review comments

src/test/ui/borrowck/return-local-binding-from-desugaring.rs

@@ -0,0 +1,33 @@
+// To avoid leaking the names of local bindings from expressions like for loops, #60984
+// explicitly ignored them, but an assertion that `LocalKind::Var` *must* have a name would
+// trigger an ICE. Before this change, this file's output would be:
+// ```
+// error[E0515]: cannot return value referencing local variable `__next`
+//   --> return-local-binding-from-desugaring.rs:LL:CC
+//    |
+// LL |     for ref x in xs {
+//    |         ----- `__next` is borrowed here
+// ...
+// LL |     result
+//    |     ^^^^^^ returns a value referencing data owned by the current function
+// ```
+// FIXME: ideally `LocalKind` would carry more information to more accurately explain the problem.
+
+use std::collections::HashMap;
+use std::hash::Hash;
+
+fn group_by<I, F, T>(xs: &mut I, f: F) -> HashMap<T, Vec<&I::Item>>
+where
+    I: Iterator,
+    F: Fn(&I::Item) -> T,
+    T: Eq + Hash,
+{
+    let mut result = HashMap::new();
+    for ref x in xs {
+        let key = f(x);
+        result.entry(key).or_insert(Vec::new()).push(x);
+    }
+    result //~ ERROR cannot return value referencing local binding
+}
+
+fn main() {}

src/test/ui/borrowck/issue-63026.stderr → src/test/ui/borrowck/return-local-binding-from-desugaring.stderr

@@ -1,5 +1,5 @@
 error[E0515]: cannot return value referencing local binding
-  --> $DIR/issue-63026.rs:15:5
+  --> $DIR/return-local-binding-from-desugaring.rs:30:5
    |
 LL |     for ref x in xs {
    |                  -- local binding introduced here