Account for `impl Trait`

Address #49287

src/librustc_resolve/diagnostics.rs

@@ -1519,9 +1519,21 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
         for missing in &self.missing_named_lifetime_spots {
             match missing {
                 MissingLifetimeSpot::Generics(generics) => {
-                    let (span, sugg) = match &generics.params {
-                        [] => (generics.span, format!("<{}>", lifetime_ref)),
-                        [param, ..] => (param.span.shrink_to_lo(), format!("{}, ", lifetime_ref)),
+                    let (span, sugg) = if let Some(param) = generics
+                        .params
+                        .iter()
+                        .filter(|p| match p.kind {
+                            hir::GenericParamKind::Type {
+                                synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
+                                ..
+                            } => false,
+                            _ => true,
+                        })
+                        .next()
+                    {
+                        (param.span.shrink_to_lo(), format!("{}, ", lifetime_ref))
+                    } else {
+                        (generics.span, format!("<{}>", lifetime_ref))
                     };
                     err.span_suggestion(
                         span,
@@ -1592,7 +1604,8 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                     Applicability::MaybeIncorrect,
                 );
             };
-            let suggest_new = |err: &mut DiagnosticBuilder<'_>, sugg: &str| {
+            let suggest_new =
+                |err: &mut DiagnosticBuilder<'_>, sugg: &str| {
                     err.span_label(span, "expected named lifetime parameter");
 
                     for missing in self.missing_named_lifetime_spots.iter().rev() {
@@ -1603,9 +1616,16 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                         MissingLifetimeSpot::Generics(generics) => {
                             msg = "consider introducing a named lifetime parameter".to_string();
                             should_break = true;
-                            match &generics.params {
-                                [] => (generics.span, "<'a>".to_string()),
-                                [param, ..] => (param.span.shrink_to_lo(), "'a, ".to_string()),
+                            if let Some(param) = generics.params.iter().filter(|p| match p.kind {
+                                hir::GenericParamKind::Type {
+                                    synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
+                                    ..
+                                } => false,
+                                _ => true,
+                            }).next() {
+                                (param.span.shrink_to_lo(), "'a, ".to_string())
+                            } else {
+                                (generics.span, "<'a>".to_string())
                             }
                         }
                         MissingLifetimeSpot::HigherRanked { span, span_type } => {
@@ -1622,7 +1642,8 @@ impl<'tcx> LifetimeContext<'_, 'tcx> {
                         }
                     });
                         for param in params {
-                        if let Ok(snippet) = self.tcx.sess.source_map().span_to_snippet(param.span)
+                            if let Ok(snippet) =
+                                self.tcx.sess.source_map().span_to_snippet(param.span)
                             {
                                 if snippet.starts_with("&") && !snippet.starts_with("&'") {
                                     introduce_suggestion

src/test/ui/suggestions/impl-trait-missing-lifetime.rs

@@ -0,0 +1,2 @@
+fn f(_: impl Iterator<Item = &'_ ()>) {} //~ ERROR missing lifetime specifier
+fn main() {}

src/test/ui/suggestions/impl-trait-missing-lifetime.stderr

@@ -0,0 +1,14 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/impl-trait-missing-lifetime.rs:1:31
+   |
+LL | fn f(_: impl Iterator<Item = &'_ ()>) {}
+   |                               ^^ expected named lifetime parameter
+   |
+help: consider introducing a named lifetime parameter
+   |
+LL | fn f<'a>(_: impl Iterator<Item = &'a ()>) {}
+   |     ^^^^                          ^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0106`.