Correct detection of elided lifetimes in impl-trait.

Author: cjgillot

compiler/rustc_hir_analysis/src/collect/lifetimes.rs

@@ -1195,8 +1195,10 @@ impl<'a, 'tcx> LifetimeContext<'a, 'tcx> {
                     // Fresh lifetimes in APIT used to be allowed in async fns and forbidden in
                     // regular fns.
                     if let Some(hir::PredicateOrigin::ImplTrait) = where_bound_origin
-                        && let hir::LifetimeName::Param(_) = lifetime_ref.res
-                        && lifetime_ref.is_anonymous()
+                        && let hir::LifetimeName::Param(param_id) = lifetime_ref.res
+                        && let Some(generics) = self.tcx.hir().get_generics(self.tcx.local_parent(param_id))
+                        && let Some(param) = generics.params.iter().find(|p| p.def_id == param_id)
+                        && param.is_elided_lifetime()
                         && let hir::IsAsync::NotAsync = self.tcx.asyncness(lifetime_ref.hir_id.owner.def_id)
                         && !self.tcx.features().anonymous_lifetime_in_impl_trait
                     {

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

@@ -60,4 +60,9 @@ mod in_path {
     //~| ERROR missing lifetime specifier
 }
 
+// This must not err, as the `&` actually resolves to `'a`.
+fn resolved_anonymous<'a, T>(f: impl Fn(&'a str) -> &T) {
+    f("f")
+}
+
 fn main() {}