Move "elided lifetime in path" to subdiagnostic struct

This requires nested subdiagnostics.

compiler/rustc_errors/src/diagnostic_impls.rs

@@ -348,3 +348,11 @@ pub struct IndicateAnonymousLifetime {
     pub count: usize,
     pub suggestion: String,
 }
+
+#[derive(Subdiagnostic)]
+pub struct ElidedLifetimeInPathSubdiag {
+    #[subdiagnostic]
+    pub expected: ExpectedLifetimeParameter,
+    #[subdiagnostic]
+    pub indicate: Option<IndicateAnonymousLifetime>,
+}

compiler/rustc_errors/src/lib.rs

@@ -41,8 +41,8 @@ pub use diagnostic::{
     SubdiagMessageOp, Subdiagnostic,
 };
 pub use diagnostic_impls::{
-    DiagArgFromDisplay, DiagSymbolList, ExpectedLifetimeParameter, IndicateAnonymousLifetime,
-    SingleLabelManySpans,
+    DiagArgFromDisplay, DiagSymbolList, ElidedLifetimeInPathSubdiag, ExpectedLifetimeParameter,
+    IndicateAnonymousLifetime, SingleLabelManySpans,
 };
 pub use emitter::ColorConfig;
 pub use rustc_error_messages::{
@@ -1912,27 +1912,24 @@ impl Level {
 }
 
 // FIXME(eddyb) this doesn't belong here AFAICT, should be moved to callsite.
-pub fn add_elided_lifetime_in_path_suggestion<G: EmissionGuarantee>(
+pub fn elided_lifetime_in_path_suggestion(
     source_map: &SourceMap,
-    diag: &mut Diag<'_, G>,
     n: usize,
     path_span: Span,
     incl_angl_brckt: bool,
     insertion_span: Span,
-) {
-    diag.subdiagnostic(diag.dcx, ExpectedLifetimeParameter { span: path_span, count: n });
-    if !source_map.is_span_accessible(insertion_span) {
-        // Do not try to suggest anything if generated by a proc-macro.
-        return;
-    }
-    let anon_lts = vec!["'_"; n].join(", ");
-    let suggestion =
-        if incl_angl_brckt { format!("<{anon_lts}>") } else { format!("{anon_lts}, ") };
+) -> ElidedLifetimeInPathSubdiag {
+    let expected = ExpectedLifetimeParameter { span: path_span, count: n };
+    // Do not try to suggest anything if generated by a proc-macro.
+    let indicate = source_map.is_span_accessible(insertion_span).then(|| {
+        let anon_lts = vec!["'_"; n].join(", ");
+        let suggestion =
+            if incl_angl_brckt { format!("<{anon_lts}>") } else { format!("{anon_lts}, ") };
 
-    diag.subdiagnostic(
-        diag.dcx,
-        IndicateAnonymousLifetime { span: insertion_span.shrink_to_hi(), count: n, suggestion },
-    );
+        IndicateAnonymousLifetime { span: insertion_span.shrink_to_hi(), count: n, suggestion }
+    });
+
+    ElidedLifetimeInPathSubdiag { expected, indicate }
 }
 
 pub fn report_ambiguity_error<'a, G: EmissionGuarantee>(

compiler/rustc_lint/src/context/diagnostics.rs

@@ -2,7 +2,7 @@
 #![allow(rustc::untranslatable_diagnostic)]
 
 use rustc_ast::util::unicode::TEXT_FLOW_CONTROL_CHARS;
-use rustc_errors::{add_elided_lifetime_in_path_suggestion, Diag};
+use rustc_errors::{elided_lifetime_in_path_suggestion, Diag};
 use rustc_errors::{Applicability, SuggestionStyle};
 use rustc_middle::middle::stability;
 use rustc_session::lint::BuiltinLintDiag;
@@ -74,13 +74,15 @@ pub(super) fn builtin(sess: &Session, diagnostic: BuiltinLintDiag, diag: &mut Di
             diag.span_note(span_def, "the macro is defined here");
         }
         BuiltinLintDiag::ElidedLifetimesInPaths(n, path_span, incl_angl_brckt, insertion_span) => {
-            add_elided_lifetime_in_path_suggestion(
-                sess.source_map(),
-                diag,
-                n,
-                path_span,
-                incl_angl_brckt,
-                insertion_span,
+            diag.subdiagnostic(
+                sess.dcx(),
+                elided_lifetime_in_path_suggestion(
+                    sess.source_map(),
+                    n,
+                    path_span,
+                    incl_angl_brckt,
+                    insertion_span,
+                ),
             );
         }
         BuiltinLintDiag::UnknownCrateTypes(span, note, sugg) => {

compiler/rustc_resolve/src/errors.rs

@@ -1,4 +1,4 @@
-use rustc_errors::{codes::*, Applicability, MultiSpan};
+use rustc_errors::{codes::*, Applicability, ElidedLifetimeInPathSubdiag, MultiSpan};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{
     symbol::{Ident, Symbol},
@@ -907,6 +907,8 @@ pub(crate) struct ExplicitAnonymousLivetimeReportError {
 pub(crate) struct ImplicitElidedLifetimeNotAllowedHere {
     #[primary_span]
     pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) subdiag: ElidedLifetimeInPathSubdiag,
 }
 
 #[derive(Diagnostic)]

compiler/rustc_resolve/src/late.rs

@@ -1883,20 +1883,18 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     //     async fn foo(_: std::cell::Ref<u32>) { ... }
                     LifetimeRibKind::AnonymousCreateParameter { report_in_path: true, .. }
                     | LifetimeRibKind::AnonymousWarn(_) => {
-                        let mut err =
-                            self.r.dcx().create_err(errors::ImplicitElidedLifetimeNotAllowedHere {
-                                span: path_span,
-                            });
                         let sess = self.r.tcx.sess;
-                        rustc_errors::add_elided_lifetime_in_path_suggestion(
+                        let subdiag = rustc_errors::elided_lifetime_in_path_suggestion(
                             sess.source_map(),
-                            &mut err,
                             expected_lifetimes,
                             path_span,
                             !segment.has_generic_args,
                             elided_lifetime_span,
                         );
-                        err.emit();
+                        self.r.dcx().emit_err(errors::ImplicitElidedLifetimeNotAllowedHere {
+                            span: path_span,
+                            subdiag,
+                        });
                         should_lint = false;
 
                         for id in node_ids {