macros: subdiagnostic derive

Add a new derive, `#[derive(SessionSubdiagnostic)]`, which enables
deriving structs for labels, notes, helps and suggestions.

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

Author: davidtwco

compiler/rustc_error_messages/locales/en-US/typeck.ftl

@@ -82,3 +82,11 @@ typeck-value-of-associated-struct-already-specified =
 
 typeck-address-of-temporary-taken = cannot take address of a temporary
     .label = temporary value
+
+typeck-add-return-type-add = try adding a return type
+
+typeck-add-return-type-missing-here = a return type might be missing here
+
+typeck-expected-default-return-type = expected `()` because of default return type
+
+typeck-expected-return-type = expected `{$expected}` because of return type

compiler/rustc_errors/src/diagnostic.rs

@@ -78,6 +78,13 @@ impl<'source> Into<FluentValue<'source>> for DiagnosticArgValue<'source> {
     }
 }
 
+/// Trait implemented by error types. This should not be implemented manually. Instead, use
+/// `#[derive(SessionSubdiagnostic)]` -- see [rustc_macros::SessionSubdiagnostic].
+pub trait AddSubdiagnostic {
+    /// Add a subdiagnostic to an existing diagnostic.
+    fn add_to_diagnostic(self, diag: &mut Diagnostic);
+}
+
 #[must_use]
 #[derive(Clone, Debug, Encodable, Decodable)]
 pub struct Diagnostic {
@@ -768,6 +775,13 @@ impl Diagnostic {
         self
     }
 
+    /// Add a subdiagnostic from a type that implements `SessionSubdiagnostic` - see
+    /// [rustc_macros::SessionSubdiagnostic].
+    pub fn subdiagnostic(&mut self, subdiagnostic: impl AddSubdiagnostic) -> &mut Self {
+        subdiagnostic.add_to_diagnostic(self);
+        self
+    }
+
     pub fn set_span<S: Into<MultiSpan>>(&mut self, sp: S) -> &mut Self {
         self.span = sp.into();
         if let Some(span) = self.span.primary_span() {

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -530,6 +530,11 @@ impl<'a, G: EmissionGuarantee> DiagnosticBuilder<'a, G> {
         name: impl Into<Cow<'static, str>>,
         arg: DiagnosticArgValue<'static>,
     ) -> &mut Self);
+
+    forward!(pub fn subdiagnostic(
+        &mut self,
+        subdiagnostic: impl crate::AddSubdiagnostic
+    ) -> &mut Self);
 }
 
 impl<G: EmissionGuarantee> Debug for DiagnosticBuilder<'_, G> {

compiler/rustc_errors/src/lib.rs

@@ -370,8 +370,8 @@ impl fmt::Display for ExplicitBug {
 impl error::Error for ExplicitBug {}
 
 pub use diagnostic::{
-    Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId, DiagnosticStyledString,
-    IntoDiagnosticArg, SubDiagnostic,
+    AddSubdiagnostic, Diagnostic, DiagnosticArg, DiagnosticArgValue, DiagnosticId,
+    DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
 };
 pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
 use std::backtrace::Backtrace;

compiler/rustc_macros/src/lib.rs

@@ -1,5 +1,6 @@
-#![feature(proc_macro_diagnostic)]
 #![feature(allow_internal_unstable)]
+#![feature(let_else)]
+#![feature(proc_macro_diagnostic)]
 #![allow(rustc::default_hash_types)]
 #![recursion_limit = "128"]
 
@@ -77,3 +78,18 @@ decl_derive!(
         suggestion_hidden,
         suggestion_verbose)] => session_diagnostic::session_diagnostic_derive
 );
+decl_derive!(
+    [SessionSubdiagnostic, attributes(
+        // struct/variant attributes
+        label,
+        help,
+        note,
+        suggestion,
+        suggestion_short,
+        suggestion_hidden,
+        suggestion_verbose,
+        // field attributes
+        skip_arg,
+        primary_span,
+        applicability)] => session_diagnostic::session_subdiagnostic_derive
+);