 Mar 2022 09:02:31 +0100
Subject: [PATCH] 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              |  1 +
 .../rustc_macros/src/session_diagnostic.rs    | 12 ++++++++++-
 src/test/ui-fulldeps/session-derive-errors.rs | 20 +++++++++++++++++++
 .../ui-fulldeps/session-derive-errors.stderr  | 14 ++++++++++++-
 4 files changed, 45 insertions(+), 2 deletions(-)

diff --git a/compiler/rustc_macros/src/lib.rs b/compiler/rustc_macros/src/lib.rs
index b571fdb4ae2..67b56d2e9db 100644
--- a/compiler/rustc_macros/src/lib.rs
+++ b/compiler/rustc_macros/src/lib.rs
@@ -67,6 +67,7 @@ decl_derive!(
         warning,
         error,
         // field attributes
+        skip_arg,
         primary_span,
         label,
         suggestion,
diff --git a/compiler/rustc_macros/src/session_diagnostic.rs b/compiler/rustc_macros/src/session_diagnostic.rs
index 73cd840f0eb..fec9a820d0f 100644
--- a/compiler/rustc_macros/src/session_diagnostic.rs
+++ b/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! {
diff --git a/src/test/ui-fulldeps/session-derive-errors.rs b/src/test/ui-fulldeps/session-derive-errors.rs
index 422f3894991..96d78c2d00e 100644
--- a/src/test/ui-fulldeps/session-derive-errors.rs
+++ b/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,
+}
diff --git a/src/test/ui-fulldeps/session-derive-errors.stderr b/src/test/ui-fulldeps/session-derive-errors.stderr
index 78ed255523f..21bf8e060ba 100644
--- a/src/test/ui-fulldeps/session-derive-errors.stderr
+++ b/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`.