Rollup merge of #63051 - estebank:borrow-ice, r=matthewjasper

Avoid ICE when referencing desugared local binding in borrow error

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, the binding generated by desugaring the for loop would leak into the diagnostic (#63027):
```
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
```

Ideally `LocalKind` would carry more information to more accurately explain the problem, but for now, in order to avoid the ICE (fix #63026), we accept `LocalKind::Var` without a name and produce the following output:

```
error[E0515]: cannot return value referencing local binding
  --> $DIR/return-local-binding-from-desugaring.rs:30:5
   |
LL |     for ref x in xs {
   |                  -- local binding introduced here
...
LL |     result
   |     ^^^^^^ returns a value referencing data owned by the current function
```

src/librustc_mir/borrow_check/conflict_errors.rs

@@ -1140,19 +1140,18 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 bug!("try_report_cannot_return_reference_to_local: not a local")
             };
             match self.body.local_kind(*local) {
-                LocalKind::ReturnPointer | LocalKind::Temp => {
-                    (
-                        "temporary value".to_string(),
-                        "temporary value created here".to_string(),
-                    )
-                }
-                LocalKind::Arg => {
-                    (
-                        "function parameter".to_string(),
-                        "function parameter borrowed here".to_string(),
-                    )
-                },
-                LocalKind::Var => bug!("local variable without a name"),
+                LocalKind::ReturnPointer | LocalKind::Temp => (
+                    "temporary value".to_string(),
+                    "temporary value created here".to_string(),
+                ),
+                LocalKind::Arg => (
+                    "function parameter".to_string(),
+                    "function parameter borrowed here".to_string(),
+                ),
+                LocalKind::Var => (
+                    "local binding".to_string(),
+                    "local binding introduced here".to_string(),
+                ),
             }
         };
 

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/return-local-binding-from-desugaring.stderr

@@ -0,0 +1,12 @@
+error[E0515]: cannot return value referencing local binding
+  --> $DIR/return-local-binding-from-desugaring.rs:30:5
+   |
+LL |     for ref x in xs {
+   |                  -- local binding introduced here
+...
+LL |     result
+   |     ^^^^^^ returns a value referencing data owned by the current function
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0515`.