A custom error message for lending iterators

compiler/rustc_resolve/messages.ftl

@@ -234,6 +234,10 @@ resolve_items_in_traits_are_not_importable =
 resolve_label_with_similar_name_reachable =
     a label with a similar name is reachable
 
+resolve_lending_iterator_report_error =
+    associated type `Iterator::Item` is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type.
+    .note = you can't create an `Iterator` that borrows each `Item` from itself, but you can instead create a new type that borrows your existing type and implement `Iterator` for that new type.
+
 resolve_lifetime_param_in_enum_discriminant =
     lifetime parameters may not be used in enum discriminant values
 

compiler/rustc_resolve/src/errors.rs

@@ -882,6 +882,15 @@ pub(crate) struct ElidedAnonymousLivetimeReportError {
     pub(crate) suggestion: Option<ElidedAnonymousLivetimeReportErrorSuggestion>,
 }
 
+#[derive(Diagnostic)]
+#[diag(resolve_lending_iterator_report_error)]
+pub(crate) struct LendingIteratorReportError {
+    #[primary_span]
+    pub(crate) lifetime: Span,
+    #[note]
+    pub(crate) ty: Span,
+}
+
 #[derive(Subdiagnostic)]
 #[multipart_suggestion(
     resolve_elided_anonymous_lifetime_report_error_suggestion,

compiler/rustc_resolve/src/late.rs

@@ -9,7 +9,7 @@
 use crate::{errors, path_names_to_string, rustdoc, BindingError, Finalize, LexicalScopeBinding};
 use crate::{BindingKey, Used};
 use crate::{Module, ModuleOrUniformRoot, NameBinding, ParentScope, PathResult};
-use crate::{ResolutionError, Resolver, Segment, UseError};
+use crate::{ResolutionError, Resolver, Segment, TyCtxt, UseError};
 
 use rustc_ast::ptr::P;
 use rustc_ast::visit::{visit_opt, walk_list, AssocCtxt, BoundKind, FnCtxt, FnKind, Visitor};
@@ -1703,10 +1703,28 @@ fn resolve_anonymous_lifetime(&mut self, lifetime: &Lifetime, elided: bool) {
                                 break;
                             }
                         }
-                        self.r.dcx().emit_err(errors::ElidedAnonymousLivetimeReportError {
-                            span: lifetime.ident.span,
-                            suggestion,
-                        });
+
+                        // Is it caused by user trying to implement a lending iterator?
+                        if !self.in_func_body
+                            && let Some((module, _)) = &self.current_trait_ref
+                            && let Some(ty) = &self.diag_metadata.current_self_type
+                            && let crate::ModuleKind::Def(DefKind::Trait, trait_id, _) = module.kind
+                            && def_id_matches_path(
+                                self.r.tcx,
+                                trait_id,
+                                &["core", "iter", "traits", "iterator", "Iterator"],
+                            )
+                        {
+                            self.r.dcx().emit_err(errors::LendingIteratorReportError {
+                                lifetime: lifetime.ident.span,
+                                ty: ty.span(),
+                            });
+                        } else {
+                            self.r.dcx().emit_err(errors::ElidedAnonymousLivetimeReportError {
+                                span: lifetime.ident.span,
+                                suggestion,
+                            });
+                        }
                     } else {
                         self.r.dcx().emit_err(errors::ExplicitAnonymousLivetimeReportError {
                             span: lifetime.ident.span,
@@ -4824,3 +4842,15 @@ pub(crate) fn late_resolve_crate(&mut self, krate: &Crate) {
         }
     }
 }
+
+/// Check if definition matches a path
+fn def_id_matches_path(tcx: TyCtxt<'_>, mut def_id: DefId, expected_path: &[&str]) -> bool {
+    let mut path = expected_path.iter().rev();
+    while let (Some(parent), Some(next_step)) = (tcx.opt_parent(def_id), path.next()) {
+        if !tcx.opt_item_name(def_id).map_or(false, |n| n.as_str() == *next_step) {
+            return false;
+        }
+        def_id = parent;
+    }
+    return true;
+}

tests/ui/lifetimes/no_lending_iterators.rs

@@ -0,0 +1,12 @@
+struct Data(String);
+
+impl Iterator for Data {
+    type Item = &str;
+    //~^ ERROR 4:17: 4:18: associated type `Iterator::Item` is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type.
+
+    fn next(&mut self) -> Option<Self::Item> {
+        Some(&self.0)
+    }
+}
+
+fn main() {}

tests/ui/lifetimes/no_lending_iterators.stderr

@@ -0,0 +1,14 @@
+error: associated type `Iterator::Item` is declared without lifetime parameters, so using a borrowed type for them requires that lifetime to come from the implemented type.
+  --> $DIR/no_lending_iterators.rs:4:17
+   |
+LL |     type Item = &str;
+   |                 ^
+   |
+note: you can't create an `Iterator` that borrows each `Item` from itself, but you can instead create a new type that borrows your existing type and implement `Iterator` for that new type.
+  --> $DIR/no_lending_iterators.rs:3:19
+   |
+LL | impl Iterator for Data {
+   |                   ^^^^
+
+error: aborting due to 1 previous error
+