Do not ICE on lifetime error involving closures

src/librustc_mir/borrow_check/diagnostics/conflict_errors.rs

@@ -882,9 +882,27 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 err.span_label(
                     drop_span,
                     format!(
-                        "...but `{}` will be dropped here, when the function `{}` returns",
+                        "...but `{}` will be dropped here, when the {} returns",
                         name,
-                        self.infcx.tcx.hir().name(fn_hir_id),
+                        self.infcx
+                            .tcx
+                            .hir()
+                            .opt_name(fn_hir_id)
+                            .map(|name| format!("function `{}`", name))
+                            .unwrap_or_else(|| {
+                                match &self
+                                    .infcx
+                                    .tcx
+                                    .typeck_tables_of(self.mir_def_id)
+                                    .node_type(fn_hir_id)
+                                    .kind
+                                {
+                                    ty::Closure(..) => "enclosing closure",
+                                    ty::Generator(..) => "enclosing generator",
+                                    kind => bug!("expected closure or generator, found {:?}", kind),
+                                }
+                                .to_string()
+                            })
                     ),
                 );
 

src/test/ui/lifetimes/unnamed-closure-doesnt-life-long-enough-issue-67634.rs

@@ -0,0 +1,3 @@
+fn main() {
+    [0].iter().flat_map(|a| [0].iter().map(|_| &a)); //~ ERROR `a` does not live long enough
+}

src/test/ui/lifetimes/unnamed-closure-doesnt-life-long-enough-issue-67634.stderr

@@ -0,0 +1,15 @@
+error[E0597]: `a` does not live long enough
+  --> $DIR/unnamed-closure-doesnt-life-long-enough-issue-67634.rs:2:49
+   |
+LL |     [0].iter().flat_map(|a| [0].iter().map(|_| &a));
+   |                                             -   ^- ...but `a` will be dropped here, when the enclosing closure returns
+   |                                             |   |
+   |                                             |   `a` would have to be valid for `'_`...
+   |                                             has type `&i32`
+   |
+   = note: functions cannot return a borrow to data owned within the function's scope, functions can only return borrows to data passed as arguments
+   = note: to learn more, visit <https://doc.rust-lang.org/book/ch04-02-references-and-borrowing.html#dangling-references>
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0597`.