errors: add emit_note/create_note

Add `Noted` marker struct that implements `EmissionGuarantee` so that
`emit_note` and `create_note` can be implemented for struct diagnostics.

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

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -255,6 +255,56 @@ impl EmissionGuarantee for () {
     }
 }
 
+/// Marker type which enables implementation of `create_note` and `emit_note` functions for
+/// note-without-error struct diagnostics.
+#[derive(Copy, Clone)]
+pub struct Noted;
+
+impl<'a> DiagnosticBuilder<'a, Noted> {
+    /// Convenience function for internal use, clients should use one of the
+    /// `struct_*` methods on [`Handler`].
+    pub(crate) fn new_note(handler: &'a Handler, message: impl Into<DiagnosticMessage>) -> Self {
+        let diagnostic = Diagnostic::new_with_code(Level::Note, None, message);
+        Self::new_diagnostic_note(handler, diagnostic)
+    }
+
+    /// Creates a new `DiagnosticBuilder` with an already constructed
+    /// diagnostic.
+    pub(crate) fn new_diagnostic_note(handler: &'a Handler, diagnostic: Diagnostic) -> Self {
+        debug!("Created new diagnostic");
+        Self {
+            inner: DiagnosticBuilderInner {
+                state: DiagnosticBuilderState::Emittable(handler),
+                diagnostic: Box::new(diagnostic),
+            },
+            _marker: PhantomData,
+        }
+    }
+}
+
+impl EmissionGuarantee for Noted {
+    fn diagnostic_builder_emit_producing_guarantee(db: &mut DiagnosticBuilder<'_, Self>) -> Self {
+        match db.inner.state {
+            // First `.emit()` call, the `&Handler` is still available.
+            DiagnosticBuilderState::Emittable(handler) => {
+                db.inner.state = DiagnosticBuilderState::AlreadyEmittedOrDuringCancellation;
+                handler.emit_diagnostic(&mut db.inner.diagnostic);
+            }
+            // `.emit()` was previously called, disallowed from repeating it.
+            DiagnosticBuilderState::AlreadyEmittedOrDuringCancellation => {}
+        }
+
+        Noted
+    }
+
+    fn make_diagnostic_builder(
+        handler: &Handler,
+        msg: impl Into<DiagnosticMessage>,
+    ) -> DiagnosticBuilder<'_, Self> {
+        DiagnosticBuilder::new_note(handler, msg)
+    }
+}
+
 impl<'a> DiagnosticBuilder<'a, !> {
     /// Convenience function for internal use, clients should use one of the
     /// `struct_*` methods on [`Handler`].

compiler/rustc_errors/src/lib.rs

@@ -374,7 +374,7 @@ pub use diagnostic::{
     AddToDiagnostic, DecorateLint, Diagnostic, DiagnosticArg, DiagnosticArgFromDisplay,
     DiagnosticArgValue, DiagnosticId, DiagnosticStyledString, IntoDiagnosticArg, SubDiagnostic,
 };
-pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee};
+pub use diagnostic_builder::{DiagnosticBuilder, EmissionGuarantee, Noted};
 use std::backtrace::Backtrace;
 
 /// A handler deals with errors and other compiler output.
@@ -988,7 +988,11 @@ impl Handler {
     }
 
     pub fn has_errors(&self) -> Option<ErrorGuaranteed> {
-        if self.inner.borrow().has_errors() { Some(ErrorGuaranteed(())) } else { None }
+        if self.inner.borrow().has_errors() {
+            Some(ErrorGuaranteed(()))
+        } else {
+            None
+        }
     }
     pub fn has_errors_or_lint_errors(&self) -> Option<ErrorGuaranteed> {
         if self.inner.borrow().has_errors_or_lint_errors() {

compiler/rustc_session/src/parse.rs

@@ -12,7 +12,7 @@ use rustc_data_structures::sync::{Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, ColorConfig, Handler};
 use rustc_errors::{
     fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticId, DiagnosticMessage,
-    EmissionGuarantee, ErrorGuaranteed, IntoDiagnostic, MultiSpan, StashKey,
+    EmissionGuarantee, ErrorGuaranteed, IntoDiagnostic, MultiSpan, Noted, StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -354,6 +354,17 @@ impl ParseSess {
         self.create_warning(warning).emit()
     }
 
+    pub fn create_note<'a>(
+        &'a self,
+        note: impl IntoDiagnostic<'a, Noted>,
+    ) -> DiagnosticBuilder<'a, Noted> {
+        note.into_diagnostic(&self.span_diagnostic)
+    }
+
+    pub fn emit_note<'a>(&'a self, note: impl IntoDiagnostic<'a, Noted>) -> Noted {
+        self.create_note(note).emit()
+    }
+
     pub fn create_fatal<'a>(
         &'a self,
         fatal: impl IntoDiagnostic<'a, !>,

compiler/rustc_session/src/session.rs

@@ -28,7 +28,7 @@ use rustc_errors::json::JsonEmitter;
 use rustc_errors::registry::Registry;
 use rustc_errors::{
     error_code, fallback_fluent_bundle, DiagnosticBuilder, DiagnosticId, DiagnosticMessage,
-    ErrorGuaranteed, FluentBundle, IntoDiagnostic, LazyFallbackBundle, MultiSpan,
+    ErrorGuaranteed, FluentBundle, IntoDiagnostic, LazyFallbackBundle, MultiSpan, Noted,
 };
 use rustc_macros::HashStable_Generic;
 pub use rustc_span::def_id::StableCrateId;
@@ -489,6 +489,15 @@ impl Session {
     pub fn emit_warning<'a>(&'a self, warning: impl IntoDiagnostic<'a, ()>) {
         self.parse_sess.emit_warning(warning)
     }
+    pub fn create_note<'a>(
+        &'a self,
+        note: impl IntoDiagnostic<'a, Noted>,
+    ) -> DiagnosticBuilder<'a, Noted> {
+        self.parse_sess.create_note(note)
+    }
+    pub fn emit_note<'a>(&'a self, note: impl IntoDiagnostic<'a, Noted>) -> Noted {
+        self.parse_sess.emit_note(note)
+    }
     pub fn create_fatal<'a>(
         &'a self,
         fatal: impl IntoDiagnostic<'a, !>,