diff --git a/Cargo.lock b/Cargo.lock
index 6009b11f904..ef4800a2261 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -4158,6 +4158,7 @@ dependencies = [
  "rustc_errors",
  "rustc_feature",
  "rustc_lexer",
+ "rustc_macros",
  "rustc_session",
  "rustc_span",
  "tracing",
diff --git a/compiler/rustc_error_messages/locales/en-US/parser.ftl b/compiler/rustc_error_messages/locales/en-US/parser.ftl
index 24e59a93cea..3143b81b609 100644
--- a/compiler/rustc_error_messages/locales/en-US/parser.ftl
+++ b/compiler/rustc_error_messages/locales/en-US/parser.ftl
@@ -1,3 +1,7 @@
 parser-struct-literal-body-without-path =
     struct literal body without path
     .suggestion = you might have forgotten to add the struct literal inside the block
+
+parser-maybe-report-ambiguous-plus =
+    ambiguous `+` in a type
+    .suggestion = use parentheses to disambiguate
diff --git a/compiler/rustc_macros/src/session_diagnostic.rs b/compiler/rustc_macros/src/session_diagnostic.rs
index ff7506979fc..9466d0f34bc 100644
--- a/compiler/rustc_macros/src/session_diagnostic.rs
+++ b/compiler/rustc_macros/src/session_diagnostic.rs
@@ -308,7 +308,7 @@ fn into_tokens(self) -> proc_macro2::TokenStream {
             {
                 fn into_diagnostic(
                     self,
-                    #sess: &'__session_diagnostic_sess rustc_session::Session
+                    #sess: &'__session_diagnostic_sess rustc_session::parse::ParseSess
                 ) -> rustc_errors::DiagnosticBuilder<'__session_diagnostic_sess, #param_ty> {
                     use rustc_errors::IntoDiagnosticArg;
                     #implementation
diff --git a/compiler/rustc_parse/Cargo.toml b/compiler/rustc_parse/Cargo.toml
index a823607ab0e..c6ca260e983 100644
--- a/compiler/rustc_parse/Cargo.toml
+++ b/compiler/rustc_parse/Cargo.toml
@@ -13,6 +13,7 @@ rustc_ast_pretty = { path = "../rustc_ast_pretty" }
 rustc_data_structures = { path = "../rustc_data_structures" }
 rustc_feature = { path = "../rustc_feature" }
 rustc_lexer = { path = "../rustc_lexer" }
+rustc_macros = { path = "../rustc_macros" }
 rustc_errors = { path = "../rustc_errors" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index acc0d7a6ee0..63772c098b5 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -21,6 +21,7 @@
 use rustc_errors::{
     Applicability, DiagnosticBuilder, DiagnosticMessage, Handler, MultiSpan, PResult,
 };
+use rustc_macros::SessionDiagnostic;
 use rustc_span::source_map::Spanned;
 use rustc_span::symbol::{kw, Ident};
 use rustc_span::{Span, SpanSnippetError, DUMMY_SP};
@@ -241,6 +242,16 @@ fn emit_many<G: EmissionGuarantee>(
         err.multipart_suggestions(msg, suggestions.map(|s| s.patches), applicability);
     }
 }
+
+#[derive(SessionDiagnostic)]
+#[error(slug = "parser-maybe-report-ambiguous-plus")]
+struct AmbiguousPlus {
+    pub sum_ty: String,
+    #[primary_span]
+    #[suggestion(code = "({sum_ty})")]
+    pub span: Span,
+}
+
 // SnapshotParser is used to create a snapshot of the parser
 // without causing duplicate errors being emitted when the `Parser`
 // is dropped.
@@ -1196,15 +1207,7 @@ pub(super) fn maybe_report_ambiguous_plus(
         ty: &Ty,
     ) {
         if matches!(allow_plus, AllowPlus::No) && impl_dyn_multi {
-            let sum_with_parens = format!("({})", pprust::ty_to_string(&ty));
-            self.struct_span_err(ty.span, "ambiguous `+` in a type")
-                .span_suggestion(
-                    ty.span,
-                    "use parentheses to disambiguate",
-                    sum_with_parens,
-                    Applicability::MachineApplicable,
-                )
-                .emit();
+            self.sess.emit_err(AmbiguousPlus { sum_ty: pprust::ty_to_string(&ty), span: ty.span });
         }
     }
 
diff --git a/compiler/rustc_session/src/parse.rs b/compiler/rustc_session/src/parse.rs
index 6a36ae63c68..e933fe1cb24 100644
--- a/compiler/rustc_session/src/parse.rs
+++ b/compiler/rustc_session/src/parse.rs
@@ -3,13 +3,14 @@
 
 use crate::config::CheckCfg;
 use crate::lint::{BufferedEarlyLint, BuiltinLintDiagnostics, Lint, LintId};
+use crate::SessionDiagnostic;
 use rustc_ast::node_id::NodeId;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_data_structures::sync::{Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, ColorConfig, Handler};
 use rustc_errors::{
     error_code, fallback_fluent_bundle, Applicability, Diagnostic, DiagnosticBuilder,
-    ErrorGuaranteed, MultiSpan,
+    DiagnosticMessage, ErrorGuaranteed, MultiSpan,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -287,4 +288,23 @@ pub fn save_proc_macro_span(&self, span: Span) -> usize {
     pub fn proc_macro_quoted_spans(&self) -> Vec<Span> {
         self.proc_macro_quoted_spans.lock().clone()
     }
+
+    pub fn emit_err<'a>(&'a self, err: impl SessionDiagnostic<'a>) -> ErrorGuaranteed {
+        err.into_diagnostic(self).emit()
+    }
+
+    pub fn emit_warning<'a>(&'a self, warning: impl SessionDiagnostic<'a, ()>) {
+        warning.into_diagnostic(self).emit()
+    }
+
+    pub fn struct_err(
+        &self,
+        msg: impl Into<DiagnosticMessage>,
+    ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
+        self.span_diagnostic.struct_err(msg)
+    }
+
+    pub fn struct_warn(&self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'_, ()> {
+        self.span_diagnostic.struct_warn(msg)
+    }
 }
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index b4548129689..e8279f6fed2 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -212,7 +212,7 @@ pub struct PerfStats {
 pub trait SessionDiagnostic<'a, T: EmissionGuarantee = ErrorGuaranteed> {
     /// Write out as a diagnostic out of `sess`.
     #[must_use]
-    fn into_diagnostic(self, sess: &'a Session) -> DiagnosticBuilder<'a, T>;
+    fn into_diagnostic(self, sess: &'a ParseSess) -> DiagnosticBuilder<'a, T>;
 }
 
 impl Session {
@@ -334,7 +334,7 @@ pub fn struct_err(
         &self,
         msg: impl Into<DiagnosticMessage>,
     ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
-        self.diagnostic().struct_err(msg)
+        self.parse_sess.struct_err(msg)
     }
     pub fn struct_err_with_code(
         &self,
@@ -414,10 +414,10 @@ pub fn err(&self, msg: impl Into<DiagnosticMessage>) -> ErrorGuaranteed {
         self.diagnostic().err(msg)
     }
     pub fn emit_err<'a>(&'a self, err: impl SessionDiagnostic<'a>) -> ErrorGuaranteed {
-        err.into_diagnostic(self).emit()
+        self.parse_sess.emit_err(err)
     }
     pub fn emit_warning<'a>(&'a self, warning: impl SessionDiagnostic<'a, ()>) {
-        warning.into_diagnostic(self).emit()
+        self.parse_sess.emit_warning(warning)
     }
     #[inline]
     pub fn err_count(&self) -> usize {